RU2294541C1 - Method for determining speed of thermo-capillary flow near side surface of washer-shaped vial - Google Patents

Abstract

FIELD: possible use for diagnostics of flow of liquids in the channel of micro-fluid pump or optical switch.

SUBSTANCE: in accordance to method, thermo-capillary flow is excited by means of a light beam. Static curvature and total curvature of visually observed side surface of washer-shaped vial are measured, deformation of which surface is caused by thermo-capillary flow. On basis of results of measurement, speed of the flow is determined.

EFFECT: simplified diagnostics of current in micro-scale.

5 dwg

Description

The invention relates to the field of non-contact methods for diagnosing the flow of liquids at the microscale and can be used to determine the flow velocity at the surface of a bubble moving in the channel of a microfluidic pump or optical switch [1-2].

There is a method [3] for measuring the flow rate of a liquid, which consists in the following: a liquid stream is seeded with tracer particles labeled with a fluorescent dye, then the cross section of the stream is irradiated with successive pulses of laser light focused by a cylindrical lens into the sheet and, simultaneously with the supply of pulses, perform image capture. Further, on the basis of these images, using a computer program, the position of the selected particle is recorded on a sequence of frames and its displacement is measured for the period between two pulses, then the speed is calculated.

However, in microfluidics this method has several disadvantages. The choice of tracer particles is critical for each case studied. On the one hand, the particles must be small enough to track the streamlines and not block the flow, and on the other hand, large enough to damp the Brownian motion, which introduces an error in the velocity measurements. In addition, a complex data processing algorithm requires a specialized computer program.

When diagnosing the flow induced by small (up to 10 ° C) local thermal disturbances (for example, convection at the microscale caused by the thermal action of the light beam [1, 4]), the application of this method imposes restrictions on the choice of the wavelength of the inducing radiation, which should not heat the tracer particles and interfere with them.

The aim of the present invention is to simplify the method for determining the velocity of thermocapillary flow at the surface of the washer-like bubble.

The goal is achieved by measuring the static curvature and the total curvature of the visually observed lateral surface of the bubble, the deformation of which is caused by the thermocapillary flow of the induced light beam. In this case, the total curvature of the observed visually deformed surface of the bubble is found according to the principle of superposition of curvatures [5, 6].

A detailed mechanism for the formation of total curvature (Figure 1) and the derivation of the expression for the speed are as follows. The lateral surface of the washer bubble 1, sandwiched between two transparent for radiation plates 2 has a constant static curvature χ _S , Figure 1 (a). At the moment when the absorbing liquid 3 is adjacent to the lateral surface of the bubble 1 and is heated by the light beam 4, Fig. 1 (b), the surface tension on the surface of the bubble decreases, and a field of thermocapillary forces 5 arises, causing the fluid to be carried away from the irradiation zone. As a result, the lateral surface of the bubble acquires a dynamic curvature χ _d , which is folded according to the superposition principle [5, 6] with the curvature χ _S before irradiation. As a result, the visually observed deformation 6 of the lateral surface of the bubble 1 has a total curvature [6]

Dynamic curvature χ _d creates excessive capillary pressure in the bubble p _σ = _σ · χ _d and return fluid flows to the dynamic pressure p _i = ρu ^2/2, generating two coherent vortex 7 in a liquid [4]. Here u is the desired velocity of the thermocapillary flow near the surface of the washer bubble, which, from the condition of the balance of these pressures and the superposition principle (1), has the form

fluid density ρ and its surface tension coefficient σ are tabulated values.

Further, the velocity u of the thermocapillary flow is found by measuring the total curvature χ of the side surface of the bubble and its static curvature χ _{S of} Fig. 1 (a).

Figure 2 shows a diagram for measuring the curvature χ. Using a photograph of a deformed bubble obtained by photographing or filming, and assuming that the deformed surface 6 is a section of a certain circle 8, this circle is completed in any graphic editor and its radius of curvature R = χ ^{-1 is} measured. Knowing the static curvature of the bubble χ _S = R _S ^-1 , the velocity of thermocapillary flow u is calculated by formula (2).

Figure 3 shows the frames of the bubbles: (a) - the light beam is projected into the wetting film under the bubble and does not have a thermal effect on the side surface; (b) - a light beam heats the near-surface region of the bubble, which causes a thermocapillary flow that deforms the lateral surface of the bubble. Here, the curvature measurement was carried out in the MSWord editor, where these frames were inserted and the circle was completed using the WordArt tool and its curvature was measured.

Example. The table shows the average values of the velocity of thermocapillary flow in different liquids, excited by a light beam at the surface of the bubbles, obtained by the proposed method.

Acetone Ethanol Butanol 10% ethanol-water mixture u, mm / s 250 160 180 142

In the experiments, we used bubbles with R _S from 0.4 to 0.6 mm, clamped in a cell with a gap of 50 μm, filled with colored liquids, which were irradiated with focused radiation of a mercury lamp. The absorbed power of the liquids was the same and equal to 30 mW.

The difference between the values obtained by the proposed method and estimated using tracer particles is less than 15%.

Thus, the proposed method, characterized by simplicity, has several advantages: it does not require tracer particles, additional probing beams and optics for them, and also does not require software for calculating the speed.

The method can be used to determine the velocity of thermocapillary flows caused not only by the local action of a light beam on the surface of the bubble, but also due to local heating of this surface by resistive heaters. In this case, it is only necessary to illuminate the bubble with diffused light.

LITERATURE

1. Sato M., Horie M., Kitano N. et. al. Thermocapillary optical switch. // Hitachi Cable Review. 2001. No.20. P.19-24.

2. Jun T.K., Kim C.-J. Valveless pumping using traversing vapor bubbles in microchannels. // J. Applied Physics. 1998. Vol. 83. No. 11. P.5658-5664.

3. URL: http://www.lavison.de/download/educational/pivintroduction.pdf

4. Bezugly B.A., Ivanova N.A. Manipulating a gas bubble in a Hele-Shaw cell using a beam of light. // Letters to the ZhTF. 2002. Vol. 28. Issue 19. S.71-75.

5. Bezugly B.A., Tarasov O.A., Fedorets A.A. A modified method of an inclined plate for measuring the wetting angle. // Colloidal journal. 2001. No.6. S.735-741.

6. Tarasov O.A. Additivity of the dynamic curvature of the thermocapillary recess and the static curvature of the wetting meniscus. // Colloidal journal. 2005.V. 67. No. 2. S.1-9.

Claims (1)

A method for determining the velocity of a thermocapillary flow near the lateral surface of a washer bubble, wherein the thermocapillary flow is excited by a light beam, characterized in that the static curvature and the total curvature of the visually observed lateral surface of the bubble are measured, the deformation of which is caused by the thermocapillary flow, and the speed of this flow is found from the measurement results.

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