A flexible film with controllable droplet transport path, preparation method thereof, and application thereof

Abstract

The present invention provides a flexible film with controllable droplet transport path, and its preparation method and application, which belong to the field of droplet transport technology. The present invention uses silica gel as a flexible matrix, and adds a certain amount of softener to the silica gel to control the softness of the film. When a magnet is placed under the flexible film, since the film is flexible and contains magnetic particles, after the magnet is applied, the magnetic particles will carry the surrounding polymer matrix along the magnetic induction lines, causing the originally smooth surface to become a rough surface, while the surface farther away from the magnet remains smooth. The wettability of the rough surface is lower, and the wettability of the smooth surface is higher. When a water droplet is placed in a wettability gradient area, the water droplet will spontaneously move from a low wettability area (rough area) to a high wettability area (smooth surface), thereby realizing the driving of the droplet. The flexible film provided by the present invention can realize the transmission of droplets along any path without generating large macroscopic deformation, and is more practical.

Description

Technical Field

[0001] The present invention relates to the field of droplet transport technology, and in particular to a flexible film with a controllable droplet transport path, a preparation method thereof, and applications thereof. Background Art

[0002] Droplet transport has important applications in many aspects of human life, such as transporting droplets collected from the air to a water storage area, dissipating heat from condensers or chip surfaces through droplet transport, and enabling targeted drug delivery in clinical medicine. These applications are all closely related to the micro / nanostructure of the material surface and the design of the droplet transport pathway.

[0003] Preparing wettability gradients, shape gradients or anisotropic microstructure arrays on the surface of materials is the first choice for researchers to design and prepare functional surfaces for droplet transport. However, surfaces prepared using traditional chemical modification or physical etching methods can only achieve unidirectional transport of droplets and are prone to large contact angle hysteresis. Although researchers have prepared a variety of lubricating surfaces with small contact angle hysteresis by using the surface of bionic pitcher plants, the wettability gradient or anisotropic microstructure array cannot be achieved because the lubricating liquid fills the entire surface, resulting in droplet transport only being achieved under the action of gravity or shape gradient. However, in the fields of microfluidics, bioanalysis equipment, etc., there are high requirements for the controllability of droplet transport paths. Therefore, the research on smart surfaces with adjustable functions has become the focus of scientific researchers.

[0004] Magnetic fields are widely used to manipulate magnetic surfaces due to their advantages such as contactlessness and fast response. Researchers have used magnetic fields to regulate the tilt or bending deformation of microstructure arrays and prepared a variety of surfaces that can exhibit wettability gradients or anisotropic microstructures for reversible transport of droplets. For example, Chen et al. (Chen, G.; Gao, YB; Li, MZ; Ji, B.; Tong, R.; Law, MK; Wen, WJ; Zhou, BP Rapid and Flexible Actuation of Droplets via ALow adhesive and Deformable Magnetically Functionalized Membrane. J Mater Sci 2018, 53(18), 13253-13263.) designed a micrometer-scale film that is fixed at the periphery and suspended in the middle. When a magnetic field is applied, the film directly above the magnetic field will be attracted downward by the magnet and produce a macroscopic deformation (depression). Droplets placed on the surface will automatically roll to the lowest point on the surface of the film under the action of gravity. The maximum volume of droplets that can be transported is 20 μL. However, the macroscopic deformation makes this type of film unsuitable for many engineering projects. Summary of the Invention

[0005] The purpose of the present invention is to provide a flexible film with controllable droplet transport path, as well as its preparation method and application. The film provided by the present invention can realize the transmission of droplets along any path without generating large macroscopic deformation, and is more practical.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a method for preparing a flexible film with a controllable droplet transport path, comprising the following steps:

[0008] Mixing the main agent corresponding to the silica gel, a curing agent and magnetic powder to obtain a suspension containing magnetic particles; the mass of the magnetic powder is 20 to 140% of the total mass of the main agent corresponding to the silica gel and the curing agent;

[0009] Mixing the suspension containing magnetic particles and a silica gel softener to obtain a suspension containing magnetic particles and a softener; the mass of the silica gel softener is less than 40% of the total mass of the main agent and the curing agent corresponding to the silica gel;

[0010] forming a film from the suspension containing the magnetic particles and the softener, and curing the film to obtain a solid film;

[0011] The solid film is immersed in lubricating oil, and after being taken out, a flexible film with a controllable droplet transport path is obtained.

[0012] Preferably, the soaking time is 5 to 30 minutes.

[0013] Preferably, the lubricating oil comprises silicone oil.

[0014] Preferably, the mass of the magnetic powder is 80-120% of the total mass of the main agent and the curing agent corresponding to the silica gel.

[0015] Preferably, the mass of the silicone softener is 10 to 40% of the total mass of the main agent and curing agent corresponding to the silicone.

[0016] Preferably, the silica gel is platinum silica gel, and the mass ratio of the main agent and the curing agent corresponding to the platinum silica gel is 1:1.

[0017] Preferably, the magnetic powder is one or more of carbonyl iron powder, ferroferric oxide, ferrous oxide, neodymium iron boron and hydroxy iron powder; and the particle size of the magnetic powder is 1 to 3 μm.

[0018] The present invention provides a flexible film with controllable droplet transport path prepared by the preparation method described in the above scheme, comprising a silica gel matrix, a softener and magnetic powder; the softener and magnetic powder are dispersed in the silica gel matrix, and the surface of the flexible film is coated with lubricating oil.

[0019] Preferably, the thickness of the flexible film with controllable droplet transport path is 250-2000 μm.

[0020] The present invention provides the application of the flexible film described in the above solution for transporting droplets under the action of a magnetic field.

[0021] The present invention provides a preparation method of a flexible film with a controllable droplet transport path, comprising the following steps: mixing a main agent corresponding to silica gel, a curing agent and magnetic powder to obtain a suspension containing magnetic particles, wherein the mass of the magnetic powder is 20-140% of the total mass of the main agent and the curing agent corresponding to silica gel; mixing the suspension containing magnetic particles with a silica gel softener to obtain a suspension containing magnetic particles and the softener, wherein the mass of the silica gel softener is less than 40% of the total mass of the main agent and the curing agent corresponding to silica gel; forming the suspension containing magnetic particles and the softener into a film, and curing the film to obtain a solid film; and immersing the solid film in lubricating oil and taking it out to obtain the flexible film with a controllable droplet transport path.

[0022] The present invention uses silica gel as a flexible matrix and adds a certain amount of softener to the silica gel to control the softness of the film. When a magnet is placed under the flexible film, since the film is flexible and contains magnetic particles, after the magnet is applied, the magnetic particles will carry the surrounding polymer matrix along the magnetic induction line, causing the originally smooth surface ( Figure 2 The initial state) becomes a rough surface ( Figure 2 The second state in the magnet), while the surface farther from the magnet remains smooth (such as Figure 15 The rough area A and the smooth area B in the figure). The wettability of the rough surface is lower, while the wettability of the smooth surface is higher. Figure 15 Region C, between regions A and B in the figure, is the transition region from low to high wettability (this transition region is called the wettability gradient region). When a water droplet is placed in the wettability gradient region, it will spontaneously move from the low wettability region (rough region) to the high wettability region (smooth surface).

[0023] Based on this method of driving droplets, the present invention uses a ring magnet (such as Figure 18, the magnetic field intensity is larger at point b on the edge of the ring, and the magnetic field intensity is 0 at point a in the center of the ring) is placed at the bottom of the surface, which will induce a rough ring-shaped area on the surface, and the wettability of the area from the inside to the outside tends to gradually weaken, showing a radial wettability gradient (the wettability at the center is the strongest, and the wettability at the edge is the weakest). When a water droplet is placed on this wettability gradient surface, the water droplet will spontaneously move toward the center of the ring. Since the wettability at the center is higher than that of the surrounding areas, the water droplet will be confined to the center. When the annular magnet is moved, the originally smooth area of ​​the surface becomes rough with the arrival of the magnetic field, and the originally rough area becomes smooth due to the distance from the magnetic field. The result is that the annular area with a radial wettability gradient will move with the movement of the bottom magnetic field. During the movement, the water droplet will always be confined to the annular area, thereby realizing the transfer of water droplets along any path.

[0024] Although the surface of the flexible film of the present invention changes from smooth to rough under the action of the magnetic field, the change is only on the micron scale, so the movement of water droplets can be considered to be completely on the horizontal plane. The droplets can be transferred along any path without generating large macroscopic deformation, which is more practical. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The SEM images of carbonyl iron powder at different magnifications;

[0026] Figure 2 These are the three states of the flexible film surface;

[0027] Figure 3 are three-dimensional confocal microscope images and line roughness profiles, (a1) to (a3) ​​are matrices 500 / CIP 100 The three states of the / softener0 surface: (a1) initial state, Ra = 0.16 μm, (a2) second state, Ra = 0.46 μm, (a3) ​​third state, Ra = 0.15 μm. (b1) to (b3) are matrices. 500 / CIP 100 / softener 40 Three states of the surface: (b1) initial state, Ra = 0.16 μm, (b2) second state, Ra = 1.71 μm, (b3) third state, Ra = 0.16 μm;

[0028] Figure 4 This is the effect of softener content on film surface roughness;

[0029] Figure 5 The effect of magnetic particle content on roughness (a) and the schematic diagram of films with different magnetic particle contents in a magnetic field (b);

[0030] Figure 6 is the effect of film thickness on roughness;

[0031] Figure 7 is a matrix 500 / CIP 100 / softener 40 A picture of the contact angle of the droplet in its initial state before the surface is immersed in lubricating oil;

[0032] Figure 8 is a matrix 500 / CIP 100 / softener 40 A picture of the contact angle of the droplet in the second state before the surface is immersed in lubricating oil;

[0033] Figure 9 is a matrix 500 / CIP 100 / softener 40 Three-dimensional confocal microscopy images and line roughness profiles of the rough surface in the initial state after the surface was immersed in lubricating oil for different times, (a) immersion for 5 minutes, (b) immersion for 10 minutes, (c) immersion for 15 minutes, (d) immersion for 20 minutes, (e) immersion for 25 minutes, and (f) immersion for 30 minutes;

[0034] Figure 10 is a matrix 500 / CIP 100 / softener 40 Three-dimensional confocal microscopy images and line roughness profiles of the rough surface in the second state after the surface was immersed in lubricating oil for different times, (a) immersion for 5 minutes, (b) immersion for 10 minutes, (c) immersion for 15 minutes, (d) immersion for 20 minutes, (e) immersion for 25 minutes, and (f) immersion for 30 minutes;

[0035] Figure 11 is a matrix 500 / CIP 100 / softener 40 The roughness of the surface at different immersion times;

[0036] Figure 12 is a matrix 500 / CIP 100 / softener 40 Contact angle and sliding angle of the surface when immersed for different times;

[0037] Figure 13 Stability of contact angle for repeated application and removal of magnets;

[0038] Figure 14 Stability of the sliding angle for repeated application and removal of magnets;

[0039] Figure 15 Schematic diagram of directional transport of droplets;

[0040] Figure 16 This is a picture taken during the continuous droplet transport experiment;

[0041] Figure 17 Schematic diagram of the formation of a movable wettability gradient;

[0042] Figure 18 This is a photo of a ring magnet;

[0043] Figure 19 This is a three-dimensional confocal microscopy image of a circular area on the surface;

[0044] Figure 20 It is the cross-sectional profile of the surface annular area;

[0045] Figure 21 It is a star-shaped transport path;

[0046] Figure 22 It is a W-shaped transport path;

[0047] Figure 23 It is a Z-shaped transport path;

[0048] Figure 24 It is a straight-line transport path with an inflection point. DETAILED DESCRIPTION

[0049] The present invention provides a method for preparing a flexible film with a controllable droplet transport path, comprising the following steps:

[0050] Mixing the main agent corresponding to the silica gel, a curing agent and magnetic powder to obtain a suspension containing magnetic particles; the mass of the magnetic powder is 20 to 140% of the total mass of the main agent corresponding to the silica gel and the curing agent;

[0051] Mixing the suspension containing magnetic particles and a silica gel softener to obtain a suspension containing magnetic particles and a softener; the mass of the silica gel softener is less than 40% of the total mass of the main agent and the curing agent corresponding to the silica gel;

[0052] forming a film from the suspension containing the magnetic particles and the softener, and curing the film to obtain a solid film;

[0053] The solid film is immersed in lubricating oil, and after being taken out, a flexible film with a controllable droplet transport path is obtained.

[0054] In the present invention, unless otherwise specified, all raw materials used are commercially available products well known in the art.

[0055] The invention mixes a main agent corresponding to silica gel, a curing agent and magnetic powder to obtain a suspension containing magnetic particles.

[0056] In the present invention, the silicone is preferably platinum silicone; the corresponding base and curing agent are both commercially available products well known in the art. In the present invention, when the silicone is platinum silicone, the base is Smooth-on-30B and the curing agent is Smooth-on-30A, both purchased from Beijing Sanjing Xinde Technology Co., Ltd. In the present invention, the mass ratio of the base to curing agent for the platinum silicone is preferably 1:1.

[0057] In the present invention, the magnetic powder is preferably one or more of carbonyl iron powder, ferrosoferric oxide, ferrous oxide, neodymium iron boron and hydroxy iron powder. In an embodiment of the present invention, it is specifically carbonyl iron powder. The particle size of the magnetic powder is preferably 1 to 3 μm. In the present invention, the mass of the magnetic powder is 20 to 140% of the total mass of the main agent and curing agent corresponding to the silica gel, preferably 80 to 120%, and more preferably 100%. The present invention is beneficial to improving the response of the flexible film to the magnetic field by controlling the addition amount of magnetic powder within the above range. In an embodiment of the present invention, the carbonyl iron powder was purchased from BASF SE, Germany. In addition, it can also be purchased on Taobao.com.

[0058] The present invention has no special requirements on the mixing method of the main agent, curing agent and magnetic powder corresponding to the silica gel, as long as the three can be mixed evenly. In the embodiment of the present invention, the stirring is specifically for 3 minutes.

[0059] After obtaining the suspension containing magnetic particles, the present invention mixes the suspension containing magnetic particles with a silica gel softener to obtain a suspension containing magnetic particles and the softener.

[0060] The present invention does not have any special limitation on the silicone softener, and any silicone softener known in the art can be used. In the embodiment of the present invention, the silicone softener is purchased from Beijing Sanjing Xinde Technology Co., Ltd. and is a softener directly corresponding to the silicone (Smooth-on) in this embodiment, and is named Softener.

[0061] In the present invention, the mass of the silicone softener is less than 40% of the total mass of the main agent and curing agent corresponding to the silicone, preferably 10-40%, more preferably 30-40%, and most preferably 40%. Controlling the amount of silicone softener added within this range ensures that the surface of the flexible film returns to its initial state after the magnetic field is removed, thereby achieving reversible transport of droplets.

[0062] The present invention has no special requirements on the mixing method of the magnetic particle-containing suspension and the silica gel softener, and any mixing method known in the art can be used. In the embodiment of the present invention, mechanical stirring is specifically performed for 3 minutes.

[0063] After obtaining the suspension containing magnetic particles and a softener, the present invention forms the suspension containing magnetic particles and a softener into a film, and solidifies the film to obtain a solid film.

[0064] The present invention has no special requirements for the film forming method, and any film forming method known in the art can be used. In the embodiment of the present invention, the suspension is poured onto a glass sheet and the film is formed by scraping with a blade.

[0065] The present invention has no special requirements for the curing conditions, and curing conditions well known in the art can be used. In the embodiment of the present invention, the glass sheet containing the liquid film is placed in a drying oven and heated at 70°C for 10 minutes to cure the liquid film into a solid film.

[0066] After obtaining the solid film, the present invention soaks the solid film in lubricating oil, and then takes it out to obtain a flexible film with a controllable droplet transport path.

[0067] In the present invention, the lubricant preferably comprises silicone oil. The present invention has no specific requirements for the amount of lubricant used; it only requires that the solid film be submerged. In the present invention, the immersion time is preferably 5 to 30 minutes, more preferably 10 to 20 minutes, and most preferably 15 minutes. The present invention improves contact angle hysteresis of the solid film by immersing the film in lubricant for a period of time. By controlling the immersion time, the present invention ensures that the droplets are easily driven onto the surface of the flexible film.

[0068] The present invention provides a flexible film with controllable droplet transport path prepared by the preparation method described in the above scheme, comprising a silica gel matrix, a softener and magnetic powder; the softener and magnetic powder are dispersed in the silica gel matrix, and the surface of the flexible film is coated with lubricating oil.

[0069] In the present invention, the thickness of the flexible film with controllable droplet transport path is preferably 250-2000 μm, more preferably 250-1000 μm, and even more preferably 500 μm. By controlling the thickness of the flexible film within the above range, the present invention ensures that the flexible film has good magnetic response.

[0070] The present invention provides the application of the flexible film described in the above solution for transporting droplets under the action of a magnetic field.

[0071] The present invention uses silica gel as a flexible matrix and adds a certain amount of softener to the silica gel to control the softness of the film. When a magnet is placed under the flexible film, since the film is flexible and contains magnetic particles, after the magnet is applied, the magnetic particles will carry the surrounding polymer matrix along the magnetic induction line, causing the originally smooth surface ( Figure 2 The initial state) becomes a rough surface ( Figure 2 The second state in the magnet), while the surface farther from the magnet remains smooth (such as Figure 15 The rough area A and the smooth area B in the figure). The wettability of the rough surface is lower, while the wettability of the smooth surface is higher. Figure 15 Region C, between regions A and B in the figure, is the transition region from low to high wettability (this transition region is called the wettability gradient region). When a water droplet is placed in the wettability gradient region, it will spontaneously move from the low wettability region (rough region) to the high wettability region (smooth surface).

[0072] Based on this method of driving droplets, the present invention uses a ring magnet (such as Figure 18 , the magnetic field intensity is larger at point b on the edge of the ring, and the magnetic field intensity is 0 at point a in the center of the ring) is placed at the bottom of the surface, which will induce a rough ring-shaped area on the surface, and the wettability of the area from the inside to the outside tends to gradually weaken, showing a radial wettability gradient (the wettability at the center is the strongest, and the wettability at the edge is the weakest). When a water droplet is placed on this wettability gradient surface, the water droplet will spontaneously move toward the center of the ring. Since the wettability at the center is higher than that of the surrounding areas, the water droplet will be confined to the center. When the annular magnet is moved, the originally smooth area of ​​the surface becomes rough with the arrival of the magnetic field, and the originally rough area becomes smooth due to the distance from the magnetic field. The result is that the annular area with a radial wettability gradient will move with the movement of the bottom magnetic field. During the movement, the water droplet will always be confined to the annular area, thereby realizing the transfer of water droplets along any path.

[0073] The flexible film with controllable droplet transport path, its preparation method and application provided by the present invention are described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0074] Example 1

[0075] In the first step, the main agent and curing agent of platinum silicone rubber are mixed in a mass ratio of 1:1, and carbonyl iron powder (particle size of 1-3 μm, SEM image as shown) accounting for 100 wt% of the total mass of the main agent and curing agent of platinum silicone rubber is added. Figure 1 As shown), stir with a stirrer for 3 minutes until uniform, to obtain a suspension containing magnetic particles;

[0076] The second step is to add the main agent of platinum silica gel and 40wt% of the total amount of curing agent of silica gel softener to the suspension, and stir with a stirrer for 3 minutes until uniform, to obtain a suspension containing magnetic particles and softener;

[0077] The third step is to pour the suspension onto a glass slide and use a scraper method to scrape the suspension into a liquid film with a thickness of 500 μm (because the viscosity of the suspension is relatively high, the change in thickness caused by the flow of the liquid can be ignored);

[0078] The fourth step is to put the glass sheet containing the liquid film into a drying oven and heat it at 70℃ for 10 minutes. The liquid film is solidified into a solid film (referred to as matrix). 500 / CIP 100 / softener 40 surface);

[0079] In the fifth step, the cured film is immersed in silicone oil for 15 minutes to obtain a flexible film (the thickness can be considered to be equivalent to the thickness of the liquid film).

[0080] Example 2

[0081] The difference from Example 1 is that the amount of the silicone softener used is 10 wt % of the total amount of the platinum silicone main agent and the curing agent.

[0082] Example 3

[0083] The difference from Example 1 is that the amount of the silicone softener used is 20 wt % of the total amount of the platinum silicone main agent and the curing agent.

[0084] Example 4

[0085] The difference from Example 1 is that the amount of the silicone softener used is 30 wt % of the total amount of the main agent and curing agent of the platinum silicone.

[0086] Example 5

[0087] The difference from Example 1 is that the immersion time in the silicone oil is 5 minutes.

[0088] Example 6

[0089] The difference from Example 1 is that the immersion time in the silicone oil is 10 minutes.

[0090] Example 7

[0091] The difference from Example 1 is that the immersion time in the silicone oil is 20 minutes.

[0092] Example 8

[0093] The difference from Example 1 is that the immersion time in the silicone oil is 25 minutes.

[0094] Example 9

[0095] The difference from Example 1 is that the immersion time in the silicone oil is 30 minutes.

[0096] Example 10

[0097] The difference from Example 1 is that the amount of carbonyl iron powder added is 20 wt % of the total mass of the main agent and curing agent of the platinum silica gel.

[0098] Example 11

[0099] The difference from Example 1 is that the amount of carbonyl iron powder added is 40 wt % of the total mass of the main agent and curing agent of the platinum silica gel.

[0100] Example 12

[0101] The difference from Example 1 is that the amount of carbonyl iron powder added is 60 wt % of the total mass of the main agent and curing agent of the platinum silica gel.

[0102] Example 13

[0103] The difference from Example 1 is that the amount of carbonyl iron powder added is 80 wt % of the total mass of the main agent and curing agent of the platinum silica gel.

[0104] Example 14

[0105] The difference from Example 1 is that the amount of carbonyl iron powder added is 120 wt % of the total mass of the main agent and curing agent of the platinum silica gel.

[0106] Example 15

[0107] The difference from Example 1 is that the amount of carbonyl iron powder added is 140 wt % of the total mass of the main agent and curing agent of the platinum silica gel.

[0108] Example 16

[0109] The difference from Example 1 is that the thickness of the liquid film is 250 μm.

[0110] Example 17

[0111] The difference from Example 1 is that the thickness of the liquid film is 1000 μm.

[0112] Example 18

[0113] The difference from Example 1 is that the thickness of the liquid film is 1500 μm.

[0114] Example 19

[0115] The difference from Example 1 is that the thickness of the liquid film is 2000 μm.

[0116] Comparative Example 1

[0117] The difference from Example 1 is that the silicone softener is not included. The surface of the film obtained in step 4 is recorded as matrix. 500 / CIP 100 / softener0 surface.

[0118] Comparative Example 2

[0119] The difference from Example 1 is that the amount of the silica gel softener is 50 wt %.

[0120] Comparative Example 3

[0121] The difference from Example 1 is that the amount of the silica gel softener is 60 wt %.

[0122] 1. Characterization of flexible film surface morphology and wettability

[0123] (1) For a flexible film, when no external magnetic field is applied, the surface is in its original state, which is defined as the initial state; when an external magnetic field with an intensity of 0.4 T is applied to the surface, this state is defined as the second state; when the external magnetic field is removed, the state of the surface is defined as the third state, as shown in Figure 2 shown.

[0124] Figure 3 (a1) to (a3) ​​show the matrix 500 / CIP 100 / softener0 (matrix represents platinum silica gel matrix, 500 means thickness of 500 μm, CIP is the abbreviation of carbonyl iron powder, 100 means mass fraction of 100 wt%, softener means softener, and 0 means the content of softener in the film is 0). The three-dimensional confocal microscopy image and line roughness profile of the film surface before oil immersion (experimental preparation step 5) show that the line roughness (Ra) of the film surface in its initial state is about 0.16 μm, as shown in Figure 2. Figure 3 When the surface is in the second state, Ra increases rapidly from 0.16μm to 0.46μm, as shown in (a1). Figure 3 When the surface is in the third state, that is, the magnetic field is removed, the surface returns to the initial state, and Ra becomes 0.15μm (which can be considered to be consistent with the initial state of 0.16μm), as shown in (a2). Figure 3 As shown in (a3).

[0125] The above analysis shows the roughness of the film without softener in three states. The following analysis shows the roughness of the film with softener content of 10wt%, 20wt%, 30wt%, 40wt%, 50wt% and 60wt%. Figure 4 . It can be found that the Ra of all surfaces in the initial state is about 0.16μm. However, when these surfaces are in the second state, the Ra increases rapidly with the increase of the softener content. When the surface is in the third state, that is, the magnet is removed at this time, the Ra of the film surface with a softener content of less than or equal to 40wt% returns to the initial state (Ra=0.16μm), and when the content is greater than 40wt%, the Ra is greater than 0.16μm, indicating that even without a magnetic field, the film surface cannot be completely restored to its initial state, causing the surface to show residual deformation. In order for the surface to exhibit a larger Ra when a magnetic field is applied, and the Ra can return to its initial state after the magnetic field is removed, it can be seen that the optimal content of the softener should be 40wt%.

[0126] Figure 3 (b1) to (b3) show the three-dimensional confocal microscopy images and line roughness profiles of the film surface in three states when the softener content is 40wt%. The surface is represented by matrix 500 / CIP 100 / softener 40 express.

[0127] (2) The effect of the amount of carbonyl iron powder on the roughness Figure 5 As shown. Figure 5 It can be seen that when the surface is in the initial state, the roughness increases with the increase of magnetic particle content. When the surface is in the third state, the roughness returns to the corresponding initial state because the magnetic field has been removed, so the two curves basically coincide. When the surface is in the second state, the roughness first increases and then decreases (see the attached Figure 1 b and the next paragraph), when the roughness takes the maximum value, it means that the surface is most sensitive to the magnetic field, so the quality score is selected as 100% in this study.

[0128] Each magnetic particle has its own inherent magnetic pole, the direction of which is its own inherent property and will not change with external conditions (such as Figure 5 (b) Indicated by the white arrows inside the particles). When magnetic particles are in a magnetic field, each magnetic particle will be affected by a magnetic moment (i.e., a dipole moment). The direction of this moment is related to the direction of the external magnetic field and the direction of its own magnetic poles (the direction of the moment is shown in Figure 5(Indicated by the black curved arrows outside the particles in (b)). Because the magnetic poles of the magnetic particles in this experimental preparation are randomly distributed, the magnetic moment orientation of each particle varies; some are counterclockwise, while others are clockwise. When the magnetic particle content is low, the film's response to the magnetic field becomes increasingly pronounced as the content increases, leading to a gradual increase in roughness in the second state. However, at a certain concentration, the unequal orientation of all magnetic moments leads to a decrease in roughness rather than a further increase.

[0129] (3) Figure 6 is the effect of film thickness on roughness, Figure 6 It can be seen that when the surface is in the second state, the roughness first increases and then decreases with the increase of film thickness (the reason is Figure 5 Therefore, when the film thickness is 500 μm, it has a better magnetic response.

[0130] (4) Wetting is the most direct parameter to characterize whether the droplet can be transported. Figure 7 and Figure 8 Shows the matrix 500 / CIP 100 / softener 40 The wettability of the surface in the initial state and the second state. It can be seen that the contact angles (CA) of the surface in the initial and second states are 111.6° and 116.8°, respectively. However, the sliding angles are both large. Even if the surface is rotated 180 degrees and inverted, the droplets will not fall. (The sliding angle represents the contact angle hysteresis of the surface. The smaller the value, the smaller the hysteresis, and the easier it is for the droplets to move on the surface. The larger the value, the greater the hysteresis, and the less likely it is for the droplets to move on the surface.) Therefore, in order to reduce the sliding angle of the surface, the film needs to be immersed in lubricating oil (the fifth step in the experimental preparation) so that its surface is covered with a layer of lubricating oil.

[0131] The matrix 500 / CIP 100 / softener 40 The surface is immersed in the lubricating oil solution for a certain period of time, due to the swelling effect, Figure 3 The defect-free uniform surface shown in (b1) is destroyed and a rough surface appears. Figure 9 Displays the matrix 500 / CIP 100 / softener 40 3D confocal microscopy images and line roughness profiles of the surface in the initial state after being immersed in lubricating oil for 5 min, 10 min, 15 min, 20 min, 25 min, and 30 min, respectively. Figure 10A three-dimensional confocal microscope image and a line roughness profile diagram in the second state when immersed for the above time are shown. Figure 9 and Figure 10 The change of surface Ra with immersion time is shown in Figure 11 .Depend on Figures 9 to 11 It can be seen that with the increase of immersion time, the Ra of the surface in the initial state and the second state gradually increases.

[0132] Figure 9 and Figure 10 The contact angles and sliding angles of all surfaces in Figure 12 .from Figure 12 It can be seen that the contact angle of the surface in the initial state changes little with the increase of immersion time, but the contact angle in the second state first increases and then decreases. However, the sliding angle in both states shows a trend of first decreasing and then increasing. In order to more easily drive the droplet to move, the larger the difference in the contact angle of the surface in the two states, the better, and the smaller the sliding angle, the better (detailed introduction later). Therefore, from Figure 12 It can be seen that the optimal soaking time is 15 minutes.

[0133] By repeatedly applying and removing the magnet, the stability of the surface switching between the two states was studied, e.g. Figure 13 and Figure 14 As shown in the figure, when a magnetic field with a strength of 0.4 T is applied, the contact angle is about 117° and the sliding angle is 9.5°. When the magnetic field is removed, the contact angle is about 105° and the sliding angle is 12°. This process is repeated 200 times. Figure 13 and Figure 14 It shows that the contact angle and sliding angle in both states are relatively stable.

[0134] 2. Movable wettability gradient and directional sliding behavior of droplets

[0135] Place the magnet on the bottom of the surface, e.g. Figure 15 The surface of the film in the area above the magnet is in the second state (A area), represented by the dotted box, while the surface of the area away from the magnetic field is in the initial state (B area). Whether in the horizontal or vertical direction, the magnetic field strength gradually decreases as it moves away from the magnet. Under the action of the magnetic field, the surface roughness of the flexible film will gradually decrease as the magnetic field strength decreases, and the wettability will increase as the surface roughness decreases. Therefore, it can be seen that Figure 15 In the transitional area C, the roughness decreases from left to right, and the wettability increases. That is, the contact angle decreases from 117° to 105° (a smaller contact angle indicates stronger wettability). Therefore, the surface of area C has a wettability gradient.

[0136] A droplet is placed at the boundary between area A and area C. Because the droplet is in the wettability gradient area, an unbalanced capillary force F will be generated. c Acting on the three-phase contact line of the droplet (the contact area between the droplet and the surface is a circular surface, and the boundary ring of this circle is the three-phase contact line), according to the existing literature, this unbalanced capillary force can be written as

[0137] F c =lγ(cosθ r -cosθ l ) Formula (1)

[0138] where γ is the surface tension of the droplet, θ r and θ l are the apparent contact angles on the right and left sides of the droplet, respectively, and l is the length of the boundary line between the different wettability areas covered by the droplet. It can be seen from formula (1) that when θ r The smaller and θ l When the value is larger, it may cause a larger F c , this F c It is the driving force used to drive the droplets to move. Figure 12 Display θ r The minimum is 105°, θ l The maximum is 117°. In addition to the driving force, the droplet is also subject to some other resistance to hinder the movement of the droplet, such as the resistance F of the oil d and friction force F f , can be written as

[0139] F d =kRηv,F f =mgμ Formula (2)

[0140] where k, R, η, v, m, g, and μ represent the constant coefficient, droplet radius, lubricating oil viscosity, droplet transport velocity, droplet mass, gravitational acceleration, and friction coefficient, respectively.

[0141] Obviously, when F c >F d +F f When the droplet begins to move, the transport behavior of the droplet is as follows Figure 16 (af), the black dotted box in the figure indicates the position of the magnet at the bottom of the surface. c The droplet moves from left to right. When the droplet leaves area C, F c When it decreases to 0, the droplet begins to slow down to a standstill due to the effect of resistance.

[0142] At this time, move the magnet from left to right, such as Figure 16 (gh), the originally smooth area is transformed into a rough surface due to the effect of the magnetic field, as shown in the schematic diagram Figure 17As shown in Figure 2, a movable wettability gradient is formed at this time. When the newly formed C area (wettability gradient) moves to the droplet position, F is regenerated. c , when F c >F d +F f When the droplet starts to move again, the experimental effect is as follows Figure 16 (hj). Repeatedly moving the magnet enables continuous forward transport of the droplets.

[0143] 3. Transport of droplets along arbitrary controllable paths

[0144] Although the above method can achieve continuous transport of droplets, the transport path can only be a straight line, which is not sufficient to complete many droplet transport tasks. In practice, droplets are often required to be transported along any path. For example, cells or drugs need to be transported through flexible paths to avoid existing defects or components on the material surface.

[0145] Replace the rectangular magnet in the previous section with a ring magnet, such as Figure 18 As shown in the figure, the magnetic field strength at the geometric center point a of the magnet and the edge point b of the magnet are 0T and 0.4T respectively. When the magnet is placed at the bottom of the material surface, the response of the magnetic particles inside the film to the magnetic field causes an annular wettability gradient area (annular C area) to be generated on the surface, as shown in the figure. Figure 19 As shown, it is obvious that the green area inside the ring is the initial state with high infiltration. Its three-dimensional confocal microscopy image and Figure 9 The red ring area is in the second state, with lower infiltration, and its three-dimensional confocal microscopy image and Figure 10 Therefore, an annular wettability gradient with increasing wettability is formed from the edge to the geometric center.

[0146] When the droplet is placed in this wettability gradient region, due to the influence of F c Under the action of , the droplet will spontaneously move toward the geometric center point a. Figure 20 The profile of the cross section of the annular region of the surface is shown. It can be seen that there is a height difference of about 40 μm between the edge and the center of the ring. According to the geometric dimensions, it can be concluded that θ in the figure is smaller than the sliding angle of the surface, thus proving that the spontaneous movement of the droplet is caused by the wettability gradient F c When the magnet is moved, the annular wettability gradient follows the magnet, keeping the droplet within the annular structure and enabling the droplet to be transported along any path. Figure 21-24The researchers demonstrated how the surface could transport droplets from point A to point B along star-shaped, W-shaped, Z-shaped, and straight paths with inflection points by manipulating a ring magnet. Experimental measurements revealed that this surface could transport droplets ranging in volume from 4.5 to 23 μL.

[0147] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing a flexible film with controllable droplet transport path, characterized in that: The following steps are involved: Mixing the main agent corresponding to the silica gel, a curing agent and magnetic powder to obtain a suspension containing magnetic particles; the mass of the magnetic powder is 20 to 140% of the total mass of the main agent corresponding to the silica gel and the curing agent; Mixing the suspension containing magnetic particles and a silica gel softener to obtain a suspension containing magnetic particles and a softener; the mass of the silica gel softener is less than 40% of the total mass of the main agent and the curing agent corresponding to the silica gel; forming a film from the suspension containing the magnetic particles and the softener, and curing the film to obtain a solid film; The solid film is immersed in lubricating oil, and after being taken out, a flexible film with a controllable droplet transport path is obtained.

2. The preparation method according to claim 1, characterized in that The soaking time is 5 to 30 minutes.

3. The preparation method according to claim 1 or 2, characterized in that The lubricating oil includes silicone oil.

4. The preparation method according to claim 1, characterized in that The mass of the magnetic powder is 80-120% of the total mass of the main agent and the curing agent corresponding to the silica gel.

5. The preparation method according to claim 1, characterized in that The mass of the silica gel softener is 10-40% of the total mass of the main agent and the curing agent corresponding to the silica gel.

6. The preparation method according to claim 1, characterized in that The silica gel is platinum silica gel, and the mass ratio of the main agent and the curing agent corresponding to the platinum silica gel is 1:

1.

7. The preparation method according to claim 1, characterized in that The magnetic powder is one or more of carbonyl iron powder, ferroferric oxide, ferrous oxide, neodymium iron boron and hydroxy iron powder; and the particle size of the magnetic powder is 1 to 3 μm.

8. The flexible film with controllable droplet transport path prepared by the preparation method according to any one of claims 1 to 7 comprises a silica gel matrix, a softener and magnetic powder; the softener and magnetic powder are dispersed in the silica gel matrix, and the surface of the flexible film is coated with lubricating oil.

9. The flexible film according to claim 8, characterized in that The thickness of the flexible film with controllable droplet transport path is 250-2000 μm.

10. Use of the flexible film according to claim 8 or 9 for transporting liquid droplets under the action of a magnetic field.

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