Internal flow in polymer solution droplets deposited on a lyophobic surface during a receding process

Area of Science:

• Fluid dynamics
• Polymer science
• Surface science

Background:

• Droplet evaporation on surfaces involves contact line receding and pinning.
• Internal fluid flow dynamics in evaporating polymer solutions are not fully understood.
• Previous studies on pure liquids show different flow patterns compared to polymer solutions.

Purpose of the Study:

• To investigate the internal flow dynamics during the receding and pinning of polymer solution droplets on a lyophobic surface.
• To understand the influence of initial solute concentration on evaporation rate and flow patterns.
• To elucidate the underlying mechanisms driving the observed circulation flow.

Main Methods:

• Experimental setup for observing droplet receding and internal flow.
• Deposition of polystyrene-acetophenone and -anisole solutions on a lyophobic surface.
• Numerical simulation using a hemispherical droplet model with density and surface tension distributions.

Main Results:

• Evaporation rate per unit area during receding is largely independent of initial solute concentration.
• Average solute concentration at pinning increases with initial concentration.
• A convective circulation flow, upward at the axis, is observed and enhanced by increasing concentration, contrary to viscosity effects.

Conclusions:

• The observed circulation flow in evaporating polymer solutions is experimentally confirmed and numerically reproduced.
• The flow is initiated by the solutal Rayleigh effect and dominated by the solutal Marangoni effect.
• Both solutal effects are enhanced with increasing concentration due to evaporative mass balance.