Applications of Marangoni Forces in Actuating Solid Phase Objects.

Abstract

The Marangoni effect develops due to surface tension variations at the liquid/gas interface caused by temperature gradient in the liquid. Spatially localized temperature rise reduces the localized surface tension, resulting in surface flows away from the heat source and subsurface flows in opposite direction. This phenomenon shows potential in droplet/particle manipulation for microfluidic applications. In this work, a series of experiments is performed to address several important questions to further evaluate the utility of this effect. The questions address how to spatially localize suspension particles on blank substrates, sort floating particles according to size, and actuate millimeter-sized solid objects.

Using a heater array suspended about 500 micrometer above the liquid, Marangoni flows are shown to spatially localize sedimentations of microscale suspended particles. The sedimentation patterns and accumulation levels depend on the temperature gradient at the liquid surface, number of active heaters and type of liquid used. For example, a single active heater is used to generate a temperature elevation of 6.9 K at the surface of silicone oil DC-704, resulting in the localized sedimentation of suspended 25 micrometer pollen over a region of 2.9 squared millimeter beneath the active heater.

Marangoni flows in evaporating liquid droplets can be utilized to sort cenospheres with sizes in a continuous spectrum from 5-200 micrometer. By heating the droplets from below, spheres 100-200 micrometer are deposited at the center and <50 micrometer spheres at the droplet periphery. The physical separation of large and small spheres is possible by using perforated metal plates. Cenospheres about 200 micrometer in diameter are subsequently modified by a focused ion beam to form hemispherical shells, and the fundamental wine glass mode resonance is investigated.

Activating the suspended heater array in a certain configuration rotates a millimeter-scale rotary structure mounted on a hub and completely immersed in the liquid. With a maximum temperature gradient of 36.6 K/mm at the surface of a liquid with viscosity 5 cSt, the structure takes 28 s to make a complete rotation. The angular velocity of the structure depends on the temperature gradient and viscosity of the liquid.