Microscale fluid flow induced by thermoviscous expansion along a traveling wave
Franz M. Weinert, Jonas A. Kraus, Thomas Franosch, and Dieter Braun
Recently, flow at the scale of millimeters and below has attracted significant attention, stimulated by the rapid advances to manipulate and to control small-scale devices. In our Letter we propose a novel mechanism to generate net flow in a thin fluid chamber, i.e. a viscous liquid confined between two plates separated by a distance of the order of a few micrometers.
The driving of the fluid flow is provided by imposing a traveling temperature wave. We show analytically within a thin-film approximation that such unsteady heating leads to net fluid flow. Then we corroborate our analytic approach by a finite-element calculation. Last we provide first experimental evidence that there is indeed net flow and that the fundamental dependences have been correctly identified.
The basic mechanism may be summarized as follows: Due to thermal expansion of the liquid the motion of the heating results in a pressure modulation. The pressure gradients induce a potential flow which, however, does not lead to net fluid flow. Yet, the temperature dependence of the shear viscosity gives rise to a small net mass transport typically opposite to the motion of the heat source.