Numerical Study of a Liquid Droplet Movement in a Microchannel under Laser Heat Source
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Abstract
In this study, the numerical methods are used to simulate the liquid droplet migration in a microchannel. A 40 mW heat source is placed at the distance of 1 mm from the initial position of a water droplet. To determine the exact position of a liquid droplet in a microchannel and clearly observe the surface tension of a droplet during the actuation process, we employed the finite element method with the two-phase level set technique. Both the upper wall and the bottom wall of the microchannel are set to be an ambient temperature. When the heat source is turned on, a pair of asymmetric thermocapillary convection vortices is formed inside the droplet. The thermocapillary force caused by the temperature gradient inside the droplet and the capillary force caused by the pressure gradient drive the droplet to move in a microchannel. The numerical results show the temperature gradient inside a microchannel due to laser heat source affects the behavior of a droplet movement. The actuation velocity of the water droplet increases significantly, and then decreases continuously. The dynamic contact angle is strongly affected by the oil flow motion and the thermo capillary momentum inside the droplet. The advancing contact angle is larger than the receding contact angle during the actuation process.
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References
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[4] F. Brochard; Motions of droplets on solid surfaces induced by chemical or thermal gradients; Langmuir 5 (1989) 432-438.
[5] N. Anantharaju, M.V. Panchangula, S. Vedantam; Three-phase contact line topology on dynamic contact angle on heterogeneous surface; Langmuir 23 (2007) 11673-11676.
[6] M.L. Ford, A. Nadim; Thermocapillary migration of an attached drop on a solid surface; Phys. Fluids 6 (1994) 3183-3185.
[7] T.-L. Le, J.-C. Chen, B.-C. Shen, F.-S. Hwu and H.-B. Nguyen; Numerical investigation of the thermocapillary actuation behavior of a droplet in a microchannel; Int. J. Heat Mass Transfer 83 (2015) 721-730.
[8] T.-L. Le, J.-C. Chen, F.-S. Hwu and H.-B. Nguyen; Numerical study of the migration of a silicone plug inside a capillary tube subjected to an unsteady wall temperature gradient; Int. J. Heat Mass Transfer 97 (2016) 439-449.
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[10] M.R.S. Vincent, R. Wunenburger and J.P. Delville; Laser switching and sorting for high speed digital microfluidics; Applied Physics Letters 92 (2008) 154105.
[11] S. Ramos and A. Tanguy; Pinning-depinning of the contact line on nanorough surfaces; Eur. Phys. J. E 19 (2006) 433-440.
[12] J.U. Brackbill, D.B. Kothe and C. Zemach; A continuum method for modeling surface tension; J. Comput. Phys. 100 (1991) 335-354.
[13] J.-C. Chen, C.-W. Kuo and G.P. Neitzel; Numerical simulation of thermocapillary nonwetting; Int. J. Heat Mass Transfer 49 (2006) 4567-4574.
[14] P. Tabeling; Investigating slippage, droplet breakup, and synthesizing microcapsules in microfluidic system; Phys. Fluids 22 (2010) 021302.
[15] J. Koplik, J.R. Banavar and J.F. Willemsen; Molecular dynamics of fluid flow at solid surfaces; Phys. Fluids A 1 (1989) 781-794.
[16] E. Olsson and G. Kreiss; A conservative level set method for two phase flow; J. Comput. Phys. 210 (2005) 225-246.
[17] E. Olsson, G. Kreiss and S. Zahedi; A conservative level set method for two phase flow II; J. Comput. Phys. 225 (2007) 785-807.