Retention Forces of a Liquid Slug in a Rough Capillary Tube with Symmetric or Asymmetric Features

On surfaces with asymmetric "sawtooth" features, liquid slugs or drops tend to move preferentially in one direction. In this theoretical study, the imbalance of capillary forces that leads to directionally biased wetting is examined. Capillary tubes with symmetric and asymmetric sawtooth features were used to estimate the ratios of retention forces in opposing directions. Our analysis suggests that the difference between the retention force in one direction versus the other can be maximized by increasing feature asymmetry and minimizing inherent hysteresis of the materials of construction. This work has implications for small channels or surfaces of fluid-handling components found in microfluidic devices and fuel cells.