Method for manufacturing an emitter for an electrospray generator

Abstract

Method for manufacturing an emitter for an electrospray generator - Patents.com A first aspect of the invention relates to a method for manufacturing an emitter for an electrospray generator, the method comprising the steps of providing a substrate, presenting a plate and at least one protrusion, and thereafter nanotexturing an outer surface of the at least one protrusion. According to a second aspect, the invention also relates to an emitter resulting from the manufacturing method described above, to an electrospray generating device comprising an emitter according to the invention, and to an electric space propulsion device comprising at least one such electrospray generating device.

Description

[Technical Field]

[0001] A first aspect of the present invention relates to a method for manufacturing an emitter for an electrospray generator, the method comprising the steps of providing a substrate, providing a plate and at least one protrusion, and then , even if the nanowires are arranged on the protrusions, the nanowires originate from the low relief surface in a direction perpendicular to the plate; and nanotexturing an outer surface of the at least one protrusion.

[0002] According to a second aspect, the present invention also relates to an emitter resulting from the above manufacturing method, to an electrospray generator comprising an emitter according to the invention, and to an electric space propulsion device comprising at least one electrospray generator. [Background technology]

[0003] Electrospray, also known as electrodynamic spraying, is the electrostatic acceleration of a conducting liquid that has been used to provide and create molecular or ion flows in various technological fields such as biomedical applications, space propulsion applications, food applications, pharmaceutical applications, spectroscopy, and air purification devices.

[0004] To apply this technique, a device called an electrospray emitter or generator typically uses high-voltage power to disperse a conductive liquid, such as a conductive liquid metal, colloid, or ionic liquid. Ideally, the dispersed liquid reaches the tip of the emitter and sprays in a cone shape.

[0005] However, until the early 2000s, these techniques were not considered interesting enough to be implemented, especially in the field of space propulsion, as technological advances have led to smaller types of satellites requiring lower power propulsion systems.

[0006] Electrospray devices generally include an emitter and a counter electrode. Both of these elements provide the liquid with the ability to atomize into nano- or microdroplets, pure ion emissions, or a mixture of both. The liquid is primarily defined by its density, viscosity, surface tension, dielectric constant, and conductivity.

[0007] When an electric field is applied to the liquid / solid interface of a conducting liquid, charges are transferred to the interface where the conductivity is discontinuous, causing the liquid to move. For a sufficiently conducting liquid, the electric field tangential to the liquid surface is zero, and in the steady state, the remaining normal component of the electric field balances the local surface tension. The normal component of the electric field is determined by, among other parameters that are singular as r → 0, where r is the radius of the corresponding cone-like structure generated by the charge transfer. -1 / 2 Here we show the formula that depends on

[0008] In electrospray configurations, it is desirable to use a tip termination structure with as small a radius as possible to concentrate the electric field lines.

[0009] When a volume of conducting liquid is subjected to an electric field, the shape of the liquid begins to deform from that dictated by surface tension alone. As the voltage is increased, the effect of the electric field becomes more pronounced. When this effect of the electric field begins to exert a force on the droplet comparable in magnitude to that exerted by surface tension, a conical shape with a convex surface and a rounded tip begins to form. This results in a conical shape approaching the name "Taylor cone." When a certain threshold voltage is reached, the slightly rounded tip inverts, producing a liquid jet called a cone jet. The release of this cone jet is the beginning of the electrospray process, where ions can be transferred to the gas phase. It is generally recognized that a voltage slightly above the threshold must be used to achieve a stable cone jet. As the voltage is further increased, other states of droplet collapse are observed.

[0010] In this context, the dimensionless flow parameter

number

[0011] Depending on the fluid and field conditions, the ejection may be in the form of a jet, droplets, ions, or a mixture of droplet and ion ejection. For the present invention, the ejection is required to be purely ionic. Near the tip of the cone, the electric field may be strong enough to liberate ions directly from the surface, which is called field evaporation.

[0012] The electric field at the tip of the emitter is a function of the applied field, the geometry of the emitter and extractor, and the actual shape of the liquid meniscus, and may be expressed as f V, the product of the normalized angular emission profile and the applied voltage. The lowest voltage that allows field evaporation is known as the critical voltage.

[0013] The tip radius is the radius of the meniscus, which deforms under the influence of the electric field and thus determines the electric field. Determining this radius is difficult because it is not a parameter set by the manufacturing process. Nevertheless, the geometry of the emitter and extractor may dictate a specific shape in which the electric field is intended to be concentrated, and therefore is strongly related to the electric field at the tip.

[0014] In purely ionic electrospray, the volumetric flow rate of the liquid feeding the emitter must compensate for electrostatic evaporation, while some flow restriction is typically also required to ensure pure ionic conditions.

[0015] According to the prior art, several types of emitters are known.

[0016] capillary type Capillary emitters are based on the use of a capillary tube through which a liquid is delivered, the end of which terminates in a sharp microstructure, resulting in a concentrated electric field at the meniscus of the fluid exiting the capillary.

[0017] The main technical challenges of capillary emitters are: A minimum flow pressure is required to deliver a volumetric flow to the electrospray emitter; otherwise, if the pressure is lower, emission will cease or become erratic. - The narrow fluid passages in capillary electrosprays tend to become clogged throughout their operation, either by impurities in the liquid or by debris generated within the device. - Capillary electrospray exhibits low fluidic impedance, typically resulting in droplet or mixed conditions, but generally not ionic conditions.

[0018] Porous type Porous emitters were introduced as an alternative to capillary emitters in an attempt to overcome some of the limitations of these devices, primarily the reliance on a pressure feed that could be replaced by a passive feed in the porous medium. The clogging problem could also be alleviated because there are now multiple paths to the emitter tip within the porous medium.

[0019] The main technical challenges of porous emitters are: - High fluid impedance is required to achieve pure ionic conditions, which requires small pore sizes that compete with the mitigation of clogging that has been proposed as one of the advantages of the technology. The length scale of the Taylor cone of the emission region depends on the pore size. This requires that the small pore radii required for high fluid impedance to achieve pure ionic conditions can result in multiple emission regions where the electric field is concentrated close to the microstructure tip. This can result in larger dispersion angles and substantially lower angular dispersion efficiency than other emitters. - A capacitive double layer has been observed at the interface between the emitter and the ionic liquid. With longer emission times or higher currents, the electrochemical window limit can be exceeded, leading to chemical reactions between the ionic liquid and the material that have been shown to etch and roughen the emitter tip, causing a degradation of the emission characteristics over time.

[0020] Wet outer surface An alternative solution to capillary electrospray is the externally wetting emitter, in which the emitter surface is coated with a wicking material to transport the liquid to the emitter tip by capillary forces. This type of emitter can have the hydraulic impedance required to achieve pure ionic conditions. For example, surface coatings made from black silicon are known to work, but the morphology of the coating, particularly the surface distribution and aspect ratio, cannot be controlled by typically applied methods, and therefore hydraulic impedance cannot be engineered by adjusting the height or separation of the nanostructures. Furthermore, coatings can undergo mechanical degradation over time under the stresses of device operation.

[0021] It is concluded that to achieve high-throughput, surface-wetting electrospray operating at limited voltages, the emitter design must be adapted to reduce both the onset voltage and hydraulic impedance, as long as the hydraulic impedance value is high enough to maintain ionic conditions. Since hydraulic impedance assumes an "optimum" value that maximizes extracted current while allowing ion emission, the main problem that arises is to establish a surface texture that allows control of hydraulic impedance.

[0022] None of the above types of emitters adequately solve this problem. In contrast, the present invention describes a method for fabricating an externally wetted nanotextured emitter that allows for accurate determination of hydraulic impedance with passive liquid supply while ensuring high mechanical strength. In addition, the resulting device has been shown to overcome each of the shortcomings described in the prior art.

[0023] Document WO2014 / 193995A1 discloses an electrospray system and related methods.

[0024] A paper by Hill Frances Ann et al., published in the Journal of Microelectromechanical Systems, IEEE Service Center, us, Vol. 23, No. 5, May 16, 2014, discloses "High-Throughput Ionic Liquid Ion Sources Using Arrays of Microfabricated Electrospray Emitters with Integrated Extractor Grid and Carbon Nanotube Flow Control Structures." Summary of the Invention [Problem to be solved by the invention]

[0025] The present invention provides a method for manufacturing an emitter for an electrospray generator according to claim 1, an emitter for an electrospray generator according to claim 19, an electrospray generator according to claim 32, and an electric space propulsion device according to claim 33. Advantageous embodiments are defined in the dependent claims. [Means for solving the problem]

[0026] According to a first aspect of the present invention, there is provided a method for manufacturing an emitter for an electrospray generator, said method comprising: - providing a substrate, said substrate comprising: Plate and providing a substrate comprising at least one protrusion having a base disposed on a first surface of the plate, the at least one protrusion terminating in a tip; - nanotexturing an exterior surface of the at least one protrusion, wherein the step of nanotexturing the exterior surface comprises: a) covering the surface with a suspension comprising a carrier fluid and particles; b) removing the carrier fluid from the suspension and partially protecting the surface; the aboveleaving the mask with the particles; c) Not protected by the above particles the above To remove the substrate to a specified depth 、 The main etching process In the direction perpendicular to the plate Steps to take the main etching step is configured to reveal an etched low relief surface with nanowires arranged on the protrusions but originating from the low relief surface in a direction perpendicular to the plate. and nanotexturing the surface.

[0027] The present invention provides a method for manufacturing an emitter for an electrospray generator, the method comprising a first step of providing a substrate, the substrate comprising a plate exhibiting at least one protrusion disposed on a first surface of the plate, the line of contact between the at least one protrusion and the plate defining a base of the at least one protrusion.

[0028] At least one protrusion terminates in a tip to facilitate focusing of the electric field to cause the generation of ions or the ejection of charged droplets, and the focusing of the electric field is strong enough to liberate ions of the conductive liquid or colloid directly from the surface of the tip.

[0029] The electric field at the tip of the emitter is a function of the applied electric field, the taper angle, and the emitter and extractor geometry, such as the emitter height and tip radius, where the taper angle is defined as the angle between the perpendicular from the plate to the tip of the protrusion and the side of the protrusion.

[0030] Advantageously, decreasing the tip radius and increasing either or both of the taper angle and cone height all result in an increase in the electric field at the tip of the emitter and a decrease in the onset voltage required to function.

[0031] Next, the present invention provides a method for manufacturing an emitter for an electrospray generating device, which presents a second step of nanotexturing the outer surface of at least one protrusion.

[0032] By nanotexturing it is understood that the surface is covered with nano-sized structures.

[0033] The process of nanotexturing the exterior surface of at least one protrusion includes a first step of covering the surface with a suspension comprising a carrier fluid and particles, the first step of the process of nanotexturing the exterior surface of at least one protrusion tending to uniformly cover the entire plate to provide an optimal foundation for subsequent steps of the process.

[0034] The carrier fluid of the suspension covering at least one protrusion of the plate is a solvent in which the particles do not dissolve, such as water, ethanol, or any solvent that is polar and does not affect the particles transported by the suspension. According to an embodiment, the suspension is a colloidal suspension, which allows a very uniform suspension of the colloidal particles.

[0035] Both parameters, concentration of particles in suspension and standing time, can be modified to obtain optimum characteristics and properties of the emitter under specific operating conditions.

[0036] Next, nanotexturing the exterior surface of the at least one protrusion includes a second step of removing the carrier fluid from the suspension, leaving a mask including a plurality of particles that partially protects the first side of the plate.

[0037] The second step of the process allows for uniform redistribution of the suspension over the first surface of the plate to cover the surface with a controlled amount of particles after removing excess suspension.

[0038] Finally, the step of nanotexturing the outer surface of at least one protrusion includes a third step of performing a main etching step to remove a predetermined depth of the substrate not protected by the previously deposited particle mask, thereby generating a nanostructure.

[0039] Such coatings of nanostructures exhibit relevant parameters such as packing density and nanostructure diameter, both of which provide the ability to achieve variations in emitter operating conditions.

[0040] The third step of the process of nanotexturing the outer surface of at least one protrusion is a main etching process that removes a predetermined depth of the substrate that is not protected by the particles previously applied in the first and second steps of the process. This etching process serves to form nanowires on the first surface of the plate and in the protrusion of the substrate. The nanowires are all oriented perpendicular to the substrate because the etching process is configured to etch in this direction.

[0041] In a preferred embodiment, the main etch is done in Si.

[0042] The combination of the aforementioned steps, combining a suspension of particles, preferably a colloidal suspension, with deep reactive ion etching, results in nanotexturing of the substrate and precise control over the geometry of the nanostructures, more particularly the nanowires, that are formed.

[0043] A series of steps to nanotexture the outer surface of at least one protrusion and the first side of the plate also precisely control multiple properties and characteristics of the nanowires, such as their density, orientation, diameter, and aspect ratio. Controlling key parameters such as density and aspect ratio is important for minimizing the required starting voltage and controlling the hydraulic resistance of the substrate, allowing for maximizing current flow while maintaining pure ionic conditions.

[0044] Advantageously, by controlling all parameters during the process, the resulting nanostructures are mechanically robust: the nanowires can be made relatively thick, preferably in the range of several hundred nanometers, and, being inscribed in the substrate, the non-separated parts thereof, so that they are firmly attached to the substrate and consequently cannot be ejected during the emitter's operating mode under high electric fields.

[0045] In addition, the mechanical strength of the nanostructures also prevents buckling of the nanowires, which can occur due to the strong capillary forces of the ionic liquid during the wetting and flow process described above.

[0046] In certain embodiments, at least one region of the first surface of the plate connected to at least one protrusion is nanotextured.

[0047] At least one connected area of ​​the first side of the plate presents at least one protrusion, presenting at least one nanotextured area and, accordingly, the presence of nanostructures, called nanowires due to the resulting shape of the etched surface, covering at least one protrusion of the substrate.

[0048] In a preferred embodiment, the entire protrusion and the first face of the plate are nanotextured, thus presenting a plurality of nanowires on their surface.

[0049] In certain embodiments, the suspension is a colloidal suspension comprising colloidal particles.

[0050] In a preferred embodiment, the suspension is a colloidal suspension comprising colloidal particles in a solvent as a carrier. Additionally, the suspension covers at least one protrusion to provide control over the hydraulic resistance of the nanotextured region. According to the previous embodiment, the suspension also covers at least one connected region of the substrate to provide further control over the hydraulic resistance of the nanotextured region feeding the tip of the protrusion.

[0051] In a preferred embodiment, the entire substrate is uniformly covered with the suspension and therefore completely covered with colloidal particles.

[0052] In certain embodiments, the particles are nano / microparticles.

[0053] Advantageously, the particles are nano- or microparticles and can be implemented on both nano- and microstructures. In particular, surface-wetting electrospray is based on microstructures, such as emitters, implemented to provide nanostructures, which allow the liquid to be wicked up to the microstructure tip by capillary forces, and also create the hydraulic resistance required for flow restriction at the substrate, thus providing ionic conditions.

[0054] In certain embodiments, nanotexturing the exterior surface comprises: - before step a), covering the surface to be nanotextured with a first cover layer, - after step b) and before step c), performing a pre-etching step for each main etching step according to step c) in order to remove the part of the first cover layer not protected by the particle mask, resulting in the transfer of the suspension mask to the first cover layer.

[0055] The main step of nanotexturing the exterior surface can also be preceded by an additional step of covering the surface to be nanotextured with a first cover layer applied to the substrate to facilitate the subsequent main etching step.

[0056] The step of nanotexturing the outer surface may also comprise another additional step between the second and third main steps of the above-mentioned process, in which a pre-etching step is performed for each of the main etching steps of the third main step in order to remove the part of the first cover layer that is not protected by the particle mask applied in the second step of the process, so that the suspension mask is transferred to the first cover layer.

[0057] Advantageously, the implementation of this additional step of performing a pre-etch step increases the selectivity of the mask to the substrate and provides the ability to use the patterned first cover layer as a mask during the main etch, allowing for a longer main etch step that does not damage the mask as much as if it were performed with only particles as the mask.

[0058] In certain embodiments, the first cover layer is one of the following materials: Au, Al, Cr, Ti, Ni, Pt, Co, Fe, W, Ta, Cu, Zn, any possible alloys thereof, SiO x , Si x N y , Al2O3, any metal oxide, or any combination thereof.

[0059] In a preferred embodiment, the material of the first cover layer is Au, which provides easier etching during reactive ion etching when combined with certain types of plasma, such as Ar.

[0060] Additionally, in the same preferred embodiment, the Au layer is treated with oxygen plasma to provide a negatively charged surface, allowing electrostatic self-assembly of positively charged particles carried in suspension. This step also applies to any cover layer.

[0061] In particular, an aqueous suspension containing beads is used to produce a self-assembled monolayer of well-separated particles on the charged Au surface of the substrate.

[0062] Advantageously, the surface density of the deposited particles, or so-called colloids, can be adapted by controlling the surface charge of the Au (or beads) as well as the particle concentration in suspension and its leaving time, since the concentration of particles per unit area depends on the number of particles transferred to the surface.

[0063] Also in the same preferred embodiment, the substrate is washed to remove any remaining floating colloids after applying the suspension, and dried using critical point drying techniques to avoid capillary forces during drying that may alter the correct distribution of colloids on the surface.

[0064] The particle monolayer is then used as a mask in a reactive ion etching step to form an array of Au disks, or any other material if used, on the substrate. These Au disks then serve as a mask in a main etching step to form an array of nanowires on the substrate. The material surrounding the disk is removed downwards, so that each Au disk produces a nanowire. The resulting surface with nanowires may show the mask disks atop the nanowires.

[0065] Advantageously, by carrying out the preceding sequence of steps, the formed array of nanowires is perpendicular to the first face of the substrate plate.

[0066] Furthermore, by controlling the sequence of steps above, it is possible to tailor the hydraulic resistance of the coating. For example, a large aspect ratio allows for a reduced hydraulic resistance, thereby achieving a device with higher current handling capacity. Such a device with higher current handling capacity can also be achieved by using either thinner nanowires, which result in higher transmittance and lower resistance, or a lower surface density of nanowires.

[0067] Additionally, the implementation of the aforementioned techniques allows for cost-effective, large-scale uniform etching, thereby forming nanowire populations with controlled densities.

[0068] Furthermore, a lower density of nanowires leads to greater spacing between individual nanowires, which, according to some preliminary experiments, may increase the effective length scale and allow for the initiation of ion emission at lower onset voltages.

[0069] In certain embodiments, the first cover layer is deposited by physical vapor deposition.

[0070] Depositing the first cover layer by physical vapor deposition facilitates subsequent pre-etching and reactive ion etching, improving performance of the subsequent main etch. The time spent during deposition allows for control of the resulting thickness of the first cover layer.

[0071] In certain embodiments, the colloidal suspension comprises one of the following particles: a polymer, preferably latex or polymethyl methacrylate (PMMA), Si, SiO2, ZnO, Zn, FexOy, Al2O3, Au, Pt.

[0072] In embodiments, the selected particles of the colloidal suspension are those that are resistant to the Si etching process, such as SiO2, Au, or Pt.

[0073] In certain embodiments, the suspension composition includes electrically charged particles to facilitate particle adhesion and distribution on the first cover layer.

[0074] By using charged particles, the particles adhere to and are uniformly distributed on the first cover layer, and in a preferred embodiment, the particles are selected for their charge to enhance the performance of the covering step of the process.

[0075] Controlling the charge makes it possible to control the number of particles that are transferred to the first cover layer and therefore the density of the resulting nanowire population.

[0076] In certain embodiments, the particles are uniformly distributed on the first cover layer.

[0077] Applying and setting a voltage with an opposite charge to the particles contained within the suspension results in a uniform distribution of the particles across the first cover layer, facilitating the subsequent main etch step of the process. The use of a colloidal suspension allows for a uniform distribution of particles within the suspension, aiding in a more uniform distribution of particles in the first cover layer.

[0078] In certain embodiments, the first cover layer, the substrate, or both, comprises: - Pre-surface chemical or plasma treatment, - voltage supply, or - Both have a surface charge.

[0079] In a preferred embodiment, the substrate and projected charged particles use the inherent surface charge of the substrate to self-assemble.

[0080] The substrate, the first cover layer, or both, already have a surface charge created by the discontinuities in the material. Further embodiments include treating the surface with a surface chemical or plasma treatment before projecting the charged particles. According to other embodiments, the first cover layer, the substrate, or both, are set to a voltage with a charge opposite to that of the charged particles, providing greater control over the movement of the charged particles to the surface.

[0081] In another preferred embodiment, a pre-surface chemistry, such as coating with a monolayer of polyelectrolyte monolayer, or a plasma treatment, such as oxygen plasma, is applied to diffuse the projected charged particles prior to depositing the first cover layer.

[0082] In the preferred embodiment, the selected particles are superficially charged with an opposite charge compared to the superficial charge of the substrate.

[0083] In a particular embodiment, the pre-etching step is based on the projection of a plasma, preferably an Ar plasma, perpendicular to the plate.

[0084] The projection of the plasma, preferably of Ar, bombards the surface of the substrate and mechanically extracts atoms from said surface.

[0085] In a preferred embodiment, the projected Ar plasma flow is directional, particularly vertically diffusing, to reach a kinetic energy capable of extracting the atoms, resulting in the formation of nanodisks on the substrate surface, the diameter of which corresponds to the diameter of the nanowires formed after the main etch.

[0086] In the same preferred embodiment, the perpendicular projection of the plasma results in nanowires perpendicular to the surface of the plate, with the nanowires generated on the protrusions maintaining the same orientation as the nanowires disposed on the main surface of the substrate, resulting in a good interface between the plate surface and the protrusion surface, and operationally improving delivery conditions as the fluid interacts with the continuously nanotextured surface.

[0087] In a specific embodiment, the main etching step is an anisotropic etching step of the substrate.

[0088] Advantageously, and in the preferred embodiment of the main etch performed with Si, the anisotropic etching step aids in the formation of substantially vertical nanowires.

[0089] In certain embodiments, the main etch step is performed using a combination of fluorinated gases.

[0090] In certain embodiments, the main etching step includes at least one step of inducing a plasma using the following gases, performed simultaneously or in any order: - SF6 - C4F8 - A combination of both gases.

[0091] In a preferred embodiment, a plasma is ignited using the aforementioned fluorinated gases, first SF6, then C4F8, or a unique combination of both gases, while setting a voltage and directed perpendicularly towards the substrate.

[0092] The above gas combination series provides the ability to perform anisotropic etching as the main etching step of the process, facilitating the ability to obtain better aspect ratios for the nanowire geometric features.

[0093] In certain embodiments, the diameter of the particles is in the range of 50 nm to 5000 nm, most preferably in the range of 100 nm to 3000 nm, and most preferably in the range of 200 nm to 1000 nm.

[0094] The diameter of the particles establishes the diameter of the nanowires formed, which in turn affects the size and surface density of the nanowires formed.

[0095] In certain embodiments, the surface density of the particles deposited on the first cover layer or substrate is in the range of 0.001 to 50 particles per square micron, and most preferably in the range of 0.05 to 10 particles per square micron.

[0096] The surface density of the particles deposited on the first cover layer, if any, or on the substrate depends on the particle concentration contained in the suspension applied during the first step of the process, but also on the standing time. The resulting surface density of the particles is also the resulting density of the nanowires.

[0097] In certain embodiments, the suspension comprises a polar solvent.

[0098] In particular, the selected polar solvent contained in the suspension does not dissolve the particles.

[0099] Also advantageously, the selected polar solvent provides the stable dispersion of charged particles required to ensure electrostatic self-assembly.

[0100] A second aspect of the present invention is an emitter for an electrospray generator, the emitter being a substrate, the substrate comprising: - Plates and - at least one protrusion having a base disposed on the first surface of the plate, the at least one protrusion terminating in a tip; the outer surface of the at least one protrusion According to any one of the embodiments of the first aspect of the invention, It is nanotextured, The nanotextured surface is an emitter that exhibits a low relief etched surface with nanowires that are arranged on protrusions but originate from the low relief surface in a direction perpendicular to the plate.

[0101] An emitter for an electrospray generating device comprises a substrate presenting a plate and at least one protrusion having a base disposed on a first surface of the plate, the outer surface of the at least one protrusion being nanotextured, the nanotextured surface exhibiting a low-relief etched surface, and the low-relief etched surface comprising nanowires arising from the low-relief surface.

[0102] The textured surface also exhibits a smooth configuration of predetermined dimensions, since the ends of the nanowires correspond to the original surface inscribed by the etching process, which is not the case if the nanowires are elements added to the surface without control of the nanowire orientation or the top surface that defines the textured region.

[0103] In a preferred embodiment, as a result of the vertical chemical etching performed during the nanotexturing process, the nanowires, even those located on protrusions, are perpendicular to the plate, the orientation being independent of the tilt of the surface on which they are located, thus ensuring a uniform nanotextured surface.

[0104] In certain embodiments, the substrate comprises at least one liquid source.

[0105] In a preferred embodiment, the substrate comprises at least one source of liquid used as an ionic liquid propellant, advantageously provided within the substrate to provide fluid communication between the nanotextured surface of the plate and / or the protrusions of the emitter, allowing for a continuous supply of liquid.

[0106] In certain embodiments, each protrusion has at least one liquid source located on the protrusion or on a connected region of the substrate adapted to supply the base of the protrusion.

[0107] According to this embodiment, in order to maintain fluid communication between all parts of the entire substrate of the emitter and thus the emitter in an operational mode without time limitations, each protrusion presents at least one liquid source arranged on the protrusion or on a connected area of ​​the substrate adapted to supply the base of the protrusion.

[0108] In certain embodiments, the connected region of the substrate is further nanotextured.

[0109] According to this embodiment, when the liquid source is in a connected region of the substrate and the substrate is in contact with or adjacent to the protrusions, the liquid provided from the liquid source is fluidly connected to the tips of the protrusions, and capillary forces enable a wetting process of the nanotextured surface, resulting in continuous communication.

[0110] In certain embodiments, the liquid source is a perforation that fluidly connects the first surface of the plate with the opposing surface.

[0111] In a preferred embodiment, the liquid propellant exits the liquid source reservoir and passes through the substrate plate by way of at least one perforation, thus providing fluid communication between the first side and the opposite side of the plate.

[0112] In certain embodiments, the hydraulic resistance R H is 10 15 ~10 20 Pa.sm -3and more preferably in the range of 10 15 ~10 18 Pa.sm -3 and even more preferably in the range of 10 16 ~10 18 Pa.sm -3 is within the range of R H but,

number

number

number

[0113] Tailoring the density of nanowires on the surface provides efficient control of hydraulic resistance.

[0114] Additionally, decreasing the density of nanowires increases hydraulic permeability and reduces hydraulic resistance in the specific case of externally wetted emitters.

[0115] At the same time, a decrease in nanowire density leads to an increase in the average distance between nanowires, which also has consequences such as a reduction in the required starting voltage, while still ensuring ionic conditions.

[0116] In certain embodiments, the transmittance K is 10 -16 ~10 -11 m 2 and more preferably in the range of 10 -14 ~10 -11 m 2and even more preferably in the range of 10 -13 ~10 -11 m 2 and the transmittance K is within the range

number

number

[0117] In certain embodiments, the volume fraction Ψ is

number

[0118] In certain embodiments of the method of fabricating an emitter and the emitter fabricated, the substrate of the emitter is a semiconductor.

[0119] Advantageously, the method for manufacturing the emitter and the emitter present a substrate that is semiconducting, in order to facilitate the process step in which the charge of the particles employed influences the performance of the deposition of said particles on the substrate, some preliminary experiments showing that this feature reduces the required starting voltage in operating conditions.

[0120] In a particular embodiment of the method of fabricating an emitter and the emitter fabricated, the substrate is Si.

[0121] By presenting a Si substrate, the method of manufacturing the emitter and the emitter provide a mechanically stronger substrate in the particular case of an outer surface wetting emitter in particular embodiments of the Si etching process to form Si nanowires.

[0122] In certain embodiments of the method of manufacturing an emitter and the emitter manufactured, the substrate is glass.

[0123] In certain embodiments of the method of manufacturing an emitter and the emitters manufactured, at least one protrusion is conical, pyramidal, spiral, blade-shaped, pointed, or needle-shaped.

[0124] By providing at least one protrusion having a conical, pyramidal, spiral, pointed, or needle shape, the method of manufacturing the emitter provides the advantage of concentrating a high electric field to maximize pure ion conditions during the operating state of the emitter when the emitter exhibits the at least one protrusion shape.

[0125] Either the form factor concentrating the electric field lines or the applied voltage ensures that when the electric field reaches a sufficient strength, the emission state, coupled with the correct hydraulic resistance presented to the fluid and its correct temperature, reaches the ionic condition. According to the present invention, the nanowire structure obtained by the manufacturing method and the emitter obtained thereby ensures that the ionic emission state is more easily achieved.

[0126] In certain embodiments of the method of fabricating an emitter and the emitter fabricated, the tip of the emitter is a structure adapted for electric field concentration.

[0127] A third aspect of the present invention is - emitter, an electrode facing the face of the emitter plate having the at least one protrusion and spaced apart from said at least one protrusion; an electrode comprising at least one opening for passing the generated ions or droplets; a power supply for setting a voltage between the substrate and the electrode; - a liquid source in fluid communication with the nanotextured surface of the plate for supplying liquid to said surface.

[0128] The electrospray generator comprises an emitter having at least one protrusion and / or at least one connected nanotextured region, an electrode having at least one opening for passing generated ions or droplets, and a power supply for setting a voltage between the substrate and the electrode to generate an electric field.

[0129] In a preferred embodiment, the electrode is a plate parallel to the plate of the substrate of the emitter, positioned above said emitter and spaced apart from the protruding portion of the same, and is covered with holes for passing ions or droplets after the electrospray generator is in an operational mode.

[0130] In another preferred embodiment, the emitter of the electrospray generator exhibits at least one protrusion on its substrate which has been nanotextured in accordance with the process steps of the first aspect of the present invention.

[0131] Therefore, performing an etching process for nanotexturing results in preventing nanowire adhesion problems, thereby reducing long-term degradation of the electrospray generator.

[0132] A fourth aspect of the present invention is an electric space propulsion device comprising at least one electrospray generator.

[0133] The increased mechanical resistance of the substrate provided for at least one electrospray of an electric space propulsion device and the increased mechanical resistance of the nanowires, both brought about by both the main etching step and the additional pre-etching step, improves durability and survivability and reduces potential degradation caused by the launch environment, which is characterized by extremely strong mechanical vibrations.

[0134] These and other features and advantages of the present invention will be more clearly understood from the following detailed description of preferred embodiments, given by way of illustrative and non-limiting example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0135] [Figure 1] 1 is a schematic diagram of an embodiment of an electrospray generating device according to the present invention. [Figure 2] 1 is a schematic diagram of an embodiment of an electrospray generating device according to the present invention. [Figure 3] 1 is a schematic diagram of an embodiment of a structure of a multi-lobed emitter of an electrospray generating device according to the present invention. [Figures 4A-4F] 1 is a schematic diagram of an embodiment of the steps of a method for manufacturing an emitter for an electrospray generator according to the present invention. [Figure 5] 1 is a scanning electron microscope image of an embodiment of an electrospray generator according to an embodiment of the present invention. [Figure 6] 1 is a scanning electron microscope image of an embodiment of a region of an electrospray generating device exhibiting a plurality of nanowires in accordance with an embodiment of the present invention. [Figure 7] 1 is a scanning electron microscope image of an embodiment of a region of an electrospray generating device exhibiting a plurality of nanowires in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0136] As will be appreciated by those skilled in the art, aspects of the present invention may be embodied as a method for manufacturing an emitter for an electrospray generator, an emitter for an electrospray generator, an electrospray generator, or an electric space propulsion device comprising at least one electrospray generator.

[0137] 1 shows a schematic diagram of an emitter (1) for an electrospray generator (10), comprising an emitter (1) formed by a protrusion (1.1) arranged on a first side of a plate (1.3), on the one hand, the protrusion (1.1) presents a base (1.2) arranged on the first side (1.3.1) of the plate (1.3), and on the other hand, the same protrusion (1.1) terminates in a tip (1.4).

[0138] The emitter (1) is made of a substrate (S), which is preferably a semiconductor. More preferably, the substrate (S) of the emitter (1) is Si or glass. Even more preferably, the substrate (S) of the emitter (1) is Si.

[0139] Furthermore, in the particular embodiment shown in Figure 1, the protrusion (1.1) of the emitter (1) is conical in shape and terminates in a tip (1.4) to provide a structure adapted for electric field concentration. In other particular embodiments, the protrusion (1.1) of the emitter (1) may be conical, pyramidal, needle-shaped, spiral-shaped, blade-shaped, pointed, or any known shape that provides a structure adapted for electric field concentration.

[0140] 4A-4F, described below, are schematic illustrations of steps in a method for manufacturing an emitter for an electrospray generator according to a preferred embodiment of the present invention.

[0141] FIG. 4A shows the substrate (S) before undergoing any step of the method for nanotexturing at least one protrusion (1.1) of the emitter (1).

[0142] FIG. 4B shows a first step of the method for nanotexturing at least one protrusion (1.1) of the emitter (1), where the grey area represents a first cover layer extended over said at least one protrusion (1.1) according to a preferred embodiment of the present invention.

[0143] 4C shows a second step of the method, in which the first cover layer (gray area) is covered with a suspension (area represented by a tilted Cartesian coordinate pattern). As shown in FIG. 4D, after the surface is covered with the particle-containing suspension (black area), the carrier fluid is removed from the suspension (black area), creating a mask (dashed line) resulting from the deposition of particles and partially protecting the first cover layer. According to an embodiment, the carrier fluid is removed by a drying process.

[0144] According to an embodiment, before removing the carrier fluid, the particles in the suspension (black area) are charged and the substrate (S) is oppositely charged. Due to the different charge signs between the particles and the substrate (S), the particles migrate towards the surface of the substrate (S) and are attached to the substrate (S) to form a mask (dashed line).

[0145] 4D shows a third step of the method, in which the first cover layer (gray area), protected by the particle mask (dashed line), undergoes a preliminary etching step that transfers the pattern of the particle mask (dashed line) to the first cover layer (gray area). As shown in FIG. 4E, the portions of the first cover layer that are not protected by the particles are eroded in the etching step, opening the first cover layer and exposing the surface of the substrate (S), which is now partially protected by both the particle mask and the etched first cover layer.

[0146] FIG. 4E is a detailed view of the aforementioned step of the method, in which the first cover layer (grey areas) is etched in areas not protected by the particles of the suspension (dashed lines).

[0147] 4F is a detailed view of the final step of the method, in which the substrate is subjected to a main etching step, resulting in the creation of nanowires (1.9) formed by a layer of unetched substrate (S), a layer of the first cover layer (gray area) protected by particles of the pre-spread suspension (black area), and the remaining particles of the suspension (dashed line). At the end of the above steps, optionally cleaned of all particles and optionally cleaned of the pre-spread first cover layer (gray area), nanowires (1.9) are formed.

[0148] As previously described and shown in FIG. 1, after the first step of the method of the present invention in which the surface of the emitter (1.1) to be nanotextured is covered with a suspension comprising a carrier fluid and particles as shown in FIG. 4C, a region (R) of the emitter (1) is nanotextured (1.5).

[0149] 4D, the carrier fluid of the suspension (black area) is then removed, resulting in a mask (dashed line) of particles that are deposited on the surface of the substrate (S) over the region (R) of the emitter (1) that is to be nanotextured. The mask (dashed line) partially protects the region (R) of the emitter (1) that is to be nanotextured.

[0150] In a preferred embodiment, the suspension (black areas) covering the surface to be nanotextured as shown in FIG. 4C is a colloidal suspension, comprising colloidal particles that tend to be uniformly distributed in the suspension (black areas) and therefore result in a uniform distribution when deposited on the surface of the substrate (S).

[0151] Furthermore, the particles are either nanoparticles or microparticles. In certain embodiments, the colloidal suspension (black area) comprises polymer particles. In more particular embodiments, these colloidal suspensions comprise one of the following polymer particles: latex, polymethyl methacrylate (PMMA). According to other embodiments, the particles are made of Si, SiO2, ZnO, Zn, FexOy, Al2O3, Au, Pt. Furthermore, the suspension (black area) preferably comprises a polar solvent.

[0152] Finally, a main etching step is performed, as shown in Figure 4F, to remove a predetermined depth of the substrate (S) of the emitter (1) on its surface that remained unprotected by the previously generated mask (dashed line).

[0153] Preferably, the main etching step of the first cover layer applied to the emitter (1) is performed using a combination of fluorinated gases such as SF6, C4F8, or a combination of both gases, and the main etching step of the substrate (S) of the emitter (1) is an anisotropic etching process.

[0154] Furthermore, additionally, between the first and second steps of the above-described method of the present invention, the surface to be nanotextured may be covered with a first cover layer (gray area) as shown in Figure 4B.

[0155] Optionally, after removing the carrier fluid in the second main step of the method, a pre-etching step can be performed to remove part of the first cover layer (gray area) previously spread over the area to be nanotextured (R), thereby transferring the suspension mask (dashed line) onto the first cover layer (gray area), as shown in Figure 4E. The pre-etching step is based on the projection of a plasma, preferably an Ar plasma, perpendicular to the plate (1.3).

[0156] As also shown in FIG. 4E, this pre-etching is intended to remove the first cover layer (gray area) that is not protected by the mask (dashed line) made from the particles of the particle suspension (black area). The particle mask (dashed line) is transferred to the cover layer (gray area), which now becomes the new mask for the main etch that will be performed. Thus, the pre-etching step is intended to etch the cover layer (gray area) and not the substrate (S).

[0157] In a preferred embodiment, the first cover layer (gray area) is one of the following materials: Au, Al, Cr, Ti, Ni, Pt, Co, Fe, W, Ta, Cu, Zn, any possible alloys thereof, SiO x , Si x N y , Al2O3, any metal oxide, or any combination thereof.

[0158] In the same preferred embodiment, the particles are charged to attach and distribute them on the first cover layer (gray area). The first cover layer, the substrate (S) of the emitter (1), or both, have opposite charges, thereby driving the particles to the surface. If the charge on the first cover layer (gray area) naturally generated by material discontinuities, the charge on the substrate (S) of the emitter (1), or both, is insufficient, according to another embodiment, the substrate (S) of the emitter (1), the first cover layer (gray area), or both, are set to a voltage with a charge opposite to that of the particles, providing greater control over the process.

[0159] Consequently, in a more preferred embodiment, the particles are uniformly distributed on the first cover layer (grey area).

[0160] Particles located near the surface will adhere to the surface, while particles farther from the surface will experience a migration time that depends on the distance to the surface.

[0161] Thus, the surface charge, the potential applied to the substrate (S), and the waiting time are variables that allow control of the particle density per unit area.

[0162] In a more detailed view of the nanowire (1.9), said nanowire (1.9) is at least one superposition of remaining particles from the suspension (black area) previously dispersed during the first step of the method and of parts of the substrate (S) that were not etched by the main etching step.

[0163] In a preferred embodiment of the method comprising the additional step of covering the substrate (S) with a first cover layer (gray area), as shown in FIG. 4F, the nanowires (1.9) are a superposition of remaining particles from the suspension (black area) previously dispersed during the first step of the method, a layer of the first cover layer (gray area) that was not etched by the pre-etching step, and a layer of the substrate (S) protected by both the first cover layer (gray area) and a mask (dashed line), which is a new mask generated during the first step of the method according to the invention.

[0164] Additionally, in the same preferred embodiment of the method comprising the additional step of covering the substrate (S) with a cover layer (grey area), the main etching step is therefore intended to etch both the remainder of the cover layer (grey area) not etched by the pre-etching and the part of the substrate (S) of the emitter (1) not covered by the new mask in order to create nanowires (1.9).

[0165] According to the present invention, returning to Figure 1, a region (R) of the substrate (S) of the protrusion (1.1) is nanotextured (1.5), resulting in the formation of a plurality of nanowires (not shown in this figure) in said region (R) after following the main and additional steps of the method. In a preferred embodiment, as shown in Figure 1, the first face (1.3.1) of the plate (1.3) and the outer surface of the protrusion (1.1) of the emitter (1) are completely nanotextured (1.5), so that the nanotextured region (R) covers the entire outer surface of the emitter (1).

[0166] In a preferred embodiment, the nanowires resulting from the method detailed above uniformly cover the entire protrusion (1.1) and plate (1.3) of the electrospray generator (10). The nanowires emerge from the low-relief etched surface of the emitter (1) in a perpendicular orientation to the plate (1.3). When the nanowires are perpendicular to the plate (1.3), the nanowires located on the protrusion are also perpendicular to the plate (1.3) of the emitter (1), resulting in a series of nanowires all oriented vertically in accordance with the orientation shown in FIG. 1.

[0167] The emitter (1) shown in FIG. 1 generates, in its operating state, a particle beam (B) having a conical shape and emitted from the tip (1.4) of the protrusion (1.1) of the electrospray (1).

[0168] Also in operation, as shown in Figure 1, when the electrospray generator (1) emits a particle beam (B), particle emission emanates from the tip (1.4) of the protrusion (1.1) and a cone is observed at the tip of the emitter (1.1).

[0169] As shown in Figure 1, the electrospray generator (10) also presents an electrode (1.6) facing the first surface (1.3.1) of the plate (1.3) of the emitter (1). The electrode (1.6) is positioned away from the protrusion (1.1) and presents an opening for passing the generated particle beam (B).

[0170] Additionally, the electrospray generator (10) includes a power supply (1.7) between the substrate (S) of the emitter (1) and the electrode (1.6), which establishes a potential difference between the electrode (1.6) and the semiconductor substrate (S) such that the electric field generated between both elements (1.6, S) is concentrated at the tip (1.4) of the protrusion and reaches a strength that releases ionized particles from the fluid at the tip (1.4).

[0171] A continuous flow of ions emitted from the tip (1.4) of the protrusion (1.1) is guaranteed if the flow of fluid or liquid (1.8.1) from the liquid source (1.8) is also sufficient. This suggests that the capillary forces of the nanotextured surface (1.5) favor wetting of the surface and, furthermore, that the hydraulic resistance of the textured surface is not excessively high to favor sufficient fluid or liquid (1.8) flow, but is still high enough to achieve ionic conditions.

[0172] The specific nanowire structure, in which each nanowire (not shown in this figure) maintains the same orientation perpendicular to the base (1.2) whether on the protrusion (1.1), allows for a constant and stable flow of liquid (1.8.1) even at lower applied potentials. This constant flow of liquid (1.8.1) replenishes the liquid in the protrusions, ensuring continuous and stable ionic conditions.

[0173] 2 illustrates another embodiment of an emitter (1) of an electrospray generator (10), showing a reservoir containing a liquid (1.8.1) used as an ionic liquid propellant. A liquid source (1.8) is in fluid communication with the nanotextured surface (1.5) of the plate (1.3) and / or protrusion (1.1) of the emitter (1) to deliver said liquid (1.8.1) of the liquid source (1.8) to the surface.

[0174] In this particular embodiment, as shown in Figure 2, the plate (1.3) is perforated on both its sides, a first side (1.3.1) and an opposite side (1.3.2), to connect the protrusion (1.1) of the emitter (1) with the liquid (1.8.1) from the liquid source (1.8).

[0175] 3 shows a schematic diagram of the structure of the multiple lobes of the emitter (1) of the electrospray generator (10). In this particular embodiment, each lobe (1.1) of the emitter (1), which has a conical shape, presents a base (1.2) defined by its cone diameter X. Each lobe (1.1) of the emitter (1) also has a height H, a tip (1.4) radius r, and a diameter H. *, and the thickness Y of the plate (1.3) of the emitter (1).

[0176] In this particular embodiment, as depicted in Figure 3, a plurality of protrusions (1.1) are arranged on a plate (1.3), with the tips (1.4) of each protrusion (1.1) spaced apart by a pitch P. Furthermore, the plate (1.3) of the emitter (1) presents a propellant passage diameter O for allowing a liquid (not shown in this view) to pass through the plate (1.3).

[0177] The electrospray generator (10) also shows multiple sections of the electrode (1.6) in cross section, each section being separated from the other section by a distance E1 and having a thickness E2. The gap or clearance between the two sections of the electrode (1.6) in cross section is the diameter of the perforations in the electrode (1.6), which has a grid configuration made from a plate with holes. The distance E1 between the two sections of the electrode (1.6) provides sufficient space for a particle beam (not shown in this view) to emerge from each protrusion (1.1). Furthermore, each tip (1.4) of each protrusion (1.1) is separated from the electrode (1.6) by a distance E3.

[0178] On the right side of Figure 3, a more detailed focus is placed on the nanotextured region (R) exhibiting a plurality of nanowires (1.9). Each nanowire exhibits structural dimensions such as its height h, its diameter d, and the distance p between each nanowire (1.9). Preferably, the above dimensions provide those skilled in the art with the ability to calculate the density N of the wires on the nanotextured surface, and thus the volume fraction Ψ of the wires. The same detailed focus shows five nanowires (1.9) in plan view, above the cross-sectional view of two nanowires (1.9).

[0179] All structural dimensions of the aforementioned multiple emitters (1) and elements of the electrospray generator (10) are on the nano- or micrometer scale.

[0180] Figures 5, 6 and 7 are images taken by a scanning electron microscope that provides the ability to depict devices such as the present emitter (1) of electrospray (10) on a macro / nanometer scale.

[0181] FIG. 5 shows a general view of the protrusion (1.1) and region (R) of the emitter (1) covered by a number of nanowires (1.9).

[0182] Figure 6 shows a more detailed view of a portion of the surface of the protrusion (1.1) of the emitter (1) covered by the nanowire (1.9). As can be seen, the nanowire (1.9) is perpendicular to the plate (1.3) of the emitter (1).

[0183] Figure 7 shows an even more detailed view of a portion of the surface of the protrusion (1.1) of the emitter (1) covered by the nanowires (1.9). In this particular embodiment, images taken by a scanning electron microscope provide the ability to measure the height h (2.298 μm) and diameter d (285 nm) of the nanowires (1.9). This close-up also shows the residual disk of the mask created on top of each nanowire during the fabrication process.

[0184] Nanowire coatings impose hydraulic resistance on the fluid flowing across the coated surface. The design parameters of the coating, primarily the density N and diameter d of the nanowires, determine the permeability K of the coating. The height h of the nanowires (1.9), which together with the geometry of the coated surface define the height of the coating, determines the hydraulic impedance. The hydraulic impedance regulates the total volumetric flow Q, which is forced by the pressure difference Δp created by the electric force near the meniscus of the liquid (1.8.1) at the tip of the protrusion (1.1).

[0185] The specific nanowire (1.9) structure, in which all nanowires (1.9) are oriented in the same direction, preferably perpendicular to the plate, and the uniform distribution and low undulations maintained at the tips (1.4) of the protrusions (1.1) have proven to be the main reasons why the emitter (1) supports a higher flow rate under ion emission conditions than any prior art emitter (1). Under optimal conditions, the creation of a meniscus is the mechanism that directs the flow of liquid (1.8.1) through the nanowires (1.9). Nevertheless, this identical structure of the rest of the nanotextured surface (1.5) ensures that the flow feeding the tips (1.4) is sufficient to provide the required flow rate at the tips (1.4) and allows ion flow to occur.

[0186] The experimental embodiment was realized with the following dimensional parameters for the nanowire coating: [Table 1]

[0187] The ionic liquids used to realize the experiments, such as EMI-BF4, exhibit the following properties: γ=0.045N m -1 Kappa = 13.8 ρ=1280 kg m -3 k=1.2S m -1 μ = 0.040 Pa·s.

Claims

1. A method for manufacturing an emitter (1) for an electrospray generator (10), said method comprising: providing a substrate (S), said substrate (S) comprising: a plate (1.3), at least one protrusion (1.1) having a base (1.2) arranged on a first surface (1.3.1) of said plate (1.3), said at least one protrusion (1.1) terminating in a tip (1.4); - nanotexturing the outer surface of said at least one protrusion (1.1), wherein said step of nanotexturing said outer surface comprises: a) covering the exterior surface with a suspension comprising a carrier fluid and particles; b) removing the carrier fluid from the suspension to leave a mask including a plurality of the particles partially protecting the exterior surface; c) performing a main etching step in a direction perpendicular to the plate (1.3) to remove a predetermined depth of the substrate not protected by the particles, the main etching step being configured to reveal an etched low-relief surface with nanowires arising from the low-relief surface in a direction perpendicular to the plate (1.3), even the nanowires (1.9) arranged on the protrusions (1.1) arising in a direction perpendicular to the plate (1.3).

2. 2. The method according to claim 1, wherein at least one region (R) connected with said at least one protrusion (1.1) of said first face (1.3.1) of said plate (1.3) is nanotextured.

3. 3. The method of claim 1, wherein the suspension is a colloidal suspension containing colloidal particles.

4. The method of claim 1 , wherein the particles are nano / micro particles.

5. nanotexturing the exterior surface, - before step a), covering said outer surface to be nanotextured with a first cover layer; 5. The method according to claim 1, further comprising, after step b) and before step c), performing a pre-etching step for each of the main etching steps according to step c) in order to remove the part of the first cover layer that is not protected by the mask of particles, resulting in the transfer of the mask to the first cover layer.

6. The first cover layer is made of Au, Al, Cr, Ti, Ni, Pt, Co, Fe, W, Ta, Cu, Zn, any possible alloy thereof, SiO ｘ , Si ｘ N ｙ , Al ２ O ３ , any metal oxide, or any combination thereof.

7. The method of claim 5 or 6, wherein the first cover layer is deposited by physical vapor deposition.

8. 8. The method of claim 1, wherein the colloidal suspension comprises one of the following particles: polymer, Si, SiO2, ZnO, Zn, FexOy, Al2O3, Au, Pt.

9. 9. The method of claim 1, wherein the composition of the suspension comprises charged particles to facilitate deposition and distribution of the particles on the first cover layer.

10. The method of claim 1 , wherein the particles are uniformly distributed on the first cover layer.

11. the first cover layer, the substrate, or both; - pre-surface chemical or plasma treatment, - voltage supply, or - both of which provide a surface charge.

12. 6. The method of claim 5, wherein the pre-etching step is based on the projection of a plasma perpendicular to the plate.

13. 13. The method according to any one of the preceding claims, wherein the main etching step is an anisotropic etching step of the substrate (S).

14. 14. The method of any one of claims 1 to 13, wherein the main etching step is performed using a combination of fluorinated gases.

15. The main etching step - SF ６ 、 - C ４ F ８ 、 - a combination of both gases, 15. The method of claim 14, comprising at least one step of inducing a plasma with the following gases simultaneously or in any order:

16. 16. The method of any one of claims 1 to 15, wherein the particles have a diameter in the range of 50 nm to 5000 nm.

17. 17. A method according to any one of the preceding claims, wherein the surface density of the particles deposited on the first cover layer or substrate (S) is in the range of 0.001 to 50 particles per square micron.

18. 18. The method of any one of claims 1 to 17, wherein the suspension comprises a polar solvent.

19. 19. The method according to any one of claims 1 to 18, wherein the substrate (S) is a semiconductor.

20. 20. The method according to any one of the preceding claims, wherein the substrate (S) is Si.

21. 21. The method according to any one of claims 1 to 20, wherein the substrate (S) is glass.

22. 22. The method according to any one of the preceding claims, wherein said at least one protrusion (1.1) is cone-shaped, pyramidal-shaped, spiral-shaped, blade-shaped, pointed-shaped or needle-shaped.

23. 23. The method according to any one of claims 1 to 22, wherein the tip (1.4) is a structure adapted for electric field concentration.

24. An emitter (1) for an electrospray generator (10), the emitter (1) being a substrate (S), the substrate (S) comprising: - plate (1.3), - at least one protrusion (1.1) having a base (1.2) arranged on a first face (1.3.1) of said plate (1.3), said at least one protrusion (1.1) terminating in a tip (1.4); the outer surface of said at least one protrusion (1.1) is nanotextured according to any one of claims 1 to 23, An emitter (1) for an electrospray generator (10), wherein the nanotextured surface (1.5) exhibits an etched low relief surface with nanowires arising from the low relief surface in a direction perpendicular to the plate (1.3), and even the nanowires (1.9) arranged on the protrusions (1.1) arise in a direction perpendicular to the plate (1.3).

25. 25. The emitter (1) according to claim 24, wherein the substrate (S) is provided with at least one liquid source (1.8).

26. 26. An emitter (1) according to claim 25, wherein each protrusion (1.1) has at least one liquid source (1.8) arranged in a connected region (R) of the substrate (S) adapted to supply the protrusion (1.1) or the base (1.2) of the protrusion (1.1).

27. 27. The emitter (1) according to claim 26, wherein the connected region (R) of the substrate (S) is further nanotextured.

28. 28. An emitter (1) according to any one of claims 25 to 27, wherein the liquid source (1.8) is a perforation fluidly connecting the first surface (1.3.1) and the opposite surface (1.3.2) of the plate (1.3).

29. Hydraulic resistance R Ｈ But, 10 １５ ~10 ２０ Pa.s.m －３ and RH is in the range of [Equation 1] The transmittance K is calculated as follows: [Equation 2] The volume fraction Ψ is calculated as [Equation 3] where H is the height of the protrusion (1.1), h is the height of the nanotextured surface (1.5), and r ＊ 29. The emitter (1) according to any one of claims 24 to 28, wherein α is the radius of the tip (1.4) of the protrusion (1.1), α is the semi-angle of the protrusion (1.1) in a cross-sectional view, μ is the dynamic viscosity, d is the diameter of the nanowires (1.9) of the nanotextured surface (1.5), Ψ is the volume fraction of the nanowires (1.9) of the nanotextured surface (1.5), and N is the density of the nanowires (1.9).

30. The transmittance K is 10 －１６ ~10 －１１ m ２ and the transmittance K is in the range [Equation 4] The volume fraction Ψ is calculated as follows: [Equation 5] 30. The emitter (1) of claim 29, wherein d is the diameter of the nanowires (1.9) in the nanotextured surface (1.5), Ψ is the volume fraction of the nanowires (1.9) in the nanotextured surface (1.5), and N is the density of the nanowires (1.9).

31. The volume fraction Ψ is [Equation 6] 31. The emitter (1) according to any one of claims 29 to 30, wherein d is the diameter of the nanowires (1.9) of the nanotextured surface (1.5) and N is the density of the nanowires (1.9).

32. 32. The emitter (1) according to any one of claims 24 to 31, wherein the substrate (S) is a semiconductor.

33. 33. The emitter (1) according to claim 32, wherein the substrate (S) is Si.

34. 32. The emitter (1) according to any one of claims 24 to 31, wherein the substrate (S) is glass.

35. 35. The emitter (1) according to any one of claims 24 to 34, wherein the at least one protrusion (1.1) is conical, pyramidal, spiral, blade-shaped, pointed or needle-shaped.

36. 36. The emitter (1) according to any one of claims 24 to 35, wherein the tip (1.4) is a structure adapted for electric field concentration.

37. - an emitter (1) according to any one of claims 24 to 36, and an electrode (1.6) facing the first face (1.3.1) of the plate (1.3) of the emitter (1) having the at least one protrusion (1.1) and spaced apart from the at least one protrusion (1.1); - said electrode (1.6) comprising at least one opening for the passage of the generated ions or droplets; a power supply (1.7) for setting the voltage between said substrate (S) and said electrode (1.6); a liquid source (1.8) in fluid communication with the nanotextured surface (1.5) of the plate (1.3) for supplying a liquid (1.8.1) to the nanotextured surface (1.5).

38. 38. An electric space propulsion device (100) comprising at least one electrospray generator (10) according to claim 37.

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