Colloidal Deposits via Capillary Bridge Evaporation and Particle Sorting Thereof

Abstract

Evaporating droplets of colloidal suspensions leave behind particle deposits which could be effectively controlled via manipulating the surrounding conditions and particles and liquid properties. While previous studies extensively focused on sessile and pendant droplets, the present work investigates the evaporation dynamics of capillary bridges of colloidal suspensions formed between two parallel plates. We vary the wettability of the plates and the particle size and composition of the colloidal suspensions, keeping the same spacing between the plates. We employ side visualization, optical microscopy, fluorescence microscopy, and scanning electron microscopy and develop computational and theoretical models to collect the data. A computational model based on diffusion-limited evaporation is used to characterize the timescale of the evaporation of the capillary bridge. The model predictions are in good agreement with the present and prior experimental measurements. We discuss about the deposits of monodispersed particle suspension formed by the interplay of pinning of the contact line and evaporation dynamics. Multiple rings on the plates are observed due to the stick–slip motion of the contact line. The larger particles tend to form asymmetric deposits, with most particles concentrated on the bottom plates due to a considerably stronger gravitational pull than the hydrodynamic drag. This deposition is explained by estimating the competing forces on the particles during the evaporation. A regime map is proposed for classifying deposits on the particle size wettability plane. Lastly, we demonstrate size-based particle sorting of bidispersed colloidal suspensions in this framework. We describe two mechanisms: gravity-assisted and geometry-assisted sorting, which can be designed to sort particles efficiently. A regime map depicting the regions of influence of each mechanism is presented.

Malleable Patterns from the Evaporation of a Colloidal Liquid Bridge: Coffee Ring to the Scallop Shell

“…6,32 Analogous to sessile droplets, evaporation occurs differentially over the surface due to the surface curvature of the LB. 23 The evaporation flux is maximum near the glass surface and minimum at the center of the LB as in the present study; for all cases, the meniscus is concave. 23 The LB surfaces have varying curvatures depending upon the value of α (also, the curvature changes dynamically with evaporation); evaporation flux is dependent on the curvature of the surface and can influence the flow inside the LB.…”

“…23 The evaporation flux is maximum near the glass surface and minimum at the center of the LB as in the present study; for all cases, the meniscus is concave. 23 The LB surfaces have varying curvatures depending upon the value of α (also, the curvature changes dynamically with evaporation); evaporation flux is dependent on the curvature of the surface and can influence the flow inside the LB. It is thus crucial to understand the variation of the concentration field of water vapor across the surface of LB of different curvatures (i.e., different values of α) to understand its influence on flow inside the LB.…”

“…Differential evaporation across the liquid–vapor interface of a sessile droplet induces a flow inside the droplet, commonly known as evaporation-driven flow (EDF). , Analogous to sessile droplets, evaporation occurs differentially over the surface due to the surface curvature of the LB . The evaporation flux is maximum near the glass surface and minimum at the center of the LB as in the present study; for all cases, the meniscus is concave .…”

“…Also, we compare the experimentally observed v dw with the theoretical one (Figure b). To find out the theoretical velocity, we first evaluated the evaporation flux by using the following expression: …”

“…According to them, the continuous stick−slip motion of the CL resulted in symmetric spiral deposits independent of the shape of the particles. Very recently, Upadhyay and Bhardwaj 23 examined the colloidal liquid bridge by altering the wettability of solid surfaces and the size of particles along with the composition of the colloids. Their observations include the formation of multiple rings governed by the stick−slip motion of CL.…”