dc.description.abstract | An understanding of the transport behavior of a liquid droplet controlled and manipulated by the thermal gradient is very important for the development of microfluidic applications. In this study, a numerical computation is utilized to investigate the thermocapillary actuation behavior of a liquid with two different physical problems: microchannel and capillary tube. The finite element method with the two-phase level set technique, developed by Comsol Multiphysics, is used to solve the incompressible Navier-Stokes equations coupled with the energy equation. The conservative level set method, the arbitrary Lagrangian Eulerian (ALE), and the continuum surface force (CSF) method are used to track the liquid/gas interface and ensure good resolution near the free interface. Two forces are considered at the liquid/gas interface such as the capillary force acting in the normal direction, and the thermocapillary force acting in the tangential direction to the free surface.
For modeling of a liquid droplet in microchannel, the lower wall of the microchannel is subjected to a uniform temperature gradient, while the upper one is adiabatic, isothermal or heated wall. When the upper wall is set to be adiabatic, a pair of asymmetric thermocapillary convection vortices initially occurs inside the droplet but these turn into a sole thermocapillary vortex once enough time has passed. For the isothermal case, a pair of asymmetric thermocapillary convection vortices always appears inside the droplet. For the case of the upper wall temperature higher than the bottom one, the net thermocapillary momentum generated by two pairs of thermocapillary vortices assists the droplet migration during the initial stage. When time reaches a certain value, it turns to go against the droplet migration. The droplet initially accelerates for all cases. The droplet velocity then decreases dramatically for the adiabatic case while it decreases slowly for the isothermal one. For the heated upper wall case, the droplet velocity decelerates to zero velocity after it gets the maximal value. The actuation velocity of the droplet is affected by temperature gradients, contact angles and microchannel heights for adiabatic, isothermal or heated wall cases.
For a silicone plug migration inside a capillary tube, flow motion is affected by the thermocapillary effect generated by the temperature gradient along the gas-liquid interface near the receding side and the capillary force caused by the temperature difference between the ends of the liquid plug. When time is long enough, the flow mainly moves horizontally from the hot side to the cold side near the tube wall and then returns to the hot side near the center of the tube due to the capillary force effect. There is a smaller clockwise circulation near the receding contact angle caused by the thermocapillary convection. The flow motion causes significant distortion of the isotherms inside the silicone plug. The temperature gradient along the tube is enhanced by the flow motion inside the capillary tube. The liquid plug accelerates rapidly in the initial stage and then decelerates after it reaches the maximum speed. During the migration process, the receding contact angle is always greater than the advancing one. An increase in the input heat flux leads to a higher migration velocity due to the higher temperature gradient along the tube wall. When the initial contact angle is smaller, the migration velocity moves faster due to the higher capillary force. A liquid plug with a lower viscosity moves faster owing to the lower viscous force. The numerical simulation results are in good agreement with the results from previous experiments. | en_US |