When considering fluidic devices at the micron length scale, surface tension forces become dominant relative to body forces. Albeit smaller than mechanical and electrical pumps, capillary forces are commonly exploited as a mechanism to drive fluid flow. Unlike pumps, capillary driven flows are passive in nature and are not dependent on auxiliary equipment to drive fluid flow. Although beneficial from an energy standpoint, the lack of a supplementary driving potential causes the flow to be limited by the wick structure dimensions that generate the capillary forces. Subsequently, investigation into the contributions of the wick structure must be performed in order to optimize the fluid flow through a capillary structure. General capillary theory states that capillary forces increase inversely proportional to the pore radius. Consequently, arrays of vertically aligned nanopillars grown on silicon substrates are considered for fluid flow optimization due to their small pores. To simulate these nanopillars, an ab initio analysis was done on a homogenously dispersed array of vertically aligned pillars. An analytical solution to predict the maximum achievable capillary flow with respect to the structure dimensions was found through this method. Subsequently, this analytical solution can be used to produce a set of optimal geometric conditions that would induce the maximum capillary flow through a wick comprised of vertically aligned pillars. Experimental results are also presented to validate the analytical solution. Homogeneously dispersed cylindrical pillars were created on silicon wafers via reactive ion etching to reconstruct the geometry assumed by the analytical solution. The capillary limit was found for structures with varying geometric dimensions. By contrasting the empirical data with the values predicted by the analytical model, the validity of the analytical model was found to be in good agreement.
- Fluids Engineering Division
Characterization of Capillary Flow Within a Homogenously Dispersed Array of Vertical Micropillars
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Zhang, C, & Hidrovo, CH. "Characterization of Capillary Flow Within a Homogenously Dispersed Array of Vertical Micropillars." Proceedings of the ASME 2010 8th International Conference on Nanochannels, Microchannels, and Minichannels collocated with 3rd Joint US-European Fluids Engineering Summer Meeting. ASME 2010 8th International Conference on Nanochannels, Microchannels, and Minichannels: Parts A and B. Montreal, Quebec, Canada. August 1–5, 2010. pp. 655-663. ASME. https://doi.org/10.1115/FEDSM-ICNMM2010-30526
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