Emanuel Bertrand

Acceleration of the recognition rate between grafted ligands and receptors with magnetic forces

When ligands and receptors are both attached on surfaces, because of the restriction of configurational freedom, their recognition kinetics may be substantially reduced as compared with freely diffusing species. In nature, this reduction may influence the efficiency of the capture and adhesion of circulating cells. Here we show that similar consequences are observed for colloids grafted with biomolecules that are used as probes for diagnostics. We exploit Brownian magnetic colloids that self-assemble into linear chains to show also that the resulting one-dimensional confinement considerably accelerates the recognition rate between grafted receptors and their ligands. We propose that because confinement significantly augments the colliding frequency, it also causes a large increase in the attempt frequency of the recognition. This work gives the basis of a rapid, homogeneous, and highly sensitive bioanalysis method.

Wetting of alkanes on water

The wetting behavior of oil on water (or brine) has important consequences for the transport properties of oil in water-containing porous reservoirs, and consequently for oil recovery. The equilibrium wetting behavior of model oils composed of pure alkanes or alkane mixtures on brine is reviewed in this paper. Intermediate between the partial wetting state, in which oil lenses coexist on water with a thin film of adsorbed alkane molecules, and the complete wetting state, in which a macroscopically thick oil layer covers the water, these systems display a third, novel wetting state, in which oil lenses coexist with a mesoscopic (a few-nanometers-thick) oil film. The nature and location of the transitions between these wetting regimes depend on oil and brine compositions, temperature and pressure.

Effect of Wetting on Gravity Drainage in Porous Media

A series of benchmark experiments on the effect of the wetting state on the flow properties in porous media were performed, allowing us to relate the wetting properties at the pore scale to the macroscale hydrodynamics. Drainage of n-alkanes (oils) displaced by air in a model porous medium consisting of water-wet sand was studied using gamma-ray densitometry and weight measurements. The enormous advantage of our system is that we know and control the wetting properties perfectly: we can tune the wetting properties by changing the salinity of the water. This allows us to perform porous medium flow experiments for the different wetting states without changing the transport properties (viscosity and density) of the oil. Drainage is found to be more efficient, and consequently oil recovery more important for partial wetting.

Bio-specific recognition and applications: from molecular to colloidal scales

Biomolecules have the well-known ability to build reversible complexes. Indeed, antigens and antibodies or adhesion molecules are able to recognize one another with a strong affinity and a very high specificity. This paper first reviews the various techniques and related results about binding and unbinding, at the scale of a unique ligand/receptor couple. One important biotechnological application arising from these recognition phenomena concerns immuno-diagnosis, which is essentially based on the formation of these specific complexes. We show how the physics of colloids associated with the growing scientific background concerning molecular recognition helps in rationalizing and inventing new diagnostic strategies. Finally the concept of colloidal self-assembling systems as biosensors is presented as directly impacting the most important questions related to molecular recognition and their biotechnological implications.

Wetting of hydrocarbon mixtures on water under varying pressure or composition

Abstract The wetting/spreading properties of oil on water in the presence of gas are related to the Hamaker constant W of the water/oil/gas system: a negative W corresponds to a stable thick (macroscopic) oil film wetting water, whereas a positive W corresponds to oil lenses that do not wet water. A simple calculation scheme is presented in this paper, based on a combination of an approximation of the Dzyaloshinskii–Lifshitz–Pitaevskii (DLP) theory and either an adequate equation of state or refractive index measurements, to predict the wetting properties as a function of thermodynamic variables such as temperature, pressure and fluid composition. This paper also presents ellipsometry measurements of the wetting behavior of a propane/ n -hexane binary system on water at 20°C under varying pressure (controlled by the injection of the gaseous propane component into the ellipsometry cell). These measurements indicate that this system displays the same wetting behavior and goes through the same sequence of wetting transitions under increasing pressures as those encountered for pure n -alkanes (e.g., n -pentane or n -hexane) under increasing temperatures. Namely, we observed upon increasing the pressure (or the propane content in the system) a transition between the usual partial wetting state (at low pressures), and the so-called pseudo-partial wetting state, in which oil lenses coexist with an oil film with thickness ≈100 A. A continuous (or critical) transition between this pseudo-partial wetting state and complete wetting occurs for a higher pressure corresponding to the vanishing of the Hamaker constant W . Our expectation is that most condensates and volatile oils (or at least those mostly composed of alkanes) spread on brines under reservoir (high pressures and temperature) conditions.

Gravity drainage in porous media: the effect of wetting

In order to relate the wetting properties at the pore scale to the macroscale hydrodynamics, a series of benchmark experiments on the effect of the wetting on flow in porous media were performed. Gravity drainage of hexane (oil) in a model porous medium consisting of water-wet sand was studied using γ-ray densitometry and weight measurements. The advantage of our system is that we know and control the wetting properties perfectly: we can tune the wetting properties by changing the salinity of the water. This allows us to perform porous medium flow experiments for the different wetting states without changing the transport properties (viscosity and density) of the oil. Drainage is found to be more efficient and, consequently, oil recovery more important for partial wetting.