Influence of Surfactant Concentration on Spontaneous Emulsification Kinetics.

The kinetics of spontaneous emulsification is investigated on aqueous pendant drops in paraffin oil. Optical microscopy in transmission mode is used for high-spatial-resolution image recording. The influence of a lipophilic surfactant (Span 80) and two water-soluble surfactants (CTAB and SDS) is investigated. As time runs, the drop interface turns opaque due to the formation of microstructures associated with spontaneous emulsification. The time evolution of this phenomenon is shown to depend upon temperature and surfactant concentration, which leads to an overall shrinkage due to gradual water uptake and transport into paraffin oil. Spontaneous emulsification kinetics depends upon the chemical composition. Higher concentrations of Span 80 and CTAB (resp. SDS) are shown to promote (resp. hinder) water transport. This work provides new insights into the understanding of spontaneous emulsification when combining the properties of non-ionic and ionic surfactants.

Microstructure Formation in Freezing Nanosuspension Droplets.

The structural evolution of suspensions upon freezing is studied with optical microscopy in a suspended droplet configuration. Droplets are of millimeter size and consist of an aqueous mixture of silica particles, while the surrounding phase is hexane. Freeze-thaw cycles are applied to this system, and a two-step freezing mechanism is evidenced. A fast adiabatic growth of dendrites that invade the full droplets is first observed and occurs within a few milliseconds. Then, a slow process lasts for several seconds and corresponds to the release of solidification latent heat into the hexane phase. The striking feature of this work is to evidence that after the first freeze-thaw cycle flocculated microstructures are generated. When a second cycle is performed, microstructures further flocculate and generate, for dense silica suspensions, stable porous spheres of the size of the droplets. A phenomenological description based on repulsion or engulfment of particles by solidifying ice fronts is proposed.

Spontaneous Microstructure Formation at Water/Paraffin Oil Interfaces.

An experimental investigation of spontaneous emulsification is proposed with a water drop pendant in a paraffin oil (PO) solution loaded with a surfactant (SPAN80). Optical microscopy in a transmission mode is employed for high-spatial-resolution image recording. The kinetics of spontaneous emulsification is studied. It is shown to generate a darkening of the drops because of interface modification with a characteristic time that depends upon the SPAN80 concentration. For low concentrations, spontaneous emulsification is slow and produces micrometer-sized droplets, whereas for large concentrations, it is fast and bush-like microstructures are observed. These microstructures increase in size and progressively invade the complete water/PO interfaces, detach, and finally migrate into the PO phase. This transport phenomenon withdraws water from the drops and leads to a gradual shrinking of their volume. At the end of this process, they appear as deformed objects surrounded by a loose membrane.

A numerical investigation on the drainage of a surfactant-modified water droplet in paraffin oil.

A volume of fluid approach is used in numerical simulations of the settling motion of a surfactant modified water droplet in a continuous paraffin oil phase. The droplet is millimeter-sized and confined in a square two dimensional domain. The surfactant interfacial and bulk concentration-equations are solved together with the incompressible Navier-Stokes equation. The role of boundary walls in the overall settling dynamics is described. As the droplet moves downwards the interfacial shear creates non-homogeneous interfacial surfactant concentrations and Marangoni driven phenomena come into play. A decrease of the drainage velocity is then evidenced indicating that buoyancy forces are counter balanced by Marangoni induced lift-forces. The lateral migration of the droplet due to boundary wall proximity is discussed. It is shown to increase with wall proximity and to decrease when increasing the interfacial concentration. Finally, a simplified model is used to investigate the evolution of the bulk concentration assuming the surfactant is insoluble in paraffin oil and poorly soluble in water.

Use of IR thermography to investigate heated droplet evaporation and contact line dynamics.

In this paper we present the results of an experimental study investigating interfacial properties during the evaporation of sessile water droplets on a heated substrate. This study uses infrared thermography to map the droplet interfacial temperature. The measurements evidence nonuniform temperature and gradients that evolve in time during the evaporation process. A general scaling law for the interfacial temperature is deduced from the experimental observations. A theoretical analysis is performed to predict the local evaporation rates and their evolution in time. The use of energy conservation laws enabled us to deduce a general expression for the interfacial temperature. The comparison between the theory and experiments shows good agreement and allows us to rationalize the experimental observations. The thermography analysis also enabled the detection of the three-phase contact line location and its dynamics. To our knowledge, such measurements are performed for the first time using thermography.

Rheology and structure formation in diluted mixed particle-surfactant systems.

In the present work, we focus on the bulk rheology of mixtures consisting of surfactant modified silica nanoparticles in water. Depending on the ratio of surfactant and nanoparticle concentrations, significant modifications in the measured rheology are evidenced. A domain where the dispersions behave like viscoelastic media is observed. Outside this domain, the dispersions exhibit viscous properties. The changes in the bulk rheology characteristics are discussed in terms of interaction effects between the surfactant and the particles. The results obtained are seen in the context of diluted emulsions' properties and characteristics.

Infrared thermography investigation of an evaporating sessile water droplet on heated substrates.

The present study is an experimental investigation of the thermal evolution of millimeter-sized sessile water droplets deposited on heated substrates. Infrared thermography is used to record temperature profiles on the droplet interface in time as evaporation takes place. The local measurements of the interface temperature allowed us to deduce the local evaporation rate and its evolution in time. To our knowledge, this is the first time that such measurements have been performed. The deduced evaporation rate using thermography data has been validated with optical measurements. Temperature evolution is used to reveal the contact line location and transient temperature fields. Temperature differences between the apex of the droplet and the contact line are shown to decrease in time. The rate of local temperature increase at the interface is found to behave linearly with time. The slope of this linear increase turns out to be more pronounced as the substrate temperature is increased. A generalized linear trend, using dimensionless properties for the interface temperature rise, is deduced from the measurements.

Influence of substrate heating on the evaporation dynamics of pinned water droplets.

The dynamics of evaporating water droplets deposited on a heated substrate is investigated numerically. Droplets are pinned with a contact line radius of R = 1 mm. Evaporative mass flow and convection occurring inside the droplets are studied for different heating substrate sizes L S and heating temperatures T S. A simplified model neglecting hydrodynamics in air and evaporative cooling and assuming droplets to be spherical caps is simulated with a finite element method. A toruslike convective cell appears inside the droplets as evaporation takes place. For L S/ R > 1, the contact line is warmer than the apex of the droplets, and convection generates a downstream flow in the vicinity of the symmetry axis of the droplets. For L S/ R < 1, it is the apex that is warmer. Convection then generates an upstream flow. The overall evaporation time is described. It slows when L S/ R > 1.

Evaporation and Marangoni driven convection in small heated water droplets.

Evaporation dynamics of small sessile water droplets under microgravity conditions is investigated numerically. The water-air interface is free, and the surrounding air is assumed to be quasisteady. The droplet is described by Navier-Stokes and heat equations and its surrounding water/air gaseous phase with Laplace equation. In the thermodynamic conditions of the simulations presented herein, the evaporative mass flow is nonlinear. It shows a minimum that indicates the existence of qualitative changes in the evaporative regimes although the droplet is sessile. Due to temperature gradients on the free interface, Marangoni motion occurs and generates inside the droplet convection cells that furthermore exhibit small fluctuating motion as evaporation goes on.