Effect of Micropillar Array Morphology on Liquid Propagation Coefficient Enhancement.

Roughness on hydrophilic surfaces allows for fast propagation of liquids. In this paper, the hypothesis is tested which theorizes that pillar array structures with nonuniform pillar height levels can enhance wicking rates. In this work, within a unit cell, nonuniform micropillars were arranged with one pillar at constant height, while other shorter pillars were varied in height to study these nonuniform effects. Subsequently, a new microfabrication technique was developed to fabricate a nonuniform pillar array surface. Capillary rising-rate experiments were conducted with water, decane, and ethylene glycol as working liquids to determine the behavior of propagation coefficients that were dependent on pillar morphology. It is found that a nonuniform pillar height structure leads to a separation of layers in the liquid spreading process and the propagation coefficient increases with declining micropillar height for all liquids tested. This indicated a significant enhancement of wicking rates compared to uniform pillar arrays. A theoretical model was subsequently developed to explain and predict the enhancement effect by considering capillary force and viscous resistance of nonuniform pillar structures. The insights and implications from this model thus advance our understanding of the physics of the wicking process and can inform the design of pillar structures with an enhanced wicking propagation coefficient.