Abstract
The migration of a small droplet has been developed during the last two decades due to its applications in industry and high technology such as MEMS and NEMS devices, Lap-On-a- chip, transportation of fluids and so on. There have many studies on this topic in which the energy source as a driving force for the moving of a droplet is quite a difference like heating, magnetics, pressure, electric, laser, and so on. In this study, the numerical computation is used to investigate the transient thermocapillary migration of a water droplet in a micro-channel under the effect of heating source. For tracking the evolution of the free interface between two immiscible fluids, we employed the finite element method with the two-phase level set technique to solve the Navier-Stokes equations and continuity equation coupled with the energy equation. Both the upper wall and the bottom wall of the microchannel are set to be ambient temperature. 40mW heat source is placed at a distance of 1 mm from the initial position of a water droplet. When the heat source is turned on, a pair of asymmetric thermocapillary convection vortices is formed inside the droplet, and the thermocapillary on the receding side is smaller than that on the advancing side. The temperature gradient inside the droplet increases quickly at the initial times and then decreases versus time. Therefore, the actuation velocity of the water droplet first increases significantly and then decreases continuously. Furthermore, the results also indicate that the dynamic contact angle is strongly affected by the oil flow motion and the net thermocapillary momentum inside the droplet. The advancing contact angle is always larger than the receding contact angle during the actuation process.
Highlights
Microfluidics technique has significantly attracted owing to its diverse applications in Lab-on-a Chip devices (LOC), Micro-Electro-Mechanical System (MEMS) or protein crystallization [1,2,3]
Brochard 6 indicated that the contact angle of a liquid droplet at rest, static contact angle (SCA), is altered to the dynamic contact angle (DCA) when the droplet moves on a solid surface
The results showed the actuation velocity and the DCA of the droplet are strongly affected by the thermal condition of upper wall
Summary
Microfluidics technique has significantly attracted owing to its diverse applications in Lab-on-a Chip devices (LOC), Micro-Electro-Mechanical System (MEMS) or protein crystallization [1,2,3]. The droplet transport behavior in a microchannel actuated by a transient temperature gradient has already been investigated in numerous studies [6,7,8,9,10,11,12,13,14]. Brochard 6 indicated that the contact angle of a liquid droplet at rest, static contact angle (SCA), is altered to the dynamic contact angle (DCA) when the droplet moves on a solid surface. The difference in the DCA between the advancing and receding sides, so-called contact angle hysteresis (CAH), is strongly affected by the temperature gradient. The experimental results of Chen et al 7, developed from Ford and Nadim’s work 8, indicated that a fixed CAH influences the droplet velocity and threshold values much more significantly than the slip length
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