JP2012032258A - Droplet moving device and droplet moving method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a droplet moving device for easily moving a droplet to an arbitrary direction in a non-contact state without being restricted by a micro-heater or the like without making it necessary to embed any micro-heater or electrode pattern or the like in a wall surface in advance.SOLUTION: A droplet moving device 100 is provided with: a fixed surface with which a droplet A is brought into contact; and laser irradiating means for locally irradiating one end of the droplet with a laser 103 to increase the wettability of the droplet and the fixed surface. Since the contact angle of the irradiation end of the laser is decreased according to heating, the resultant force of a surface tension acting on the droplet appears so that it is possible to move the droplet to the direction of the laser irradiation end.

Description

  The present invention relates to a droplet moving apparatus and method for moving a droplet on a solid surface.

In the fields of chemical and optical equipment and medicine, there is a growing need to control the movement of microscale liquids such as heat transfer and reaction in microchannels or microfluidic devices.
For example, in a medical device such as a DNA analyzer, it is necessary to react and mix a rare and expensive reagent with a small amount of droplets, and research and development of a technique for moving a plate-like droplet is underway.

In such problems, the influence of interface phenomena such as surface tension and wettability is important. Recently, by using temperature difference, electric field, photochemical reaction, Marangoni effect, etc., surface tension and wettability are locally changed to control the movement and separation of droplets or the position of liquid columns in microchannels. A method for branching is being studied.

For example, the following Non-Patent Document 1 discloses a method in which an electrode is disposed on a flat plate and a droplet is driven by electrically changing the wettability of a wall surface. Non-Patent Document 2 below discloses a technique in which a microheater is embedded in a wall surface and driven using a change in surface tension caused by heat.

Dahuber, A.D. A. and Troian, S .; M.M. , Principles of Microfluidic Actuation by Modulation of Surface Stresses, Annual Review of FluidMechanics, 37 (2005), 425-455. Shukla, R. and Kalllam, K.K. A. Effect of liquid transparency laser-inducedmotion of drops, Transactions of ASME, Journal of Fluids Engineering, 131 (2009), 081301, 1-7

However, the methods disclosed in the above-mentioned Non-Patent Documents 1 and 2 require processing of an electrode pattern or the like on the wall surface in advance, and the limitation that only movement according to the pattern can be obtained in the moving direction of the droplet. There are drawbacks such as receiving. An object of the present invention is to provide a new droplet moving method that does not require wall surface processing.

The droplet moving device of the present invention includes a solid surface that comes into contact with the droplet, and heating means that locally heats a part of the droplet in a non-contact manner, which improves the wettability between the droplet and the solid surface. According to the present invention, as a result of the contact angle of one end heated by the heating means being reduced, a resultant surface tension acting on the droplet appears, and the droplet moves in the direction of the heated one end.

Thereby, it is not necessary to embed a micro heater, an electrode pattern, or the like in advance on the wall surface as in the prior art, and it is possible to move the droplets easily and non-contact. In addition, it is not limited to micro heaters, so it can move droplets in any direction.

The present invention may also be provided with a vibration means for vibrating the solid surface, preferably ultrasonic vibration. By applying ultrasonic vibration, the size of the contact angle history between the droplet and the solid surface can be reduced, and the resistance when the droplet moves can be reduced.

Furthermore, you may form the solid surface of this invention with a self-assembled monolayer (Self Assembled Monolayers). Since this self-assembled monomolecular film has very smooth surface irregularities, the contact angle history is reduced and the external force required to move the droplet can be reduced.

According to another aspect of the present invention, a contact angle between a liquid droplet and a solid surface with which the liquid droplet contacts is heated in a non-contact manner so that the liquid droplet moves in a desired direction. It may be a droplet moving method that locally reduces the wettability and increases the wettability of a part of the heated droplet. In this method, the solid surface may be ultrasonically vibrated.

According to the droplet moving device of the present invention, processing such as embedding a micro heater or an electrode pattern in advance on the wall surface is unnecessary, and the droplet can be moved in any direction without being limited to the micro heater or the like. it can.

Schematic configuration diagram of a droplet moving device according to the present embodiment Conceptual diagram when a droplet moves on a solid surface Schematic diagram showing the behavior of the contact line of a droplet passing through a defect on a solid surface Schematic diagram of the equipment used in the experiment Diagram showing change in contact angle with respect to vibration amplitude The figure which shows the contact angle history in experiment The figure which shows the result which measured the drop angle of the droplet for every vibration amplitude A photograph of a droplet that changes over time Diagram showing change in contact angle by laser irradiation Photograph of the moment when a droplet moves under critical conditions The figure which shows the measurement result of the movement speed of the droplet

Hereinafter, a droplet moving device according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram of a droplet moving device 100 according to the present embodiment. The droplet moving device 100 of this embodiment includes a sample flat plate 101 on which the droplet A is placed, an ultrasonic vibrator 102 that applies ultrasonic vibration to the sample flat plate 101, and a laser 103 that irradiates the droplet A with a laser beam. I have.

The surface of the sample flat plate 101 in contact with the droplet A is covered with a self-assembled monolayer 101A (Self Assembled Monolayers, hereinafter referred to as SAMS). The SAMS film 101A is an organic monomolecular film having a uniform molecular orientation formed by spontaneously gathering organic molecules. Already commercially available self-assembled monolayers can be used, and in this embodiment, a sample plate in which a self-assembled monolayer is applied to the Si wafer surface by a dipping method developed by Nippon Soda Co., Ltd. is used. .

The surface roughness of the SAMS film 101A is as smooth as about 1 nm and has an orderly orientation at the molecular level. Therefore, the contact angle history is several degrees, which is considerably smaller than a normal surface. Here, the contact angle history refers to a difference between contact angles appearing at both ends of the droplet (that is, an advancing contact angle and a receding contact angle).

The type of the SAMS film 101A is not particularly limited, and any monomolecular film may be used as long as the surface is very smooth and has a regular orientation at the molecular level.

The ultrasonic transducer 102 applies vibration with high frequency and minute amplitude to the SAMS film 101A. In this embodiment, a vibrator having an amplitude of several micrometers at several tens of kHz is used.

The laser 103 locally heats the droplet by irradiating the end of the droplet with a laser beam in a state of ultrasonic vibration. In this embodiment, a laser beam having a spot diameter of about 0.1 mm is irradiated on the inner side of the peripheral edge of the droplet from an angle of about 60 degrees with respect to the sample plate. The output of the laser 103 is not particularly limited, but can be changed, for example, in the range of 10 mW to 200 mW.

When the temperature dependence of the contact angle of the droplet is significant, the droplet movement by a mechanism different from the surface tension difference becomes possible due to the change in wettability at a high temperature.

Hereinafter, the principle of droplet movement when the apparatus of this embodiment is used will be described. FIG. 2 is a conceptual diagram when a droplet moves on a solid surface.

First, let us consider a case where a droplet attached to a horizontal solid surface receives an external force and moves in the x direction of FIG. From the consideration of the wetting behavior of the solid-gas-liquid three-phase contact line (C in FIG. 2) at the moment when the droplet moves, the resultant force D acting on the periphery of the droplet with the liquid surface tension σ is given by the following equation (1). Given

Here, θR and θA are the receding and advancing contact angles appearing on the contact line. Moreover, b is the maximum width of the droplet adhesion surface. Equation (1) holds for droplets having an arbitrary attachment surface shape. When an external force such as gravity or centrifugal force becomes larger than D in Expression (1), the droplet starts to move. From the above equation (1), it can be seen that the resistance can be reduced by reducing the contact angle history (θA−θR).

The contact angle history is said to appear due to the presence of defects such as roughness and chemical inhomogeneities existing on the solid surface. Regarding the mechanism of contact angle history, it has been pointed out that it is related to the amount of work required when contact lines pass through defects on the surface.

FIG. 3 schematically shows the behavior of a contact line having a macroscopic contact angle θR and passing through a defect on a solid surface. As a result of trapping part of the contact line in the locally existing defect portion, the resistance increases by the amount of energy corresponding to the strain formation.

The contact angle observed macroscopically is affected by the increase in energy, and an angle different from the so-called equilibrium contact angle appears depending on the moving direction of the contact line. Although a method for specifically obtaining the contact angle history itself has not yet been established, it is considered that the mechanism can be qualitatively explained by the model of FIG.

It is known that when a disturbance such as vibration is applied to a solid surface, the contact angle approaches an equilibrium contact angle that satisfies the minimum energy condition. According to the model shown in FIG. 3, when vibration is applied, the resistance when overcoming the defect is reduced.

As a result, an increase in energy is suppressed, and a reduction in contact angle history can be expected. In the present embodiment, a method for reducing the resistance of Equation (1) by applying a minute vertical vibration to a flat plate on which a droplet is placed is studied. According to the model of FIG. 3, it is a microscale defect that affects the contact angle history, and a vibration having a high frequency as much as possible is desired.

For example, the above equation (2) is considered as a representative value f (1 / s) of the liquid vibration frequency accompanying the surface tension. ρ represents density, and l represents length scale. Even if l to 1 mm is considered as a droplet scale, f> 100 Hz. When considering the scale of the defect, higher frequency vibration is expected to be effective.

The present invention moves droplets on a solid surface by a method combining the above-described solid surface vibration and laser irradiation. Since spot-like heating can be performed with a laser, droplet driving by a mechanism different from the moving method using the difference in surface tension is also possible.

That is, when the temperature dependence of the contact angle of the solid liquid is significant, it may be considered that the droplet is driven by a change in wettability at a high temperature. From equation (1), when the contact angle at the laser irradiation position is smaller than the receding contact angle at room temperature, the droplet moves toward the heating end.

In this embodiment, since the surface provided with the SAMS film is used, it is easily affected by heating. That is, the surface roughness of the SAMS film is as smooth as about 1 nm and has an orderly orientation at the molecular level, so the contact angle history is several degrees, which is considerably smaller than that of a normal surface.

For this reason, as can be seen from the equation (1), it can be expected that the external force required to drive the droplet is reduced. In addition, since the surface has an ordered structure, the influence of random thermal vibration caused by laser light on the molecular arrangement is great, and wetting can be expected to change significantly.

Next, an experiment in which a droplet is moved using the droplet moving device of this embodiment will be described. FIG. 4 shows a schematic diagram of the apparatus used in this experiment. A sample plate 101 is placed on the vibration surface of an ultrasonic vibrator 102 (Fuji Ceramic, FBL 28452HS) having a resonance frequency of 28 kHz, and an axisymmetric droplet having a predetermined volume is placed with a microsyringe.

Several types of vibration amplitudes were set by changing the voltage and current while matching the resonance frequency of the ultrasonic vibrator 102. The sample flat plate 101 was tilted while being oscillated, and an image at the moment when the droplet A fell was taken with a digital camera, and the advancing (retreating) contact angle of the droplet leading end (rear end) was measured.

The above-mentioned droplet image was taken into a PC, the shape near the wall surface was approximated by a polynomial, and the contact angle was obtained from the gradient. The contact angle was measured 6 times or more under the same experimental conditions, and the average value was adopted. The measurement accuracy of the contact angle in this experiment is about plus or minus 1 °.

In order to see the change of the resistance due to surface tension against the droplet movement of Equation (1) due to the addition of ultrasonic vibration, the tilt angle when the drop occurred was also measured. The measurement error of the falling angle is plus or minus 0. 3 degrees.

  A laser beam (Mellesgriot, Model 85-GHS-305) with a spot diameter of about 0.1 mm was irradiated inside the peripheral edge of the droplet from an angle of about 60 ° with respect to the sample plate while the wall surface was ultrasonically vibrated. The laser output W was varied from 10mW to 200mW, and the contact angle and droplet motion were observed.

As a sample flat plate, a solid surface obtained by applying a SAMS film to the surface of a Si wafer by a dipping method developed by Nippon Soda Co., Ltd. was used. Here, butyl carbitol acetate (hereinafter BCA), α-methylsilane dimer (hereinafter αMSD), and glycerin were used as test liquids.

In this experiment, 2500 ppm of Zudan III was added as a dye for αMSD and BCA for which the droplet movement experiment was performed in order to efficiently absorb the laser energy in the droplet. If the wavelength of the laser is changed, it is possible to absorb heat even for a colorless liquid. Table 1 shows the physical property values of the test liquids at the reference temperature (20 ° C.).

Here, the density was measured by Baume's hydrometer, and the surface tension was measured by Wilhelmy's suspension plate method using an electronic balance. The table also shows the measurement results of the rate of change in surface tension with temperature T (dσ / dT).

FIG. 5 shows the change of the contact angle of each test liquid with respect to the SAMs sample plate with respect to the vibration amplitude A. The volume of the droplet is 1 μL (≡1 mm ³ ). 5 (a) to 5 (c), each test liquid showed a tendency that the advancing contact angle θ _A decreased and the receding contact angle θ _R slightly decreased as the vibration amplitude increased. As a result, the contact angle history, which is the difference between the two, decreased with the vibration amplitude.

When a vibration with a large amplitude and a lower frequency is applied than in this experiment, the advancing contact angle decreases, the receding contact angle increases, and the difference between the two tends to decrease.

FIG. 6 shows the value of the contact angle history that is the difference between the forward and backward contact angles. The contact angle history (θ _A −θ _R ) decreased with amplitude, and gradually tended to approach a constant value. By applying ultrasonic vibration, the contact angle history of 3 to 4 ° in the stationary state is reduced to about 1 °, and as a result, the resistance of the equation (1) can be reduced.

Table 2 is the amplitude A = 3.2 .mu.m, contrast αMSD and BCA, show the measurement results of the contact angle when varying the droplet volume V _l. Regardless of the volume, almost the same effect of reducing the contact angle history was recognized.

FIG. 7 shows the result of measuring the drop angle of a 1 μL droplet for each vibration amplitude. In the figure, theoretical values for axisymmetric droplets are shown for comparison. The theoretical value of the falling angle was calculated from the following equation (3).

Here, φ is the inclination angle of the wall surface. When the plate is tilted after the axisymmetric droplet is placed on the horizontal plate, the droplet width is kept constant. In the above equation, φ was calculated by substituting the measured value of the droplet diameter on the horizontal plate for the droplet width b on the left side.

From FIG. 7, it can be seen that the measurement result can be roughly arranged by the equation (3). As a result of the small contact angle history, the value on the left side of Equation (3) changes sensitively to the contact angle measurement error. The difference between the theoretical value and the experimental value is mainly due to the measurement error of the contact angle.

From FIG. 7, the falling angle φ shows a tendency to decrease as the vibration amplitude increases. When the maximum amplitude in this experiment was given, the fall angle was reduced by about 15 ° from the stationary state. The vibrator used in this experiment reduced the force required to move a 1 μL droplet by about 80%.

A laser beam was irradiated to one end of the droplet in a state where vibration was applied. As a result, it was observed that the liquid droplet moved in the direction of the laser irradiation end above the output of a certain laser.

FIG. 8 is an example of a continuous photograph in which the state of droplets changing with time t (s) is photographed. The laser is applied to the right end of the axisymmetric droplet. In the initial state shown in FIG. 8A, the droplet is in contact with the advancing contact angle θ _A for each amplitude in FIG. As time elapses from the start of irradiation in FIG. 8A, the contact angle at the laser irradiation end gradually decreases. This is considered to be because the affinity of the solid-liquid was changed by the temperature rise by the laser, and the wettability was improved.

In the state shown in FIG. 8D, the contact angle θ ₁ at the irradiation end becomes considerably smaller than the contact angle θ _{2 at the} other end, and the droplet starts to move in the right direction by the action of the surface tension. That is, although the surface tension on the high temperature side is reduced, the contact angle is remarkably reduced. Therefore, the resultant force of the surface tension in the tangential direction of the wall surface acts on the right side, so that the droplet is considered to move toward the high temperature side.

Under the conditions in which the droplets moved as shown in FIG. 8, the time change of the contact angle after laser irradiation was measured. A measurement example for αMSD is shown in FIG. The dotted line in the figure represents the time when the droplet started to move. Even under different conditions such as FIGS. 9 (a) and 9 (b), the contact angle behaved similarly.

That is, the contact angle θ _{1 at} the irradiation position decreases rapidly after laser irradiation. Thereafter, the degree of decrease becomes gradual and shows a tendency to settle to a constant value. After a time of about 1 second has elapsed, the droplet starts to move. The time required for movement is shorter in FIG. 9A where the laser output is large. Due to the rapid heating in the vicinity of the contact line at the initial stage of irradiation, θ ₁ decreases rapidly.

Since the droplet is initially placed at an advancing contact angle θ _A at 20 ° C., the droplet is pulled to the right as θ ₁ decreases, resulting in θ _{2 at} the other end decreasing from θ _A , and back contact closer to the corner θ _R. Because of the deformation until the contact angle reaches θ _R , it is considered that it takes some time before the liquid droplet starts to move.

The critical condition that the droplet can move was investigated. Here, when the laser was irradiated for 1 minute and it was confirmed that the droplet moved at a speed of 0.05 mm / min or more, the critical condition was considered.

Table 3 shows the results of measuring the critical laser power at which a 1 μL droplet starts moving for various vibration amplitudes. The critical output value decreased with increasing amplitude, and showed a tendency to settle to a constant value. This is the same tendency as the contact angle history (FIG. 6) and the inclination angle (FIG. 7) of the flat plate on which the droplet falls, which is considered as a result of the effect of reducing the movement resistance due to vibration.

FIG. 10 is an example of a photograph at the moment when a droplet moves under a critical condition. At this time, it is considered that the surface tension acting on both ends approximately satisfies the following equation.

Here, σ ₁ and σ ₂ indicate the surface tension on the laser irradiation side and the other end, respectively. At the start of movement in FIG. 10, θ ₁ = 48.4 ° and θ ₂ = 52.6 °. When the surface tension when the formula (4) is established is estimated, it is estimated that there is a temperature difference of about 40 ° C. at both ends of the droplet.

FIG. 11 shows the result of measuring the moving speed of the droplet at each laser output when a vibration with the maximum amplitude A = 4.3 μm in this experiment was given. The moving speed increases linearly with respect to the output. Under this experimental condition, a maximum moving speed of 0.6 mm / s was obtained.

As described above, high-frequency vibration (28 kHz) is applied to the wall surface by an ultrasonic transducer to reduce the size of the contact angle history between the droplet and the solid surface, and then experiments are conducted on the method of moving the droplet by laser irradiation. It was.

According to this experiment, when ultrasonic vibration was applied to a SAMs plate with a small contact angle history, the effect of reducing the contact angle history by several degrees was recognized for all three liquids. At the maximum amplitude of 4.3 μm in this experiment, the drop angle of a 1 μL droplet decreased by about 15 °, and the resistance when the droplet moved could be reduced by about 80%.

When laser irradiation was performed with ultrasonic vibration applied, a phenomenon was observed in which the contact angle at the irradiation end was significantly reduced by heating. As a result, it was observed that the resultant surface tension acting on the droplet appeared and the droplet moved in the direction of the laser irradiation end. In the range of this experiment, the droplet could be moved at a maximum speed of 0.6 mm / s.

DESCRIPTION OF SYMBOLS 100 Droplet moving apparatus 101 Sample flat plate 101A SAMS film | membrane 102 Ultrasonic vibrator 103 Laser A Droplet

Claims (9)

A droplet moving apparatus for moving a droplet wetted on a solid surface,
A solid surface in contact with the droplet;
A droplet moving device comprising heating means for locally heating a part of a droplet in a non-contact manner, which improves the wettability between the droplet and the solid surface. The droplet moving apparatus according to claim 1, wherein the heating means locally heats the solid surface partially in non-contact. The droplet moving apparatus according to claim 1, wherein the heating unit is a laser irradiation unit that irradiates a laser beam. The droplet moving device according to claim 1, further comprising a vibrating unit that vibrates the solid surface. The droplet moving device according to claim 1, wherein the vibration means ultrasonically vibrates the solid surface. The droplet moving device according to claim 1, wherein the solid surface is formed of a self-assembled monomolecular film. A droplet movement method for moving a droplet on a solid surface,
A part of the liquid droplet is heated in a non-contact manner so that the liquid droplet moves in a desired direction to locally reduce the contact angle between the liquid droplet and the solid surface with which the liquid droplet contacts, A droplet movement method for increasing the wettability of a part of a droplet. 8. A part of the solid surface is heated in a non-contact manner to locally reduce the contact angle between the droplet and the solid surface with which the droplet contacts, thereby increasing the wettability of the solid surface and the droplet. Droplet movement method. 9. The droplet moving method according to claim 7, wherein the solid surface is vibrated ultrasonically.