Anna Zigelman

The deposition of colloidal particles from a sessile drop of a volatile suspension subject to particle adsorption and coagulation

Electrical double layer and van der Waals (DLVO) forces are known to determine the morphology of the deposit of colloidal particles following the evaporation of the carrier liquid. It is assumed that the adsorption of particles to the solid substrate and their coagulation in the liquid are the mechanisms connecting DLVO forces to the morphology of the deposit. We use theory to test this assertion. We model the deposition of particles from a volatile drop while accounting for the contribution of adhesion and coagulation. The rate of both mechanisms is connected to DLVO forces via the interaction-force boundary layer and the Smoluchowski theorems, respectively. We present analytical solutions for the morphology of the deposit, accounting for particle adsorption and pair-limited coagulation, and a corresponding numerical analysis for the case where particle adhesion and coagulation are concurrent. We conclude that larger aggregates of particles are found near the edge of the drop at the expense of the smaller ones in the absence of adhesion. The adhesion of particles to the substrate smears the deposit, rendering large aggregates to appear near the center of the drop. The analysis is in agreement with a previous experiment when accounting for the corresponding DLVO forces.

Influence of a Propagating Megahertz Surface Acoustic Wave on the Pattern Deposition of Solute Mass off an Evaporating Solution

We study the influence of a megahertz Rayleigh surface acoustic wave (SAW), propagating in a solid substrate, on the pattern deposition of a solute mass off an evaporating solution. An experimental procedure, where a film of a solution undergoes a controlled evaporation in a chamber, shows that the SAW alters the state of the pattern deposition. Increasing the power of the SAW supports an increase in the density of the deposited patterns. Beyond threshold conditions, the deposited patterns merge and we observe the deposition of a solid film. A simplified theory suggests that the SAW deforms the geometry of the film, which is predominantly governed by the capillary stress. The deformation of the film taking place alongside with the evaporation of the solution increases the concentration near the pinned three phase contact line at the front of the film, which is closer to the source of the SAW, on the expense of the concentration at the rear. The increased concentration translates to the deposition of solute mass over an increased area near the front of the film, which explains the experimental observation.

Signatures of van der Waals and Electrostatic Forces in the Deposition of Nanoparticle Assemblies

We evaporate aqueous suspensions in a microchamber to explore the connection between the morphology of the nanoparticle deposits at nanometer resolutions and at micrometer and hundreds of micrometers resolutions. Repulsive or weakly attractive electrical double-layer and van der Waals surface forces render the deposition of detached particles and small aggregates at nanometer resolutions. However, strongly attractive surface forces render the dense deposition of large aggregates. At greater length resolutions, the deposit morphology is further governed by evaporation-mediated transport of particles in the volatile suspension. We use experiment and theory to show that the contributions of the different mechanisms to the deposit morphology are mediated by particle coagulation and by particle adsorption to the substrate. The nanometer deposit morphology and particle transport render the morphology of the deposits at greater length resolutions, where it may take the shape of crude or smooth particulate micropatterns or continuous particulate coating layers.

Simulations of the dynamic deposition of colloidal particles from a volatile sessile drop

HYPOTHESIS The deposition of particles from a volatile liquid drop atop a substrate is primarily governed by the advection and diffusion of the particles in the liquid. Colloidal particles may further coagulate and adsorb to the substrate during the deposition process. The external geometry and the internal composition of the particulate deposit are then determined by an interplay between these four mechanisms. SIMULATION We simulate the process of deposition by solving the governing transport equations. We explore the interplay between the different mechanisms mentioned above. In particular, we study the contribution of the diffusion of colloidal particles and aggregates to the morphology of the deposit, which was neglected in a previous study. FINDINGS The rates of diffusion and coagulation of each specific aggregate are dependent on its size. Hence, the transport equation uniquely correlates to each population of aggregates. The overall transport problem, alongside the rates of particle and aggregate adsorption and liquid evaporation, determines the geometry of the deposit. Moreover, the local rate of particle coagulation determines the internal composition of the different aggregate populations in the deposit. Our results appear to be in qualitative agreement with previous experimental findings.