  The effect of slip surfaces on the laminar-turbulent separatrix of plane
Poiseuille flow is studied by direct numerical simulation. Turbulence
lifetimes, the likelihood that turbulence is sustained, is investigated for
transitional flows with various slip lengths. Slip surfaces decrease the
likelihood of sustained turbulence compared to the no-slip case, and likelihood
is further decreased as slip length is increased. A deterministic analysis of
the effects of slip surfaces on transition to turbulence is performed using
nonlinear traveling wave solutions to the Navier-Stokes equations, also known
as exact coherent solutions. Two solution families, dubbed P3 and P4, are used
since their lower-branch solutions are embedded on the boundary of the basin of
attraction of laminar and turbulent flows (Park & Graham 2015). Additionally,
they exhibit distinct flow structures -- the P3 and P4 are denoted as core mode
and critical layer mode, respectively. Distinct effects of slip surfaces on the
solutions are observed by the skin friction evolution, linear growth rate, and
phase-space projection of transitional trajectories. The slip surface modifies
transition dynamics little for the core mode, but considerably for the critical
layer mode. Most importantly, the slip surface promotes different transition
dynamics -- early and bypass-like transition for the core mode and delayed and
H-/K-type-like transition for the critical layer mode. Based on spatiotemporal
and quadrant analyses, it is found that slip surfaces promote the prevalence of
strong wall-toward motions (sweep-like events) near vortex cores close to the
channel centre, inducing an early transition, while sustained ejection events
are present in the region of the ${\Lambda}$-shaped vortex cores close to the
critical layer, resulting in a delayed transition.
