High speed microfluidic doublet flow in open pools driven by non-contact micromachined thermal sources

We report a phenomenon in which a micromachined heat source placed less than 50 μm above the surface of a liquid drives a high-speed doublet flow pattern with linear velocities reaching nearly 5 mm/sec and rotational velocities up to 1200 rpm. Tests were performed on a 50-100 μm-thick layer of water containing 3 μm polystyrene beads for flow visualization. The thermal source is a polyimide cantilever with an integrated heater near the tip, operated with input powers ranging from 0-32 mW. It has no moving parts and does not contact the liquid. The speed of the doublet flow scales with input power as well as liquid temperature, and is inversely related to the air gap between the heater and liquid surface. The orientation of the doublet flow can be reversed by changing the angle of the cantilever. A one-dimensional array of probes used in the same manner generates a linear flow pattern.