Mickaël Antoni

From Spherical to Polymorphous Dispersed Phase Transition in Water/Oil Emulsions

Optical scanning tomography is used to characterize bulk properties of transparent water-in-paraffin oil emulsions stabilized with hexadecyl-trimethylammonium bromide (CTAB) and silica nanoparticles. A flow of 500 hundred images is used to analyze each scanning shot with a precision of about 1 microm. The role of silica particles in the shape of the water droplets is investigated. Depending on the concentration of CTAB and silica nanoparticles, a transition occurs in their geometry that changes from spherical to polymorphous. This transition is controlled by the ratio R=[CTAB]/[SiO2] and is described using an identification procedure of the topology of the gray level contours of the tomographic images. The transition occurs for Rcrit approximately 3x10(-2) and is shown to correspond to a pH of the dispersed phase of 8.5.

First-order microcanonical transitions in finite mean-field models

A microcanonical first-order transition, connecting a clustered to a homogeneous phase, is studied from both the thermodynamic and the dynamical point of view for an N-body Hamiltonian system with infinite-range couplings. In the microcanonical ensemble, specific heat can be negative, but besides that, a microcanonical first-order transition displays a temperature discontinuity as the energy is varied continuously (a dual phenomenon to the latent heat in the canonical ensemble). In the transition region, the entropy per particle exhibits, as a function of the order parameter, two relative maxima separated by a minimum. The relaxation of the metastable state is shown to be ruled by an activation process induced by intrinsic finite N fluctuations. In particular, numerical evidences are given that the escape time diverges exponentially with N, with a growth rate given by the entropy barrier.

First- and second-order clustering transitions for a system with infinite-range attractive interaction

We consider a Hamiltonian system made of N classical particles moving in two dimensions, coupled via an infinite-range interaction gauged by a parameter A. This system shows a low energy phase with most of the particles trapped in a unique cluster. At higher energy it exhibits a transition towards a homogenous phase. For sufficiently strong coupling A, an intermediate phase characterized by two clusters appears. Depending on the value of A, the observed transitions can be either second or first order in the canonical ensemble. In the latter case, microcanonical results differ dramatically from canonical ones. However, a canonical analysis, extended to metastable and unstable states, is able to describe the microcanonical equilibrium phase. In particular, a microcanonical negative specific heat regime is observed in the proximity of the transition whenever it is canonically discontinuous. In this regime, microcanonically stable states are shown to correspond to saddles of the Helmholtz free energy, located inside the spinodal region.

On the microcanonical solution of a system of fully coupled particles

Abstract We study the Hamiltonian mean field (HMF) model, a system of N fully coupled particles, in the microcanonical ensemble. We use the previously obtained free energy in the canonical ensemble to derive entropy as a function of energy, using Legendre transform techniques. The temperature–energy relation is found to coincide with the one obtained in the canonical ensemble and includes a metastable branch which represents spatially homogeneous states below the critical energy. “Water bag” states, with removed tails momentum distribution, lying on this branch, are shown to relax to equilibrium on a time which diverges linearly with N in an energy region just below the phase transition.

Transition from Spherical to Irregular Dispersed Phase in Water/Oil Emulsions

Bulk properties of transparent and dilute water in paraffin oil emulsions stabilized with sodium dodecyl sulfate (SDS) are analyzed by optical scanning tomography. Each scanning shot of the considered emulsions has a precision of 1 μm. The influence of aluminum oxide nanoparticles in the structure of the water droplets is investigated. Depending on concentrations of SDS and nanoparticles, a transition occurs in their shape that changes from spherical to polymorphous. This transition is controlled by the SDS/alumina nanoparticles mixing ratio and is described using an identification procedure of the topology of the gray level contours extracted from each images. The transition occurs for a critical mixing ratio of Rcrit ≈ 0.05 which does not significantly depend on temperature and electrolyte concentration. This structural change seems to be a general feature when emulsifying dispersions and most probably involves both interfacial and bulk phenomena.

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.

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.

In memoriam: Professor Albert Sanfeld (1932-2015).

1 A. Sanfeld, Introduction to the Thermodynamics of Charged and Polarized Layers, Wiley, New York, 1968. Professor Albert Sanfeld lost a battle to illness on Tuesday, 8December 2015, at the hospital in Aix-en-Provence, France. Albert had a career spanning overmore than five decades dedicated to science, his family, and his passion for music. Over this long period, Albert has been there to help countless people when needed. Albert is known amongst his family, friends, and colleagues for his compassion and sensitivity to the human condition. He would not hesitate to pay with his time, money, and effort to assist those in need. No challengewas big enough for Albert to be tackled; he was relentless in his dedication to helping people. In his professional environment, Albert was very much respected by his senior colleagues and admired by his junior collaborators. Albert mentored numerous scientists and researchers who are now successful leaders in their field. Albert was passionate about science overall and his research in particular. He gave his wholehearted attention to his research until his last days. Evenwhen Albert was ill and tired, nothing could spark off his enthusiasm and excitement apart from a scientific challenge and open questions to be addressed. When Albert was admitted to the hospital in Aix-en-Provence for a major heart surgery and put in intensive care unit, hewas furious that his doctorswould not allowhim to bring his articles and printed material to continue his research. Albert complained that the doctors do not understand the importance of his research and the problems he was trying to solve. K. Sefiane fondly recalls those long and deep discussionswith Albert, at work or at his home in Mallemort, about philosophy, science, and

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.

Thermography Applied to Interfacial Phenomena, Potentials and Pitfalls

Infrared (IR) thermography is a non-intrusive method for temperature measurement. Its ability to produce two-dimensional temperature images makes it a powerful tool for investigating systems exhibiting spatial variation of temperature. IR temperature measurements are almost always surface measurements; the technique has therefore found use in obtaining interfacial temperatures, primarily in heat and mass transfer investigations. The reasons for the technique’s limited uptake likely stems from the requirement of accurate material emissivity data and the large number of potential sources of error. This chapter provides an overview of the underlying theory of radiative heat transfer. Key considerations and problems in the application of IR thermography are discussed with reference to some examples of recent successful applications.