Method and apparatus for measuring receding contact angle

Abstract

A method for measuring receding contact angle, comprising the steps of: forming a drop of liquid on a sample surface by applying the liquid as a continuous jet having a flow rate to the sample surface, wherein the liquid is ejected through an opening having an opening diameter for a defined dosing time, thereby defining a dosing volume; measuring at least one geometric parameter of the drop formed on the sample surface; determining a contact angle between the sample surface and the drop based on the at least one geometric parameter, wherein the flow rate and the dosing time are chosen such that the dosing volume does not exceed the flow rate times 0.11 seconds. The present application also relates to a device for measuring receding contact angle, comprising: a liquid reservoir, a dosing means, and a contact angle measuring means; wherein the dosing means is adapted to control the dosing time such that the dosing volume does not exceed the flow rate times 0.11 seconds.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method and an apparatus for measuring a receding contact angle. BACKGROUND

[0002] The contact angle between a solid surface and a liquid disposed on the surface is defined as the interior angle between the surface and the tangent to the liquid profile at the boundary between the solid surface and the liquid. When a liquid with known properties is applied to a sample surface, the contact angle provides meaningful information about the characteristics of the sample surface. Therefore, measuring contact angles is a well-established technique in surface science. At the same time, measuring contact angles is a rather difficult process. To obtain reliable results, many factors need to be taken into account. The final shape of a stationary liquid drop depends not only on the liquid, the solid, and the surface energies of the solid-liquid interface, but also on the roughness and chemical or physical heterogeneities of the surface, such as adsorbed contaminants.

[0003] Another factor that has a significant influence on the measured contact angle of a stationary drop is how the liquid is applied to the surface. This is true because the interaction of the surface with the liquid also depends on wetting kinetics and contact angle hysteresis, i.e. on whether the drop shape in the static state is the result of a wetting or dewetting process. When a liquid front advances over a previously dry surface, such that dry surface areas are wetted by the liquid, a so-called advancing contact angle is formed. When the liquid recedes from a wetted surface, such that wet surface areas become dry, a so-called receding contact angle is formed. The advancing and receding contact angles refer to angles measured in dynamic or static state. When measured in static state, these terms describe the angle of a liquid that has most recently advanced through a “dry” solid surface or receded from a wetted solid surface, respectively. Therefore, the terms “most recently advanced contact angle” and “most recently receded contact angle” are considered to be more appropriate terms, but the terms “advancing” and “receding” have been used for decades (see T., Tian, X., Korhonen, J.T., et al. Surface-wetting characterization using contact-angle measurements. Nat Protoc 13, 1521-1538 (2018), https: / / doi.org / 10.1038 / s41596-018-0003-z).

[0004] By definition, for a given interface between a surface and a liquid, the advancing contact angle is the largest contact angle that can be observed, while the receding contact angle is the smallest contact angle that can be observed. The advancing and receding contact angles, as well as the difference between the two (i.e. the so-called contact angle hysteresis), provide important and distinct types of information about the sample surface. Both contact angles describe metastable contact angles that correspond to local free energy minima in higher energy states than the most stable contact angle at the global energy minimum, the so-called Young contact angle (see the above cited The most important difference between the advancing and receding contact angles is that they are the result of a wetting or dewetting process, respectively. Although the advancing and receding contact angles describe the largest and smallest contact angles, respectively, by definition, there can be additional static contact angles that correspond to additional local energy minima. According to their strict definition, these additional static contact angles are larger than the receding contact angle and smaller than the advancing contact angle. However, since these contact angles are also the result of a wetting or dewetting process, they are referred to herein as advancing and receding contact angles. They also provide valuable information.

[0005] From the document EP 0 919 801 A1 a sophisticated method and apparatus for measuring contact angles is known. According to the known method, a liquid drop is formed at the tip of a cannula and transferred to the sample surface. This method has long been considered the gold standard. The contact angles observed with this method are often referred to as “static” or “equilibrium” contact angles. These contact angles are typically located between the advancing and receding contact angles.

[0006] The document EP 2 899 528 B1 discloses another method and apparatus for measuring contact angles. It is based on a jet of pressurized liquid that applies liquid to the surface as a continuous flow with a flow rate of 45 μl / s or less in order to keep the kinetic energy level low. This process is sometimes referred to as “liquid needle”. With this technology, a drop can be formed on the sample surface in very short time, wherein the obtained contact angle is said to be very close to the equilibrium contact angle. In fact, the contact angle measured according to this method represents the advancing contact angle or the most recently advancing contact angle.

[0007] According to document US 2010 / 0024529 A1 a contact angle measurement technique applying "ballistic drop deposition" is known. The known device dispenses a series of small volume liquid droplets to the same location on a sample surface, thereby forming a liquid drop on the surface. To prevent the advancing front of the growing volume of liquid from being held up by surface heterogeneities, each droplet is applied with sufficient kinetic energy in the form of a vibration. Thereby, the drop will rapidly grow to its equilibrium size so that the equilibrium contact angle can be measured.

[0008] Document US 2020 / 008088 A1 discloses a method and a test device for determining the volume of a liquid droplet. The method is based on a measurement of the height or diameter of a drop, for example placed on a surface. Based on this measurement and based on the contact angle of the drop on the surface (the contact angle is not measured, but assumed to correspond to a known characteristic contact angle), the volume of the drop is calculated.

[0009] Document EP 3 210 004 B1 discloses a method for measuring a "receding" contact angle. Similar to the "ballistic drop deposition" method, a drop is formed from a series of small volume liquid droplets dispensed to the same location on a sample surface. Each droplet should impart to the liquid drop an amount of kinetic energy sufficient to increase the perimeter of the drop beyond its equilibrium diameter. With dissipation of the excess energy, the perimeter of the drop is allowed to retract and a receding contact angle will be formed and measured. SUMMARY

[0010] It is thus an object of the present invention to provide a method and a device for measuring a receding contact angle. This object is solved by the method and the device according to the present application. Preferred aspects of the invention are given in the other aspects of the present application.

[0011] The method is for measuring a receding contact angle and comprises the following steps:

[0012] • forming a liquid drop on a sample surface by applying a liquid to the sample surface as a continuous jet having a flow rate, wherein the liquid is ejected through an opening having an opening diameter for a defined dosing time, thereby defining a dosing volume,

[0013] • measuring at least one geometric parameter of the drop formed on the sample surface,

[0014] • determining a contact angle between the sample surface and the drop based on the at least one geometric parameter, wherein

[0015] • the flow rate and the dosing time are chosen such that the dosing volume does not exceed the flow rate multiplied by 0.11 seconds.

[0016] The liquid can be any liquid suitable for the analysis and the interaction of the sample, in particular water or methylene iodide. The opening, through which the liquid is ejected, can have a circular cross section. In particular, the opening can be a nozzle opening. The jet has a defined length corresponding to the dosing time. The jet preferably extends without interruption between a front end and a back end. In particular, it is conceivable that only a single jet is applied to form the entire drop.

[0017] After the drop has formed on the surface and has reached a stationary state typically within a few milliseconds after the back end of the jet has reached the surface, the at least one geometric parameter, such as diameter, height, or shape of the drop profile, etc., is measured. This can be done in particular in a top view or in a side view, e.g. with optical means such as a video camera. On this basis, the contact angle can be determined as known in the art, e.g. by simply drawing a tangent to the drop profile (in particular when the drop is imaged in a side view with a video camera), and / or by applying mathematical algorithms assuming a certain drop shape (e.g. a known volume and a spherical shape).

[0018] As discussed above with reference to document EP 2 899 528 B1, applying a liquid to a sample surface as a continuous jet is a well-proven method for forming drops exhibiting an advancing contact angle. The present invention is based on the inventors' insight that drops exhibiting a receding contact angle can also be formed with a continuous jet if the following few conditions are met: First, the continuous jet should be applied such that the surface area initially wetted by the liquid is larger than the surface area wetted by the drop after the drop has reached an equilibrium state, because only then can an appropriate receding process take place. Second, the initially wetted surface area should be continuous, because otherwise the liquid will likely split into multiple micro-drops, e.g. into two or more drops of comparable size, or into one larger drop and one or more smaller "satellite" drops. This would make it difficult or even impossible to reliably measure the geometric parameters. The first condition seems to require a relatively fast / high-energy jet, while the second condition seems to require a relatively slow, gentle process.

[0019] Based on extensive experimental studies, the inventors have realized that the two partly contradictory conditions can be met simultaneously when, for a given flow rate of the jet, the dosing time is controlled such that the dosing volume does not exceed the upper limit V max [μl] = 0.11 s * flux [μl / s] (1)

[0020] V max [μl] = 0.11 s * flux [μl / s] (1)

[0021] The term "flux" refers to the flow rate of the continuous jet in μl / s. Surprisingly, drops exhibiting a receding contact angle can be formed over a wide range of flow rates, as long as the dosing time is kept short enough so that the dosing volume does not exceed the maximum volume mentioned above. If a longer dosing time is used, the drop formation will slow down to allow for an appropriate receding process. The drop will instead exhibit an advancing contact angle.

[0022] The inventors found that, when applying the limit of equation (1), drops exhibiting a receding contact angle can be formed in a reliable way for a wide variety of liquid / surface combinations. In some cases, especially for some sample surface / liquid combinations, the upper limit V max It can be helpful to apply lower values, especially using factors 0.10, 0.08, 0.06 or even 0.04 instead of the factor 0.11 in equation (1). As will be discussed later, this can result in an even more pronounced receding process and an even more stable and reliable measurement of the receding angle. It will be understood that additional limitations can be applied for practical reasons, which will be apparent to the skilled person. In particular, some adjustment of the flow rate based on the opening diameter and the properties of the liquid can be required to ensure that a continuous jet is produced.

[0023] In summary, the method of the present invention allows for a fast and reliable deposition of drops exhibiting a receding contact angle. In particular, the method can be performed quickly without moving parts, such as a sleeve that needs to be lowered very carefully onto the surface, and, if desired, in a fully automated way.

[0024] According to one aspect, the maximum sample surface area wetted during drop formation is at least 6% larger than the sample surface area wetted by the drop after the drop has reached a static state. In a further aspect, the maximum sample surface area wetted during drop formation is at least 10% or at least 20% larger than the sample surface area wetted by the drop after the drop has reached a static state. This means that the liquid initially spreads over a larger surface area until the liquid front on the surface recedes towards its equilibrium position. Experimental investigations have shown that, when the degree of reduction of the wetted surface area mentioned above is met, drops exhibiting a receding contact angle can be produced, which is practically equal to the receding contact angle measured with standard techniques.

[0025] According to an aspect, the drop formed on the sample surface after having reached a static state has a drop diameter in the range of 150% to 490% of the opening diameter, in particular in the range of 200% to 400% of the opening diameter. These values are particularly suitable for drops having a contact angle of about 90°. The opening diameter determines the diameter of the jet, so the above criteria mean that a relatively broad jet forms a relatively small drop. The drop is less than 5 times as large as the width of the jet. Experimental data show that the process can be performed very quickly and very reliably in this way, probably because a relatively high flow rate can be obtained with a relatively low flow velocity.

[0026] According to an aspect, the flow rate is chosen in the range of 20 μΙ / s to 800 μΙ / s, in particular in the range of 46 μΙ / s to 650 μΙ / s, in particular in the range of 300 μΙ / s to 500 μΙ / s. As mentioned above, a large range of flow rates is suitable. Best results are obtained in the given ranges.

[0027] According to an aspect, the dosing time is chosen such that the dosing volume is in the range of 0.1 μΙ to 15 μΙ, in particular in the range of 0.2 μΙ to 5 μΙ. These dosing volumes correspond to drops that are ideally suited for measuring the receding contact angle, because they are small enough for gravity to be neglected, while they are stable for long enough time to measure the at least one geometric parameter.

[0028] According to an aspect, the opening diameter is in the range of 0.05 mm to 0.5 mm, in particular in the range of 0.11 mm to 0.4 mm, in particular in the range of 0.15 mm to 0.35 mm. Although openings with other diameters can also be used, it has been found that, in particular in the given ranges, for various surfaces and liquids, a continuous jet with properties suitable for forming drops exhibiting a receding contact angle can be obtained with readily available hardware.

[0029] According to an aspect, the liquid is ejected from a liquid reservoir in which a defined dosing pressure is maintained. Generally, the liquid jet can be formed e.g. based on an actuator driven syringe mechanism with any suitable dosing technique. However, a pressurized liquid reservoir can facilitate a smooth, uniform, uninterrupted jet. The liquid reservoir can be a relatively large volume containing a sufficient liquid supply for a large number of drops / the entire liquid supply. However, the liquid reservoir in which the dosing pressure is controlled can also be a certain sub-volume of the available liquid supply e.g. arranged in a liquid line. The dosing pressure can be maintained within the liquid reservoir e.g. with a volume of pressurized gas arranged in or coupled to the liquid reservoir, in particular within a separate, deformable gas container within the liquid reservoir. The formation of the jet, in particular the dosing time, can be controlled by a control valve arranged between the liquid reservoir and the opening through which the liquid is ejected, e.g. arranged in a liquid line connecting these elements.

[0030] According to an aspect, the dosing pressure is controlled in the range of 100 mbar to 2000 mbar, in particular in the range of 200 mbar to 800 mbar. It has been found that within these dosing pressure ranges, a well-defined liquid jet can be obtained which has properties very suitable for forming a drop exhibiting a receding contact angle.

[0031] According to an aspect, the liquid applied to the surface forms a single drop on the surface, the drop exhibiting a receding contact angle. As explained above, preferably, the drop formation process is controlled such that the formation of more than one drop, e.g. one or more additional satellite drops, is avoided in order to reliably, preferably automatically, measure the at least one geometric parameter.

[0032] The object of the present application is also achieved by a device for measuring a receding contact angle having the features defined in the present application. The device comprises:

[0033] • a liquid reservoir,

[0034] • a dosing means adapted to apply the liquid as a continuous jet with a flow rate to a sample surface, wherein the liquid is ejected through an opening having an opening diameter within a defined dosing time, thereby defining a dosing volume,

[0035] • a contact angle measuring means adapted to measure at least one geometric parameter of the drop formed on the sample surface and to determine a contact angle between the sample surface and the drop based on the at least one geometric parameter,

[0036] • wherein the dosing means is adapted to control the dosing time such that the dosing volume does not exceed the flow rate times 0.11 seconds.

[0037] The apparatus is adapted to perform the method as defined in the present application. Further, the apparatus can be adapted to perform specific aspects of the method as indicated in the present application. With respect to the features and advantages of the apparatus, reference is also made to the description of the method, which is equally applicable to the apparatus, also in view of the aspects given below. The apparatus can be designed as a stand-alone device, which comprises a controller controlling the dosing means and / or the contact angle measuring means. However, one or more of these functions can be implemented in a separate control device, such as a personal computer connected to the device.

[0038] According to one aspect, the apparatus comprises a liquid line connecting the liquid reservoir and the opening, a valve arranged in the liquid line, a liquid pressurizing means for setting the liquid in the liquid reservoir to a dosing pressure, wherein the dosing means is adapted to define the dosing time by opening and closing the valve.

[0039] According to one aspect, the dosing means is adapted to apply a continuous jet having a flow rate in the range of 20 μΙ / s to 800 μΙ / s, in particular in the range of 46 μΙ / s to 650 μΙ / s, in particular in the range of 300 μΙ / s to 500 μΙ / s.

[0040] According to one aspect, the dosing means is adapted to control the dosing time such that the dosing volume is in the range of 0.1 μΙ to 15 μΙ, in particular in the range of 0.2 μΙ to 5 μΙ.

[0041] According to one aspect, the opening diameter is in the range of 0.05 mm to 0.5 mm, in particular in the range of 0.11 mm to 0.4 mm, in particular in the range of 0.15 mm to 0.35 mm.

[0042] According to one aspect, the dosing means is adapted to control the dosing pressure in the range of 100 mbar to 2000 mbar, in particular in the range of 200 mbar to 800 mbar. BRIEF DESCRIPTION OF DRAWINGS

[0043] In the following, the application is explained in more detail with reference to the drawings:

[0044] Figure 1 schematic diagram of an apparatus for measuring receding contact angles;

[0045] Figure 2 Six still images of a high speed video showing the first drop formation process;

[0046] Figure 3 Five still images of a high speed video showing the second drop formation process;

[0047] Figure 4 A graph showing the reduction in diameter observed for one liquid / surface combination as a function of the dosing volume;

[0048] Figure 5 A graph showing the maximum drop volume as a function of the flow rate of the liquid jet for various surface / liquid combinations; and

[0049] Figure 6 A graph showing the water receding contact angle for various liquid / surface combinations using different measurement techniques. DETAILED DESCRIPTION

[0050] Figure 1 The apparatus of Fig. 1 comprises a dosing tool 10 which is located at a defined position relative to a surface 14 of a sample 12. The dosing tool has a liquid reservoir 16 which is filled with a liquid 18. Within the liquid reservoir 16, above the liquid 18, there is a volume of pressurized gas 20. Although not shown, a separate, deformable container holding the pressurized gas 20 or the liquid 18 can be provided within the liquid reservoir 16 to ensure separation between the different fluids.

[0051] Facing the surface 14, the dosing tool 10 comprises a nozzle 22 with an opening 24 having a diameter. The nozzle 22 and the opening 24 are connected to the liquid reservoir 16 by a liquid line 26 which contains a control valve 28. A controller 30 is connected to the control valve 28 and is adapted to open and close the control valve 28 in order to control the ejection of the liquid 18 through the opening 24. In the embodiment of Fig. 1, the controller 30 is shown close to the dosing tool 10, but it can also be integrated into a common housing. The connection between the controller 30 and the liquid reservoir 16, which is shown as a dashed line, indicates that the controller 30 can also be adapted to control the dosing pressure maintained in the liquid reservoir. Figure 1

[0052] ​Once the control valve 28 is opened, the liquid 18 is ejected through the opening 24 as a continuous jet 32 of liquid 18, the jet 32 having a nearly constant width or a constant width 34 exemplified by two arrows. The width 34 is determined by the diameter of the opening 24. The jet 32 has a defined flow rate, which is determined by several factors, including the diameter of the opening 24, the dosing pressure, the viscosity of the liquid 18, and the length and diameter of the liquid line 26. The jet 32 is directed towards the surface 14 and forms a drop 36 on the surface 14. In Figure 1 , the size of the drop 36 is exaggerated, in reality, after reaching a steady state (see Figure 2 and 3 ), the drop 36 has a diameter that is preferably less than 5 times the width 34 of the jet 32.

[0053] After a predetermined dosing time, the control valve 28 is closed again by the controller 30 so that a defined dosing volume has been applied, which can be calculated by multiplying the flow rate with the dosing time.

[0054] Figure 1 The apparatus of Figure 2 further comprises a contact angle measurement tool having a camera 38 and a light source 40. The camera 38 and / or the light source 40 are controlled by the controller 30. They are arranged on opposite sides of the drop 36 so that side view images of the drop 36 can be acquired. Based on these images, the receding contact angle of the drop 36 can essentially be determined by measuring the receding contact angle directly within the side view images (see Figure 2 . However, this is only one example of a geometrical parameter suitable for determining the contact angle. It should be noted that Figure 1 the contact angle measurement tool of is only one non-limiting example, the dosing tool 10 of the apparatus can be combined with any other contact angle measurement tool.

[0055] Figure 2 The drop formation process using the apparatus as shown in Figure 1 is exemplified by the jet 32 applying the liquid 18 to the surface 14 of the sample 12. The figure comprises a series of six consecutive images shown in Figure 2 a) to 2f), which are selected from a high speed video showing the process in a side view. At the top of each image, the lower end of the nozzle 22 with the opening 24 can be seen. At the bottom of each image, the sample 12 with the surface 14 represented by a dashed line can be seen. Due to the reflective properties of the surface 14, a mirror image of the liquid 18 / drop 36 can be seen below the dashed line.

[0056] Figure 2Image a) shows a point in time shortly after the control valve 28 has opened and the jet 32 ​​has reached the surface 14, where the first amount of liquid 18 initially forms a relatively broad, flat shape on the surface 14. At this point, the control valve 28 is still open, and the jet 32 ​​continues to flow from the opening 24 in the nozzle 22. It can be clearly seen that the jet 32 ​​has a constant width before it reaches the surface 14.

[0057] exist Figure 2 In image b), control valve 28 remains open, and jet 32 ​​continues to apply liquid 18 to surface 14. The volume of the still broad, flat shape on this surface is increasing.

[0058] exist Figure 2 In image c), after the metered supply time has ended, control valve 28 has closed, and the rear end of jet 32 ​​has reached surface 14. At this point, the entire metered supply volume of liquid 18, i.e., droplet 36, is positioned on surface 14, but liquid 18 is still in motion and the shape of droplet 36 is changing. This can also be seen in... Figure 2 The understanding is obtained from images d) and e). It can be seen that as the periphery of droplet 36 recedes, the surface area of ​​surface 14 wetted by droplet 36 decreases.

[0059] exist Figure 2 In image f), drop 36 has reached its equilibrium state. This occurs due to the dewetting process, so drop 36 exhibits a receding contact angle 42, which is identified in the image between the tangent 44 of the contour drawn on the periphery of drop 36 on surface 14.

[0060] Figure 3 Also shown in a side view are a series of five images a) through e), each showing only a small portion of surface 14. In this example, the receding contact angle 42 of droplet 36 is less than... Figure 2 The retreat contact angle in the figure, and the initial diffusion ratio of the shape. Figure 2 The middle is even wider, so that drop 36 will retreat more noticeably until it reaches... Figure 3 The equilibrium state is shown in image e).

[0061] Figure 4A graph showing the results of individual dosing processes for the same surface / liquid combination, i.e. a silicon wafer with water. A nozzle 22 with an opening diameter of about 0.2 mm was used together with a fixed dosing pressure of about 350 mbar, resulting in a constant flow rate of the jet 32. The dosing time was varied by controlling the opening and closing times of the control valve 28. On the x-axis, the dosing volume is displayed, which is proportional to the dosing time. For each dosing process, a high-speed video of the illustration similar to Figure 2 and 3 was taken and the maximum diameter of the wetted area (when the jet 32 just fully reaches the surface 14) and the final diameter of the drop 36 in the equilibrium state were measured. The relative reduction of the diameter, which is displayed on the y-axis, is obtained by dividing the difference between the maximum diameter and the final diameter by the final diameter. Each data point represents the average value of several experiments with the same dosing time.

[0062] The data show that for small dosing volumes, a clear reduction of the diameter can be observed, as exemplarily illustrated in Figure 4 When the dosing volume is increased, the relative reduction of the diameter decreases. For dosing volumes larger than about 12 μl, no clear reduction of the diameter is observed, so the formed drop no longer exhibits a receding contact angle.

[0063] It has been found that the maximum dosing volume for which the diameter is sufficiently reduced in a large range of surface / liquid combinations depends on the flow rate of the jet 32. In this regard, Figure 5 additional experimental data are shown. Each data point represents one specific liquid / surface combination. The sample materials covered are displayed in the legend, including different plastics like PA (polyamide), PE (polyethylene), PMMA (polymethyl methacrylate), different wafers and tiles. Except for one data point, where the liquid used was diiodomethane (DIM), the liquid used was water. For each liquid / surface combination, a series of measurements similar to the ones shown in Figure 4 were performed and the maximum drop volume was determined for which no clear reduction of the diameter was observed. These maximum drop volumes are shown in Figure 5 This means that for each data point shown in Figure 5 several measurements were performed with drop volumes just below the depicted maximum drop volume. For these drop volumes, a clear reduction of the average diameter was observed. The upper limit of the maximum volume V max corresponding to equation (1) is plotted as line 46. For dosing times resulting in dosing volumes larger than this upper limit, no receding contact angle was observed.

[0064] Figure 6 The contact angles of various surface / liquid combinations (surface material: as indicated in the graph, liquid: water) are shown. The receding contact angles observed when applying the method of the present application are shown as white bars. The hatched bars represent the receding contact angles when using a conventional method. This conventional method uses a needle that is lowered onto the surface to apply a drop, followed by suction of a portion of the liquid through the needle to perform the receding process of the periphery of the drop. The measurements can be performed Details about this conventional method can be found in the above-mentioned document by T et al. The error bars represent the standard deviation of the mean values obtained from several different measurements for each surface / liquid combination.

[0065] The comparison shows that for all samples, both methods yield almost identical results. While a measurement of one receding contact angle with the conventional method usually takes about 15 to 20 minutes, the method of the present application can be performed in a few seconds or less if desired.

[0066] For the measurements shown in Figure 6 For the measurements shown in

[0067] It has been found that applying a dosing pressure of 1500 mbar on a dosing valve with an opening of 0.2 mm in diameter to dose water microdrops with a volume in the range of 1 to 6 pl provides receding contact angles that are, within the experimental uncertainty, the same as when measured using the traditional needle-in drop method. However, depending on the substrate, openings of only 0.2 mm in diameter or less and higher pressures were found to possibly result in an enhanced formation of satellite microdrops, which hinders automated image analysis.

[0068] In other measurements, drops presenting a receding contact angle were successfully applied to various surfaces by means of openings of 0.1 mm, 0.2 mm and 0.3 mm in combination with dosing pressures of 250 mbar, 350 mbar and 700 mbar. These parameters resulted in the following flow rates:

[0069] 0.1 mm 0.2 mm 0.3 mm 250 mbar 23.7 μl / s 115.3 μl / s 251.4 μl / s 350 mbar 33.6 μl / s 134.1 μl / s 341.6 μl / s 700 mbar 51.3 μl / s 225.7 μl / s 591.5 μl / s

[0070] List of reference signs

[0071] 10 dosing means

[0072] 12 sample

[0073] 14 surface

[0074] 16 liquid reservoir

[0075] 18 liquid

[0076] 20 pressurized gas

[0077] 22 nozzle

[0078] 24 opening

[0079] 26 liquid line

[0080] 28 control valve

[0081] 30 controller

[0082] 32 jet

[0083] 34 width

[0084] 36 drop

[0085] 38 camera

[0086] 40 light source

[0087] 42 receding contact angle

[0088] 44 tangent

[0089] 46 line

Claims

1. A method for measuring a retreating contact angle (42), the method comprising the following steps: Droplets (36) of liquid (18) are formed on the sample surface (14) by applying liquid (18) as a continuous jet (32) with a certain flow rate to the sample surface (14), wherein the liquid (18) is sprayed through an opening (24) with an opening diameter within a defined metering time, thereby defining a metering volume. Measure at least one geometric parameter of the droplet (36) formed on the sample surface (14). The contact angle (42) between the sample surface and the droplet is determined based on the at least one geometric parameter, wherein... The flow rate and the metering time are selected such that the metering volume does not exceed the flow rate multiplied by 0.11 seconds.

2. The method according to claim 1, characterized in that, The maximum sample surface area wetted during drop formation is at least 6% larger than the sample surface area wetted by the drop (36) after the drop (36) reaches a static state.

3. The method according to claim 1 or 2, characterized in that, After reaching a static state, the droplet (36) formed on the sample surface (14) has a droplet diameter in the range of 150% to 490% of the opening diameter.

4. The method according to claim 1 or 2, characterized in that, The flow rate is selected to be in the range of 20 μl / s to 800 μl / s.

5. The method according to claim 1 or 2, characterized in that, The flow rate is selected to be in the range of 300 μl / s to 500 μl / s.

6. The method according to claim 1 or 2, characterized in that, The metering time is selected such that the metering volume is in the range of 0.1 μl to 15 μl.

7. The method according to claim 1 or 2, characterized in that, The diameter of the opening is in the range of 0.05 mm to 0.5 mm.

8. The method according to claim 1 or 2, characterized in that, The diameter of the opening is in the range of 0.15 mm to 0.35 mm.

9. The method according to claim 1 or 2, characterized in that, The liquid (18) is ejected from the liquid reservoir (16) and a defined metered supply pressure is maintained in the liquid reservoir (16).

10. The method according to claim 9, characterized in that, The quantitative supply pressure is controlled within the range of 100 mbar to 2000 mbar.

11. The method according to claim 1 or 2, characterized in that, The liquid (18) applied to the sample surface (14) forms a single droplet (36) on the sample surface (14), the single droplet (36) exhibiting a receding contact angle (42).

12. An apparatus for measuring a retreat contact angle (42), comprising: Liquid storage container (16). A metering supply tool (10) is adapted to apply the liquid (18) as a continuous jet (32) with a certain flow rate to the sample surface (14), wherein the liquid (18) is sprayed through an opening (24) having an opening diameter within a defined metering supply time, thereby defining a metering supply volume. A contact angle measuring tool adapted to measure at least one geometric parameter of a droplet (36) of liquid formed on the sample surface (14), and to determine the contact angle (42) between the sample surface and the droplet (36) based on the at least one geometric parameter. The metering tool (10) is adapted to control the metering time so that the metering volume does not exceed the flow rate multiplied by 0.11 seconds.

13. The device according to claim 12, characterized in that, The device includes a liquid line (26) connecting the liquid reservoir (16) and the opening (24), a control valve (28) disposed in the liquid line (26), and a liquid pressurizing tool for setting the liquid (18) in the liquid reservoir (16) at a metered supply pressure, wherein the metered supply tool (10) is adapted to limit the metered supply time by opening and closing the control valve (28).

14. The device according to claim 12 or 13, characterized in that, The quantitative supply tool (10) is adapted to apply a continuous jet (32) with a flow rate in the range of 20 μl / s to 800 μl / s.

15. The device according to claim 12 or 13, characterized in that, The quantitative supply tool (10) is adapted to apply a continuous jet (32) with a flow rate in the range of 300 μl / s to 500 μl / s.

16. The device according to claim 12 or 13, characterized in that, The quantitative supply tool (10) is adapted to control the quantitative supply time so that the quantitative supply volume is in the range of 0.1 μl to 15 μl.

17. The device according to claim 12 or 13, characterized in that, The diameter of the opening is in the range of 0.05 mm to 0.5 mm.

18. The device according to claim 12 or 13, characterized in that, The diameter of the opening is in the range of 0.15 mm to 0.35 mm.

19. The device according to claim 12 or 13, characterized in that, The metering tool (10) is adapted to control the metering pressure within the range of 100 mbar to 2000 mbar.

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