Coupling between wetting dynamics, Marangoni vortices, and localized hot cells in drops of volatile binary solutions.

HYPOTHESIS: A sessile drop comprising a mixture of volatile solvents supports spatial variations in interfacial energy, which gives rise to solutal Marangoni flow, alongside evaporative loss of drop mass. Both the Marangoni flow and evaporation bring about a dance of concurrent and inter-connected phenomena: internal Marangoni vortices, localized hot cells, and complex wetting dynamics. EXPERIMENT: We employ Particle Image Velocimetry and Infra-Red Microscopy to visualize Marangoni vortices, temperature variations, and the wetting dynamics of drops of toluene and ethanol mixtures. FINDINGS: The intensity of the measured phenomena vary concurrently in time and in like manner according with the initial composition of drops. In particular, we observe maximum intensity levels when the initial toluene proportion in the drops is 60%, and none of these phenomena in the case of pure toluene. Moreover, the drops initially expand on the solid in response to Marangoni flow, then contract due to evaporation; between these dynamic wetting regimes, we further observe a regime of one or periodic wetting/de-wetting cycles at low toluene concentrations. Our findings indicate that both the solutal Marangoni flow and evaporation drive the different phenomena we observe and confirm the connection between Marangoni vortices and the formation of localized hot cells.

Deposition of nanoparticles from a volatile carrier liquid.

HYPOTHESIS: Traversing length scales in a volatile suspension alters the various contributions to particle deposition from conjoining and disjoining surface forces and from convective and liquid evaporative effects, which is apparent in the deposit morphology. EXPERIMENT: We investigate the particulate structures to result from the self-assembly of nanoparticles following the evaporation of a volatile carrier liquid from the level of the single particle and up to a level which is apparent to the naked eye, while quantifying the contributions of the main mechanisms that are involved in the deposition process. FINDINGS: We show that from the level of the nanoparticles in our experiment and up to a length scale of approximately 10 µm, the morphology of the deposit is particularly sensitive to particle adhesion to the substrate and to liquid evaporation. At greater length scales, the morphology of the deposit is well correlated with the finite volume of particles and with particle convection effects. The particulate structures are in the form of detached particles and particle islands, stripes, and continuous coating, which may vary at different length scales of the same deposit.

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.