Open-channel capillary flow in micro-scale helical support structures

Jerry J. Oelerich

Capillary-driven flow has been studied in a large aspect ratio channel consisting of a horizontally stretched microspring. Potential applications for this research exist in microstructured gas-liquid contacting and evaporative cooling of electronic chips. The capillary pressure limits under static conditions for an infinite liquid column in zero gravity supported by a uniform helical wire were previously shown to depend on two dimensionless geometric parameters pertaining to the helical support and to scale with the ratio of surface tension to coil diameter. These findings would be expected to hold under normal gravity at small length scales where surface tension dominates over the force of gravity. The principal hypothesis tested in this thesis is that static capillary pressure limits are expected to apply locally in the case of steady-state flow, and to therefore limit the available driving force for flow in the channel. The main goal of this work was to determine the maximum achievable flow rate in a given channel and to then relate this to the static capillary pressure limits for the channel. Experiments were conducted on a 300 micron diameter spring mounted in a fluidic circuit with diagnostics consisting of high-resolution imaging and pressure transducers. Static capillary pressure limits were measured for a liquid channel supported by this spring. Steady-state flow experiments were performed using the same support structure. The upper flow rate limit was found to depend on overall channel volume, and an optimum channel volume was identified that gave the maximum possible flow rate. Analysis of the pressure profile in the channel for each stable flow rate attained largely substantiates the hypothesis that static capillary limits dictate the range of pressures available to drive flow. Also, experimental evidence confirms that during steady-state flow, the pressure profile down the channel is essentially linear and a loss coefficient was measured. The results obtained are consistent with a prediction that the maximum possible velocity through a channel of fixed aspect ratio and dimensionless geometric factors is invariant with the scale of the channel.

Open-channel capillary flow in micro-scale helical support structures

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