  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.
