Thermocapillary Actuation of Millimeter-Scale Rotary Structures

In this paper, we describe the rotary motion, by Marangoni flow, of a millimeter-scale rotary structure immersed in a thin layer of liquid. A 16 × 8 array of 1 × 0.8 × 0.3 mm³ surface-mount resistors is suspended ≈ 500 μm above the liquid to serve as a programmable heat source. The continuous operation of resistor elements is used to impose a spatially-defined temperature gradient on the surface of the liquid. With a maximum temperature gradient of 36.6 K/mm at the surface of a 2 mm-thick film of liquid with viscosity 5 cSt, a stainless steel rotary structure with a weight of ≈ 10 mg, a diameter of 4.1 mm, and blade angle of 34° takes 28 s to make a 360° rotation. In general, the angular velocity of the rotary structure is affected by the temperature gradient of the liquid surface and liquid viscosity among several factors.