JP2019158471A - Droplet motion control method - Google Patents

Abstract

To provide a method for controlling droplet motion in a microscopic system with a new approach.SOLUTION: Provided is a droplet motion control method. The droplet motion control method includes the steps of: preparing a member having a surface on which a large number of micro wrinkles having orientation is formed; and moving the droplet in a certain traveling direction determined according to the orientation on the surface. A driving force acting on the droplet on the surface can be generated only by gravity. The driving force can be generated by inclining the surface to the horizontal face.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method and apparatus for controlling droplet motion in a microscopic system.

  In recent years, in various fields such as chemistry and medicine, there is an increasing demand for motion control technology for a minute amount of liquid. For example, in a microfluidic device called a lab-on-chip or a microreactor, reaction and mixing are performed using a very small amount of reagent. There are various advantages to manipulating a small amount of liquid in a minute system. For example, the temperature control is easy, the surface area per unit volume of the liquid is increased, the reaction efficiency at the interface is improved, the system is miniaturized and the sample is saved, saving space, For example, resource saving, energy saving, and cost reduction can be realized.

  When manipulating droplets in a minute system, the influence of interfacial phenomena such as wettability and surface tension on the solid surface of the droplet is important. Recently, many techniques for controlling the movement of droplets by changing the wettability and surface tension locally on a solid surface using a temperature difference, an electric field, a photochemical reaction, and the like have been proposed. For example, Non-Patent Document 1 discloses a method in which an electrode is disposed on a solid surface and the wettability of the solid surface is electrically changed to drive droplets on the solid surface. Patent Document 1 also relates to a technique for driving a droplet by changing wettability. Here, a laser is irradiated on the end of the droplet on a solid surface, and the droplet is moved in the direction of the end.

Dahuber, A.A. and Troian, S.M., Principles of Microfluidic Actuation by Modulation of Surface Stresses, Annual Review of Fluid Mechanics, Vol. 37 (2005), 425-455

JP 2012-32258 A

  However, various conventional droplet movement control techniques still have problems. For example, in Non-Patent Document 1, it is necessary to process an electrode pattern on a solid surface in advance, and there is inconvenience that only movement according to the electrode pattern can be obtained. On the other hand, in Patent Document 1, processing of a solid surface is unnecessary, and although motion control can be freely performed from the outside, a laser irradiator must be prepared. In addition, in order to cause the droplet to perform a desired movement, it is possible to control the position of the irradiator. Thus, although various droplet movement control methods have been proposed so far, this field is in the development stage, and further technological innovation is desired.

  It is an object of the present invention to provide a method and apparatus for controlling droplet movement in a minute system by a new approach.

  According to a first aspect of the droplet movement control method, a member having a surface on which a large number of microwrinkles having orientation is formed is prepared, and the droplet is placed on the surface according to the orientation. Moving in a fixed direction.

  A droplet movement control method according to a second aspect is a droplet movement control method according to the first aspect, wherein the member is arranged such that the surface is inclined with respect to a horizontal plane. Including doing. Moving the droplet includes moving the droplet in the traveling direction by applying a driving force by gravity to the droplet on the surface.

  The droplet movement control method according to the third aspect is the droplet movement control method according to the second aspect, and the driving force is only gravity.

  A droplet movement control method according to a fourth aspect is the droplet movement control method according to any one of the first to third aspects, wherein the member is prepared on the surface, Including arranging the member such that an angle formed by a direction indicated by the orientation with respect to a direction in which a driving force acts on the droplet is greater than 0 ° and less than 90 °.

  A droplet movement control device according to a fifth aspect includes a member having a surface on which a number of microwrinkles having orientation are formed. The droplets are moved on the surface in a certain traveling direction determined according to the orientation.

  According to the above aspect of the present invention, a surface on which a large number of microwrinkles having orientation is formed is used as a solid surface for moving droplets. According to the study by the present inventors, such a surface has anisotropy in light of factors such as wettability and contact angle that affect the movement of the droplet on the solid surface. Such a surface can be moved in a certain traveling direction determined according to the orientation of the microwrinkle. Therefore, the movement of the droplet in a minute system can be controlled by a new approach of controlling the traveling direction of the droplet based on the orientation of the microwrinkle.

The figure which shows the structure of the motion control apparatus of the droplet which concerns on one Embodiment of this invention. The schematic diagram explaining the process of forming a micro wrinkle. The figure which shows the measurement result of the sample board by a shape analysis laser microscope in case an extending | stretching distortion is 10%. The figure which shows the measurement result of the sample board by a shape analysis laser microscope in case extending | stretching distortion is 14%. The figure which shows the result of FFT with respect to the height distribution curve of a micro wrinkle in case extending | stretching distortion is 10% and 14%. The figure explaining the advancing contact angle and receding contact angle of a droplet. The figure which shows the structure of the experimental apparatus used for the drop fall experiment. The graph which shows the measured value and theoretical value of a critical angle in case extending | stretching distortion is 10% and 14%. The figure explaining angle (alpha) of a micro wrinkle. The graph which shows the measured value of the locus | trajectory of the droplet at the time of a fall when extending | stretching distortion is 10% and 14%.

  Hereinafter, a motion control method and apparatus according to an embodiment of the present invention will be described with reference to the drawings.

<1. Configuration of Droplet Motion Control Device>
FIG. 1 shows an overall configuration of a droplet motion control apparatus 1 according to the present embodiment. The motion control device 1 includes a member 2 having a surface 3 on which a large number of microwrinkles ML are formed. The surface 3 is configured to be flat, and the member 2 has a flat plate shape. A large number of micro wrinkles ML are formed to have a certain orientation, and in this embodiment, have a linear orientation. The microwrinkle ML is a fine wrinkle, and is typically a concavo-convex structure having a wavelength on the order of micrometers. The micro wrinkle ML is a groove formed on the surface 3, and extends along a certain orientation direction D1 corresponding to the orientation.

  The motion control device 1 can move droplets on the surface 3 in a certain traveling direction. Although the principle will be described later, the traveling direction of the droplet is determined according to the orientation of the microwrinkle ML. In the present embodiment, the driving force applied to the droplet to move on the surface 3 is only gravity. Therefore, a special device for generating a driving force is not required, and the device 1 is configured simply.

  In order to apply a driving force due to gravity to the droplet, the member 2 is disposed such that the surface 3 is inclined by an inclination angle φ (0 ° <φ) with respect to the horizontal plane, as shown in FIG. Further, as shown in FIG. 1, an angle formed by the orientation direction D1 of the micro wrinkle ML with respect to a direction D2 (hereinafter sometimes referred to as a gravity direction) D2 projected on the surface 3 is defined as α. At this time, the member 2 is arranged so that 0 ° <α <90 °. The gravity direction D2 is a direction in which gravity acts on the droplet on the surface 3.

<2. Principle>
Next, the principle of droplet movement control by the movement control device 1 will be described. Below, it demonstrates, referring the experiment which the present inventors conducted. In this experiment, the anisotropy related to the contact angle θ of the droplet by the microwrinkle ML was examined. As a result, it was confirmed that the direction of movement of the droplet moving on the solid surface by the driving force such as gravity can be controlled by this anisotropy.

<2-1. Formation of micro wrinkles>
In performing the experiment, microwrinkle ML was formed. FIG. 2 is a schematic diagram illustrating a process of forming the microwrinkle ML. First, a sample of polyvinyl chloride (PVC) cut to 1.0 mm (thickness) × 23.8 mm (length) × 80 mm (width) was prepared as a base material. The above base material was placed in a constant temperature bath at 90 ° C., and stretched by a tensile stress by a universal material testing machine as shown in FIG. 2A, and then cooled to room temperature while being fixed to the universal material testing machine. . Two types of stretching strains of 10% and 14% were given. Note that PVC reaches the glass transition temperature at 80 ° C. and transitions to a rubber state. Since the above base material was warped and curved when heated, it was cut into 1.0 × 22 × 22 mm test pieces as shown in FIG. 2B, and the two cut test pieces were bonded together. A sample plate was used. Lamination of the test pieces was performed by applying a hard vinyl chloride pipe adhesive to the surface of one of the test pieces.

  Next, a thin film was formed on the surface of the above sample plate (see FIG. 2C). More specifically, the sample plate was placed on a spin coater (MS-B100 manufactured by Mikasa), and 200 μl of the polymer solution was dropped onto the center of the sample plate using a micropipette. The polymer solution was a dichloroethane solution in which poly-N-vinylcarbazole (PVK) having a concentration of 1.2 wt% was dissolved. Thereafter, the spin coater was immediately rotated at 250 rpm for 10 seconds and further at 2500 rpm for 90 seconds to form a PVK spin-coated thin film having a thickness of about 100 nm on the surface of the sample plate.

  Thereafter, the sample plate on which the PVK thin film was formed was reheated to 90 ° C. and left for 3 hours (see FIG. 2D). PVK constituting the thin film has a glass transition temperature of 190 ° C., higher than that of PVC constituting the base material, and higher than the reheating temperature. Therefore, by reheating, the base material portion of the sample plate reaches the glass transition temperature, but the thin film portion does not reach the glass transition temperature. Therefore, only the base material portion was in a rubber state, the stretching strain was released and the material thinned, and the thin film portion buckled. Thereby, bending (wrinkle) occurred in the thin film portion of PVK on the surface of the base material, and micro wrinkle ML was formed (see FIG. 2 (e)).

  After the creation of the microwrinkle ML, its shape was confirmed. For confirmation of the shape, a three-dimensional height distribution of the surface (hereinafter referred to as the wrinkle surface) on which the micro wrinkle ML is formed on the above sample plate is examined using a shape analysis laser microscope (VK-X1000 manufactured by Keyence Corporation). By the method. The three-dimensional height distribution was measured in a range of about 284 μm in length and 213 μm in width (768 × 1024 pixels) on the wrinkle surface of the sample plate.

  FIG. 3A and FIG. 4A are planar images of the wrinkle surface when the sample plates S1 and S2 are measured with a shape analysis laser microscope, respectively. The sample plates S1 and S2 are sample plates with microwrinkle ML formed by applying 10% and 14% stretching strain to the base material, respectively. The x-axis shown in FIGS. 3 (a) and 4 (a) is the tensile direction when stretching strain is applied, and is the direction in which the groove of the microwrinkle ML extends. The y-axis is a direction orthogonal to the x-axis on the wrinkle surface, and is a direction in which the grooves of the micro wrinkle ML are arranged. Therefore, the cross-sectional shape of the wrinkle surface parallel to the y-axis has a waveform due to the uneven structure of the micro wrinkle ML. 3 (b) and 4 (b) are diagrams showing the three-dimensional height distribution of the wrinkle surface of the sample plates S1 and S2, respectively. FIGS. 3 (c) and 4 (c) It is the height distribution curve of the wrinkle surface in the cross section along line segment SS 'of the y direction in (a) and Fig.4 (a).

FIGS. 5A and 5B show the results of fast Fourier analysis (FFT) for the waveforms of the height distribution curves in FIGS. 3C and 4C, respectively. In the figure, the horizontal axis represents wavelength and the vertical axis represents amplitude. As a result of the FFT, the wavelength (peak wavelength) λ of the height distribution curve in the y direction of the sample plates S1 and S2 is as shown in Table 1. Table 1 also shows the results of measuring the surface roughness Ra and the maximum height Rz of the wrinkle surface based on the measurement results of the above three-dimensional height distribution for each of the sample plates S1 and S2. From the results in Table 1, it can be seen that as the stretching strain increases, the amount of change in the thin film on the wrinkle surface increases, and thus the surface roughness Ra and the maximum height Rz increase.

<2-2. Anisotropy of contact angle>
Next, the contact angle θ of the droplets on the wrinkle surfaces of the sample plates S1 and S2 was measured, and the anisotropy was examined. When the droplet is in a critical state of falling on an inclined solid surface, the forward contact angle θ _A is observed at the front end (lower end) as the direction in which the liquid wets the solid surface, and at the rear end (upper end), the contact is backward as the direction to dry. An angle θ _R is observed. FIG. 6 is a diagram for explaining the advancing contact angle θ _A and the receding contact angle θ _R of the droplet. The droplet on the solid surface touches the solid surface at any point between its advancing contact angle θ _A and the receding contact angle θ _R at all points around it. Since θ _A and θ _R are the maximum angle and the minimum angle at which the droplet can form on the solid surface, it can be said that the angle is the limit angle observed at the front and rear ends when the droplet starts to fall.

There are various methods for measuring the contact angle θ. Here, the shape of the droplet dropped on the wrinkle surface of the sample plates S1 and S2 installed horizontally was photographed, and the contact angle θ was measured from the image. When measuring the advancing contact angle θ _A , the sample liquid was pushed out with a microsyringe, and shooting was performed while placing droplets so that the solid-liquid three-phase contact line C spread on the wall surface (wrinkle surface). Distilled water was used as the sample liquid. On the other hand, at the time of measuring the receding contact angle θ _R, the liquid was sucked from the droplet on the wall surface (wrinkle surface) by the microsyringe, and photographing was performed while the contact line C was retracted. At this time, the droplet was slowly expanded and contracted so that the moving speed of the contact line C around the droplet was 1.0 mm / min or less so that the dynamic wetting effect did not appear.

Next, on the image photographed as described above, the point sequence that maximizes the gradient of the luminance value was taken as the gas-liquid interface on the droplet surface, and the shape of the gas-liquid interface was determined. The shape of the gas-liquid interface near the contact line C was approximated by a polynomial, and the contact angle θ was measured from the gradient at the position of the contact line C. The advancing contact angle θ _A was measured in two directions: a direction parallel to the groove of the microwrinkle ML (orientation direction D1) and a direction perpendicular thereto. Hereinafter, these are referred to as advancing contact angle θ _Ap and advancing contact angle θ _Ao , respectively. Similarly, the receding contact angle θ _R was also measured in two directions: a direction parallel to the groove of the microwrinkle ML (orientation direction D1) and a direction perpendicular thereto. Hereinafter, these are referred to as the receding contact angle θ _Rp and the receding contact angle θ _Ro , respectively. The contact angles θ _Ao , θ _Ap , θ _Ro and θ _Rp were measured for each of the sample plates S1 and S2, measured six times under the same experimental conditions, and the average value was taken as the measured value. The results of this measurement are shown in Table 2.

For comparison, a spin coat thin film is formed in the same manner as the sample plates S1 and S2, but forward contact is made in the same manner as the sample plates S1 and S2 with respect to the sample plate S0 on which the microwrinkle ML is not formed. Angle θ _A and receding contact angle θ _R were measured. Hereinafter, these are referred to as the forward contact angle θ _Ac and the backward contact angle θ _Rc , respectively. The results of this measurement are shown in Table 3.

In addition, before the above experiment, all the wrinkle surfaces of the sample plates S0 to S2 were co-washed with distilled water. From the results in Tables 2 and 3, the wettability in the direction perpendicular to the wrinkles is deteriorated due to the formation of the microwrinkle ML, and the advancing contact angle θ _Ao is larger than the advancing contact angle θ _{Ac of} only the spin coat. I understand that. In addition to the microwrinkle ML, scratches associated with processing are recognized on the surfaces of the sample plates S0 to S2, so that the edge of the droplet is caught by the scratch when measuring the receding contact angle θ _R of the droplet, and the receding contact is made. A phenomenon was observed in which the angle θ _R decreased. Therefore, it is considered that the standard deviation of the receding contact angle θ _R is larger than that of the advancing contact angle θ _A. From Table 2, in the microwrinkle ML formed in this experiment, an anisotropy of about 6 ° is observed in the advancing contact angle θ _A (in the case of the sample plate S1, it is 5.9 °, and the sample plate S2 In this case, an anisotropy of about 2 ° was observed in the receding contact angle θ _R (in the case of the sample plate S1, 4.0 °, in the case of the sample plate S2, 1. 9 °).

<2-3. Critical angle of falling>
Next, an inclination angle φ (hereinafter referred to as a critical angle) of the wrinkle surface with respect to the horizontal plane when the droplets fall on the wrinkle surfaces of the sample plates S1 and S2 was measured. FIG. 7 shows the configuration of the experimental apparatus used for the fall experiment. The experimental method using the same apparatus was as follows.

  First, droplets were dropped with a microsyringe on the wrinkle surface of a horizontally installed sample plate. Thereafter, the sample plate was tilted gradually and steeply by rotating the worm gear. As the tilt angle φ is increased, the front end of the droplet gradually extends downward at the beginning while the rear end remains stationary, but in the critical state where the droplet falls, the front and rear ends are Move together. The inclination angle φ of the wrinkle surface in this critical state is the critical angle. The determination of the critical state was performed by photographing the front and rear ends of the droplet with a digital camera and observing the images. More specifically, while the inclination angle φ was increased by 0.5 ° with a worm gear, the liquid was left for 30 seconds or longer at each inclination angle, and the presence or absence of movement of the front and rear ends of the droplets was confirmed. In order to accurately determine the critical state, the observation time was further extended under the condition where falling was observed, and it was confirmed that the entire droplet moved downward without stopping.

The critical angle was measured in two cases: a case where the sample plate was fixed in a direction in which the orientation direction D1 of the microwrinkle ML was parallel to the gravity direction D2, and a case where the sample plate was fixed in a direction orthogonal thereto. Hereinafter, the critical angles in each case are referred to as φ _p and φ _o . The critical angles φ _p and φ _o were measured while increasing the volume of the droplet in increments of 5 μl from 25 μl to 50 μl, each volume was measured 6 times, and the average value was taken as the measured value. This measurement was performed only on the sample plate S2. The result of this measurement is shown in FIG. In the figure, the horizontal axis indicates the volume of the droplet, and the vertical axis indicates the critical angle φ _p or φ _o .

On the other hand, theoretical values of the critical angles φ _p and φ _o were calculated for the sample plates S1 and S2. The curves in FIGS. 8A and 8B show theoretical values of critical angles φ _p and φ _o corresponding to the sample plates S1 and S2, respectively. The theoretical value was calculated as follows. First, referring back to FIG. 6, in the critical state of falling, gravity and a force due to surface tension act on the droplet on the inclined solid surface. In FIG. 6, the solid in-plane component of the surface tension σ acting on an arbitrary point A on the contact line C is represented by σ cos θ using the contact angle θ at the point A. Further, the falling direction component is represented by σ cos θ · cos τ. As shown in FIG. 6, τ is an angle formed by the normal to the contact line C in the solid surface and the falling direction. When this surface tension is integrated over the contact line C, the following equation is derived from the balance between gravity in the falling direction and force due to the surface tension. Here, ρ, g, and V on the right side are the density of the liquid, the acceleration of gravity, and the volume of the droplet, respectively.

When the liquid droplet starts to fall, a forward contact angle θ _A appears on the contact line C in front of the position where the width of the liquid droplet becomes maximum, and a backward contact angle θ _R appears on the rear side. Since ds cosτ is a horizontal shading of the line element ds, when cos θ · cos τ ds is integrated by dividing it forward and backward from the maximum width, b is the maximum adhesion width of the droplet on the solid surface, and bcos θ _A is calculated as -b cos [theta] _R. Therefore, regardless of the shape of the contact line C, the formula 1 can be rewritten as follows.

In this experiment, the theoretical value of the critical angles φ _p and φ _o was calculated by measuring the width b of the droplet placed on the wrinkle surface and substituting it into the formula 2 together with the results of Table 2. FIG. 8B shows that the measured values of the critical angles φ _p and φ _o agree well with the theoretical values. The critical angle φ _o when the droplet falls vertically to the micro wrinkle ML is larger than the critical angle φ _p when the droplet falls parallel to the micro wrinkle ML, and an anisotropy of about 10 ° was confirmed.

<2-4. Control of droplet traveling direction>
The result of FIG. 8 indicates that the wettability is worse in the direction perpendicular to the groove of the microwrinkle ML than in the direction parallel to the groove. The principle of droplet movement control according to this embodiment is to change the traveling direction of a droplet that falls by gravity based on such anisotropy of wettability. That is, the traveling direction of the droplet on the wrinkle surface is bent with respect to the gravity direction D2 according to the orientation of the micro wrinkle ML.

In order to confirm that the traveling direction of the droplet bends according to the orientation of the microwrinkle ML, an experiment was performed again using the experimental apparatus of FIG. In this experiment, unlike the measurement of the critical angle, for each of the sample plates S1 and S2, the orientation direction D1 of the microwrinkle ML makes an angle α = 30 °, 45 ° and 60 ° with respect to the gravity direction D2. The sample plate was fixed to (see FIG. 9). After setting the sample plate at each angle α, 30 μl of a droplet was dropped onto the upper end of the sample plate with a microsyringe. Thereafter, the sample plate is gradually tilted by rotating the worm gear so that the droplet tilt angle φ is less than the critical angle φ _o (theoretical value) at α = 90 ° so that the droplet does not stop on the wrinkle surface. The angle was set to 1 ° larger. In this state, the movement behavior of the droplets was observed by moving image shooting. Then, the center coordinates of the droplet were obtained from the coordinates of the upper and lower ends and the left and right ends of the droplet at each time read from the image, and the locus of the movement of the droplet was obtained. The droplet trajectory was measured three times for each α, and the average value was taken as the measured value.

  The droplet trajectories measured for the sample plates S1 and S2 are shown in FIGS. 10 (a) and 10 (b), respectively. Here, the vertical axis y indicates the coordinate in the gravity direction D2, and the horizontal axis x indicates the coordinate in the horizontal direction. As shown in FIG. 10 (a), in the sample plate S1, a behavior was observed in which the droplets tumbled in a constant direction from the gravity direction D2 under all the conditions of α. Further, as shown in FIG. 10 (b), even in the sample plate S2, a behavior in which the liquid droplet falls down in a certain direction from the gravity direction D2 was observed under all the conditions of α. However, as already described, the surface roughness Ra and the maximum height Rz are larger in the sample plate S2 than in the sample plate S1. Therefore, on the sample plate S2, the droplets are more significantly affected by the anisotropy (that is, the wettability anisotropy) of the contact angle θ of the microwrinkle ML, so that the liquid droplets are continuously in the horizontal direction. Falling down, the direction of travel was bent. In this experiment, as shown in FIG. 10, due to the anisotropy of the contact angle θ of the micro wrinkle, it was possible to continuously drop the droplets by inclining the falling direction of the droplets by about 12 ° from the gravity direction D2. Further, comparing the results for the sample plates S1 and S2, it is expected that the effect of tilting the falling direction of the droplet from the gravity direction D2 will be increased if the stretching strain applied to the sample plate is increased.

<3. Droplet Motion Control Method>
A droplet motion control method based on the above principle will be described. First, the motion control device 1 is prepared. And the member 2 is arrange | positioned so that the surface 3 (wrinkle surface) may incline with the predetermined | prescribed inclination | tilt angle (phi) with respect to a horizontal surface. At this time, the member 2 is arranged on the surface 3 so that the orientation direction D1 of the microwrinkle ML makes a predetermined angle α with respect to the gravity direction D2. At this time, the inclination angle φ and the angle α of the microwrinkle ML are set in advance by experiment or theory in accordance with the desired traveling direction of the droplet on the surface 3. In this state, a predetermined droplet is dropped at a predetermined position on the surface 3. As a result, the droplet is moved in a constant traveling direction determined according to the orientation direction D1 and the inclination angle φ by gravity. According to this method, the traveling direction of the droplet on the surface 3 can be controlled as desired by the micro wrinkle ML.

<4. Application>
The motion control device based on the above principle can be used in various scenes where the liquid traveling direction needs to be controlled in a minute system. For example, it can be applied to a microfluidic device called a lab-on-chip or a microreactor. can do.

<5. Modification>
As mentioned above, although one Embodiment of this invention was described, this invention is not limited to the said embodiment, A various change is possible unless it deviates from the meaning. For example, the following changes can be made.

<5-1>
In the above embodiment, the micro wrinkle ML is a stripe pattern having a linear orientation, but the orientation pattern of the micro wrinkle ML is not limited to this. By changing the orientation pattern in various ways, it is possible to change the traveling direction of the droplets in various ways and to form more complicated droplet movement paths.

<5-2>
In the above embodiment, only the gravity is given to the droplet as a driving force. However, instead of or in addition to gravity, a driving force may be applied to the droplet by an action such as a temperature difference, an electric field, a magnetic field, or a photochemical reaction.

1 motion control device 2 member 3 surface (wrinkle surface)
ML Micro Wrinkle D1 Gravity direction (direction in which the vertical direction is projected onto the wrinkle surface)
D2 Micro wrinkle orientation direction φ Wrinkle surface tilt angle

Claims (5)

A droplet movement control method comprising:
Providing a member having a surface on which a number of microwrinkles having orientation are formed;
Moving the droplets on the surface in a certain traveling direction determined according to the orientation,
Droplet motion control method. Providing the member includes arranging the member such that the surface is inclined with respect to a horizontal plane;
Moving the droplet includes moving the droplet in the traveling direction by applying a driving force by gravity to the droplet on the surface;
The droplet movement control method according to claim 1. The driving force is only gravity,
The droplet movement control method according to claim 2. The member is prepared so that an angle formed by a direction indicated by the orientation with respect to a direction in which a driving force acts on the droplet is greater than 0 ° and less than 90 ° on the surface. Including placing
The droplet movement control method according to claim 1. A droplet motion control device comprising:
A member having a surface on which a number of microwrinkles having orientation are formed;
The droplet is moved on the surface in a certain traveling direction determined according to the orientation.
Droplet motion control device.