Theoretical and experimental investigation of thermocapillary actuation for microplugs

This paper reports the results of theoretical and experimental investigations of reciprocating thermocapillary motion of a liquid plug in microchannels. A one-dimensional analytical model for the transport of micro plugs in a capillary was established. The model consists of a system of two transient one-dimensional equations: one for temperature spreading in the capillary wall and one for the dynamics of surface tension driven movement of the plug. Surface tension depends strongly on temperature. Thus, a transient temperature distribution leads to a gradient of surface stress across a liquid plug. This surface stress difference leads to the movement of the liquid plug. For the experimental investigation two heaters were used for the periodic temperature gradient. Each of the heaters was activated alternatively to induce the reciprocating motion of the liquid plug. For quantitative evaluation, the position of the plugs was captured and evaluated with a CCD camera. This paper focuses on analysing the results of this motion at different switching frequencies. The results show that the motion of the plug exhibits a chaotic characteristics at high switching frequencies. This actuation concept has potential applications in post-processing stages for droplet-based microfluidics. The chaotic motion can be explored for efficient mixing in microplugs.