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

abstract

  • 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%27s 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%27s 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 ø that characterizes the solid substrate. By testing different combinations of θ and ø, we corroborated that the evaporation flux increases when the substrate and the gas-liquid curves meet at corners with increasing sharpness. © 2016 American Chemical Society.
  • 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 ø that characterizes the solid substrate. By testing different combinations of θ and ø, we corroborated that the evaporation flux increases when the substrate and the gas-liquid curves meet at corners with increasing sharpness. © 2016 American Chemical Society.

publication date

  • 2016-01-01