On the effect of surfactant adsorption and viscosity change on apparent slip in hydrophobic microchannels

Substantial experimental, theoretical, as well as numerical effort has been invested to understand the effect of boundary slippage in microfluidic devices. However, even though such devices are becoming increasingly important in scientific, medical, and industrial applications, a satisfactory understanding of the phenomenon is still lacking. This is due to the extremely precise experiments needed to study the problem and the large number of tunable parameters in such systems. In this paper we apply a recently introduced algorithm to implement hydrophobic fluid-wall interactions in the lattice Boltzmann method. We find a possible explanation for some experiments observing a slip length depending on the flow velocity which is contradictory to many theoretical results and simulations. Our explanation is that a velocity dependent slip can be detected if the flow profile is not fully developed within the channel, but in a transient state. Further, we show a decrease of the measured slip length with increasing viscosity and demonstrate the effect of adding surfactant to a fluid flow in a hydrophobic microchannel. The addition of surfactant can shield the repulsive potential of hydrophobic walls, thus lowering the amount of slip with increasing surfactant concentration.