Recent experiments on coating flows and liquid drop impact both demonstrate
that wetting failures caused by air entrainment can be suppressed by reducing
the ambient gas pressure. Here, it is shown that non-equilibrium effects in the
gas can account for this behaviour, with ambient pressure reductions increasing
the gas' mean free path and hence the Knudsen number Kn. These effects first
manifest themselves through Maxwell slip at the gas' boundaries so that for
sufficiently small Kn they can be incorporated into a continuum model for
dynamic wetting flows. The resulting mathematical model contains flow
structures on the nano-, micro- and milli-metre scales and is implemented into
a computational platform developed specifically for such multiscale phenomena.
The coating flow geometry is used to show that for a fixed gas-liquid-solid
system (a) the increased Maxwell slip at reduced pressures can substantially
delay air entrainment, i.e. increase the `maximum speed of wetting', (b)
unbounded maximum speeds are obtained as the pressure is reduced only when slip
at the gas-liquid interface is allowed for and (c) the observed behaviour can
be rationalised by studying the dynamics of the gas film in front of the moving
contact line. A direct comparison to experimental results obtained in the
dip-coating process shows that the model recovers most trends but does not
accurately predict some of the high viscosity data at reduced pressures. This
discrepancy occurs because the gas flow enters the `transition regime', so that
more complex descriptions of its non-equilibrium nature are required. Finally,
by collapsing onto a master curve experimental data obtained for drop impact in
a reduced pressure gas, it is shown that the same physical mechanisms are also
likely to govern splash suppression phenomena.Comment: Accepted for publication in the Journal of Fluid Mechanic
