Control of Fluids in Microscale Devices

Pumping in microdevices due to gradients in surface tension was investigated both theoretically and experimentally. When a gradient in surface tension exists along an interface between two fluids, there is a net motion of the interface. The motion induces a flow parallel to the interface through viscous difhsion. This pheno~nenon is the Marangoni effect. In microdevices, where length scales are small, surface tension becomes a significant force, and can drive fluid flow at significant flow rates. This provides a mechanismfor steady pumping in microscale devices, without the use of moving parts. If a vapor bubble is created, and a temperature gradient is imposed, the resulting surface tension gradient will drive the flow from the hotter side of the bubble to the cooler side.
To demonstrate this effect, devices exhibiting the Marangoni effect in square channels were designed and fabricated from silicon and quartz substrates. The two substrates were then aligned, bonded and packaged fortesting. Each of the devices consisted of a 100umchannel with three heaters along one of its sides. The heaters generated a fluid vapor interface and controlled the temperature gradient along this interface. Flow could be generated in either direction, and could be switched on and off nearly instantaneously.  Pressure was measured during device operation, and flow rate was measured indirectly through its proportionality to the time derivative of pressure in a quasi-steady flow. Subsequent digital particle image velocimetry through an epi-fluorescent microscopy system confirmed the details of the fully developed flow within the channel.
The devices were operated at a variety of heater settings to evaluate their performance over a range of flow rates and pressures. A minimum energy of 140 mW was required from the central heater to generate and maintain the vapor bubble, and the flow rate increased with input energy. The efficiency of the device increased with flow rate, since the overhead to create and maintain the bubble (140 mW) dwarfs the energy expended to generate the temperame gradient (15-50 mW). The devices generated a maximum pressure head of 44 Pa, and a maximum flow rate of 9.3 nL/s, although the maximum values did not occur simultaneously. In fact, the maximum pressure will always occur when the flow rate is zero, and vice versa. The experimental values measured were in agreement with the theoretical estimates, although the actual flow rates and pressure drops were higher than those predicted by theory.
Publication date: 
May 31, 2001
Publication type: 
Ph.D. Dissertation
Debar, M. J. (2001). Control of Fluids in Microscale Devices. United States: University of California, Berkeley.

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