Surfactant-driven flow transitions in evaporating droplets

Abstract

An evaporating droplet is a dynamic system in which flow is spontaneously generated to minimize the surface energy, dragging particles to the borders and ultimately resulting in the so-called "coffee-stain effect". The situation becomes more complex at the droplet's surface, where surface tension gradients of different natures can compete with each other yielding different scenarios. With careful experiments and with the aid of 3D particle tracking techniques, we are able to show that different types of surfactants turn the droplet's surface either rigid or elastic, which alters the evaporating fluid flow, either enhancing the classical coffee-stain effect or leading to a total flow inversion. Our measurements lead to unprecedented and detailed measurements of the surface tension difference along the evaporating droplet's surface with good temporal and spatial resolution.

Solutal Marangoni flow as the cause of ring stains from drying salty colloidal drops

“…If such components are surface-active, the transport processes inside the droplet can be drastically affected. That has been shown for bulk mixtures [3][4][5][6] and surfactant solutions [7][8][9].…”

Self-wrapping of an ouzo drop induced by evaporation on a superamphiphobic surface

“…In the last two decades, numerous studies have focused on understanding the evaporation process of sessile drops on solid substrates experimentally, numerically and theoretically 14,15 . Surface properties [17][18][19][20] , thermal effects 21,22 , dispersed particles in the liquid 3,23 , surfactants at the liquid-gas interface [24][25][26] , and the liquid composition [27][28][29] were all found to have a contribution to the drop evaporation characteristics.…”

Steady-state droplet evaporation: Contact angle influence on the evaporation efficiency

“…The fluid properties are also engineered to (i) overcome such bottlenecks or (ii) extend the application space of a given base-fluid, where surfactants [51], corrosion inhibitors [52], bio-molecules [53], and colloidal particles [54][55][56][57] are commonly used additives. Surfactant and colloidal loadings typically reduce the overall evaporation rate, magnitude of convective flows, and depinning dynamics; yet, such additives can also result in shear-thinning phenomena [10,58], flow inversion [59], and suppression of the coffee-ring effect [60] if loadings of anisotropic geometries are used [61].…”