Novel liquid drop self-driving method based on Quincke rotation and wall surface effect

Abstract

The invention discloses a novel liquid drop self-driving method based on Quincke rotation and a wall surface effect, and belongs to the field of microfluid machinery. The method comprises the following steps: placing a weak conductive liquid drop on a hydrophobic solid wall surface, and applying a uniform electric field; then, the liquid drops generate Quincke rotation under the action of the electric field, and the rotation effect can cause the liquid drops to be separated from and suspend on the wall surface; and finally, viscous stress caused by a liquid pad between the bottom of the liquid drop and the wall surface acts on the bottom of the liquid drop, so that the upper part and the lower part of the liquid drop generate a speed difference, and additional translational motion, namely liquid drop self-driving, can be generated due to the existence of the speed difference for rotating the liquid drop. According to the method, the complexity of a self-driven platform and the instability of liquid drop movement in an existing method are overcome.

Description

Technical Field

[0001] The invention relates to a liquid drop self-driving method based on Quincke rotation and wall effect, and belongs to the field of microfluidic machinery. Background Art

[0002] Droplet self-propulsion is the core technology of high-tech industries such as microfluidic machinery, chips, and drug delivery. Existing technologies mainly use the physical properties of the droplet interface to produce asymmetric force distribution, thereby achieving directional movement of droplets. The implementation methods are mainly divided into the following categories: 1. Based on the unbalanced capillary stress on both sides of the droplet: for example, wedge-shaped grooves are opened on the super-hydrophobic wall, coatings with different hydrophobic properties are sprayed on the wall at intervals along the specified direction of movement, etc. to generate asymmetric capillary stress on both sides of the droplet. Although this method can realize self-driving of droplets in mechanism, from the application point of view, especially in the microfluidic machinery and chip industries, the design of super-hydrophobic walls and micro-conical grooves increases the cost and process difficulty; 2. Based on the asymmetric interface Marangoni stress: asymmetric Marangoni stress is generated on both sides of the droplet through a non-uniform temperature field or surfactant to achieve directional migration of droplets, but this method will bring about mass loss and droplet contamination; 3. Based on the dynamic characteristics of droplets in the electric field: using alternating electric fields or non-uniform electric fields to induce unstable and asymmetric electric stress on the droplet surface to drive the droplet to move. This method requires a complex control platform. Although great progress has been made in self-propulsion technology, there are still many challenges, such as mass loss (such as evaporating droplets), droplet contamination (such as surfactants), low translation speed, manufacturing cost (such as functionalized substrate forging) and complex operating platform. Therefore, in order to solve the existing problems, new droplet self-propulsion methods still need to be explored. Summary of the invention

[0003] In view of the problems of existing droplet self-driving platforms, low translation speed, and mass loss, the present invention proposes a new droplet self-driving method based on Quincke rotation and wall effect. The control platform of this method is simple (only an electrostatic field is required), and the droplets can be remotely operated to avoid droplet contamination and mass loss.

[0004] The present invention is implemented by the following technical solutions:

[0005] 1. Place a weakly conductive droplet on a super-hydrophobic solid surface and apply a uniform electric field.

[0006] Second, the droplet produces Quincke rotation under the action of the electric field. This rotation effect causes the droplet to detach and suspend on the wall.

[0007] 3. The viscous stress induced by the liquid cushion between the bottom of the droplet and the wall acts on the bottom of the droplet, causing a speed difference between the upper and lower parts of the droplet. For the rotating droplet, the existence of this speed difference will produce additional translational motion, namely, droplet self-drive.

[0008] According to the new method of droplet self-driving based on Quincke rotation and wall effect according to claim 1, it is characterized in that step 1 specifically comprises:

[0009] Two conditions must be met to activate Quincke rotation: i) the droplet and the surrounding fluid must satisfy te,i>te,o, where te = (ε / σ, ε is the dielectric constant; σ is the conductivity) is the time required for the internal charge of the fluid to move to the droplet interface under the action of the electric field, that is, the charge relaxation time; ii) the applied electric field must be greater than the critical field strength EQ that activates the droplet Quincke rotation:

[0010]

[0011] Where μ is the dynamic viscosity; S and R are the dielectric constant ratio and conductivity ratio of the droplet to the surrounding fluid, respectively.

[0012] According to the new method of droplet self-driving based on Quincke rotation and wall effect according to claim 1, it is characterized in that step 2 specifically comprises:

[0013] Based on the process described in step 1, the Quincke rotation effect is activated. The droplets adhering to the wall are deflected by the Quincke rotation effect and eventually detach from the wall. The suspended rotating droplets drive the surrounding fluid to flow, forming a flowing liquid cushion between the bottom of the droplets and the wall. The liquid cushion can prevent the droplets from adhering to the wall again.

[0014] According to the new method of droplet self-driving based on Quincke rotation and wall effect according to claim 1, it is characterized in that step three specifically comprises:

[0015] The liquid cushion existing between the stationary wall and the bottom of the rotating droplet will produce a viscous stress acting on the bottom of the droplet (~μu / h, h is the thickness of the liquid cushion; u is the characteristic velocity). This local stress causes a velocity difference between the upper and lower parts of the droplet. For a rotating droplet, the uneven distribution of the velocity magnitudes of the upper and lower parts will produce additional translational motion - that is, the droplet self-drive. It is worth noting that a thin liquid cushion can lead to a large velocity difference, resulting in rapid translation. Therefore, different self-drive performance can be obtained by adjusting h.

[0016] The present invention couples the Quincke rotation of droplets induced by electrostatic fields and the wall adhesion effect to obtain a new method for droplet self-driving. The Quincke rotation effect causes the droplets to detach from the wall, and the suspended rotating droplets drive the surrounding fluid to flow, forming a flowing liquid cushion between the bottom of the droplet and the wall. The liquid cushion can prevent the droplet from adhering to the wall again; in addition, the presence of the liquid cushion will produce viscous stress acting on the bottom of the droplet, which causes a speed difference between the upper and lower parts of the droplet. For the rotating droplet, the existence of the speed difference can produce horizontal movement, achieving the purpose of droplet self-driving along the wall. In addition, by increasing the electric field strength or the viscosity of the droplet, the thickness of the liquid cushion (h) can be reduced, thereby improving the self-driving performance of the droplet. The invention has a simple operating platform, no self-driving distance limitation, and can remotely operate droplets without contact, which is beneficial to the development of efficient and low-cost droplet self-driving devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The interfacial charge distribution of a weakly conductive liquid droplet placed on a solid wall under the action of an electric field;

[0018] Figure 2 Dynamic behavior of a droplet under the Quincke rotation effect: (a) deflection; (b) detachment; (c) translation

[0019] Figure 3 The interface velocity distribution of a Quincke spinning droplet during self-propulsion, with the inset detailing the mechanism of the transformation of rotational motion into rolling.

[0020] Figure 4 Average self-propelled velocity of the droplet |U| and thickness of the liquid cushion under different electric field strengths;

[0021] Symbols in the figure: a-droplet radius; V-applied potential; E0*-electric field strength (superscript "*" indicates a dimensionless variable); λ-viscosity ratio of droplet to surrounding fluid; Δh*-liquid cushion thickness; |U*|ave-absolute average translational velocity; DETAILED DESCRIPTION

[0022] The present invention will be further described below in conjunction with the accompanying drawings.

[0023] The new method of droplet self-propulsion of the present invention is mainly achieved through three steps:

[0024] 1. Place a weakly conductive droplet on a super-hydrophobic solid surface and apply a uniform electric field. Two conditions must be met to induce Quincke rotation: i) The droplet and the surrounding fluid must satisfy te,i>te,o (the subscripts i and o represent the droplet and the surrounding fluid, respectively), where te = (ε / σ, ε is the dielectric constant; σ is the conductivity) is the time required for the internal charge of the fluid to move to the droplet interface under the action of the electric field, that is, the charge relaxation time. Figure 1The charge distribution at the droplet interface under this condition is plotted; ii) The applied electric field must be greater than the critical field strength EQ that activates the droplet Quincke rotation:

[0025]

[0026] Where μ is the dynamic viscosity; S and R are the dielectric constant ratio and conductivity ratio of the droplet to the surrounding fluid, respectively.

[0027] Second, under the action of the electric field, the Quincke rotation effect of the droplet is activated. Figure 2 As shown in the figure, the droplet adhering to the wall is first deflected by the wall ( Figure 2 a), and finally break away from the wall ( Figure 2 b). The suspended rotating droplets drive the surrounding fluid to flow, forming a flowing liquid cushion between the bottom of the droplet and the wall ( Figure 2 c). The liquid cushion can prevent the droplet from adhering to the wall again.

[0028] 3. The liquid cushion existing between the stationary wall and the bottom of the rotating droplet generates a viscous stress acting on the bottom of the droplet (~μu / h, h is the thickness of the liquid cushion; u is the characteristic velocity). Figure 3 The interface velocity distribution during droplet self-propulsion shows that this local stress causes a velocity difference between the upper and lower parts of the droplet. For a rotating droplet, the asymmetry of the velocity between the upper and lower parts induces translational motion - that is, droplet self-propulsion (see Figure 3 (a) Inset). It is worth noting that a thin fluid cushion can lead to a large velocity difference, resulting in a fast translation. Therefore, different self-propulsion performances can be obtained by adjusting h. Figure 4 a shows the numerical experimental data of the average self-driving velocity of the droplet |U*ave| and the thickness of the liquid cushion under different viscosity ratios. This result shows that the increase of the viscosity of the droplet can reduce the thickness of the liquid cushion and thus improve the self-driving performance. In addition, increasing the electric field strength can also achieve the same effect.

Claims

1. A new method for droplet self-propelling based on Quincke rotation and wall effect, characterized in that: The steps include:

1. Place a weakly conductive droplet on a super-hydrophobic solid surface and apply a uniform electric field. Second, the droplet produces Quincke rotation under the action of the electric field. This rotation effect causes the droplet to detach and suspend on the wall.

3. The viscous stress induced by the liquid cushion between the bottom of the droplet and the wall acts on the bottom of the droplet, causing a speed difference between the upper and lower parts of the droplet. For the rotating droplet, the existence of this speed difference will produce additional translational motion, namely, droplet self-drive.

2. The new droplet self-driving method based on Quincke rotation and wall effect according to claim 1, characterized in that: Step 1 is as follows: To induce Quincke rotation, two conditions must be met: i) the droplet and its surrounding fluid must satisfy te,i>te,o (where the subscripts i and o represent the droplet and the surrounding fluid, respectively), te = (ε / σ, ε is the dielectric constant; σ is the conductivity) is the time required for the internal charge of the fluid to move to the droplet interface under the action of the electric field, that is, the charge relaxation time; ii) the applied electric field must be greater than the critical field strength EQ that activates the droplet Quincke rotation: Where μ is the dynamic viscosity; S and R are the dielectric constant ratio and conductivity ratio of the droplet to the surrounding fluid, respectively.

3. The new droplet self-driving method based on Quincke rotation and wall effect according to claim 1, characterized in that: Step 2 is as follows: Based on what was described in step 1, the Quincke rotation effect is activated. The droplets adhering to the wall are affected and deviate, and eventually detach from the wall. The suspended rotating droplets drive the surrounding fluid to flow, forming a flowing liquid cushion between the bottom of the droplet and the wall. This liquid cushion can prevent the droplet from adhering to the wall again.

4. The new droplet self-driving method based on Quincke rotation and wall effect according to claim 1, characterized in that: Step three is as follows: The liquid cushion existing between the stationary wall and the bottom of the rotating droplet will generate and act on the bottom of the droplet. Viscous stress (~μu / h, h is the thickness of the liquid cushion; u is the characteristic velocity). This local stress causes a velocity difference between the upper and lower parts of the droplet. For a rotating droplet, the uneven distribution of the velocity magnitudes of the upper and lower parts will produce additional translational motion - that is, the droplet self-drive. It is worth noting that a thin liquid cushion can lead to a large velocity difference, resulting in rapid translation. Therefore, the self-drive performance can be enhanced by adjusting h.

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