Transport of Colloids along Corners: Visualization of Evaporation-Induced Flows beyond the Axisymmetric Condition

Nonhomogeneous
evaporation fluxes have been shown to promote the
formation of internal currents in sessile droplets, explaining the
patterns that suspended particles leave after the droplet has dried
out. Although most evaporation experiments have been conducted using
spherical-cap-shaped drops, which are essentially in an axisymmetric
geometry, here we show an example of nonhomogeneous evaporation in
asymmetric geometries, which is visualized by following the motion
of colloidal particles along liquid fingers forming a meniscus at
square corners. It is found that the particle’s velocity increases
with the diffusive evaporation factor D(1−RH)cs for the three tested fluids: water, isopropyl
alcohol (IPA), and ethanol (EtOH). Here, D is the vapor
diffusivity in air, RH is
the relative amount of vapor in the atmosphere, and cs is the saturated vapor concentration. We observed that
in IPA and EtOH the internal currents promote a 3D spiral motion,
whereas in water the particle’s trajectory is basically unidirectional.
By adding 0.25 critical micelle concentration (CMC) of sodium dodecyl
sulfate (SDS) surfactant in water, a velocity blast was observed in
the whole circulation flow pattern, going from O(100)μm/s to nearly O(1000)μm/s in the longitudinal velocity component.
To assess the effect of breaking the axisymmetric condition on the
evaporation flux profile, we numerically solved the diffusive equation
in model geometries that preserve the value of the contact angle θ
but introduce an additional angle ϕ that characterizes the solid
substrate. By testing different combinations of θ and ϕ,
we corroborated that the evaporation flux increases when the substrate
and the gas–liquid curves meet at corners with increasing sharpness.