Francis Rondelez

Exponential growth of fingering instabilities of spreading films under horizontal thermal gradients

A thin liquid ribbon deposited on a wettable surface and subjected to a horizontal thermal gradient develops periodic fingers towards the cold region. We present an observation of the exponential growth regime, expected at early times, but not accessible in other driven fingering patterns induced by gravitational flows, spinning drops, or vertical thermal gradients. We have monitored the wavelength λ and the raising time τ of the unstable mode of undulation of the contact line; we find that λτ2 is independent of the ribbon thickness, in agreement with a linear stability analysis.

Hydrodynamic Thickening of Depletion Layers in Colloidal Solutions

We report a new surface phenomenon in flowing colloidal suspensions of anisotropic particles. It is characterized by a decrease in the probability of presence of the solute particles near the cell walls when the solution is submitted to Poiseuille flows exceeding a minimum shear rate. The effect has been detected on dilute aqueous solutions of xanthan, a wormlike polysaccharide chain, by using the optical evanescent-wave-induced fluorescence method. The results obtained at various shear rates are in quantitative agreement with a simple model based on the chain dynamics near the repulsive wall, and taking into account the increased molecular tumbling rate induced by the shearing flow.

Studies of phase transitions in Langmuir monolayers by fluorescence microscopy

The liquid–gas transition in monolayers of stearic acid on water has been investigated by fluorescence microscopy. The monolayer texture is made visible by the addition of stearic acid molecules labelled at the 12-position with methyl amino NBD [N-methyl-(7-nitro-2,1,3-benzoxadiazol-4-yl)]. The film texture and its evolution with time after changes have been made in the film density can be observed at areas up to 100 A2 per molecule. In the two-phase region we observe a structure similar to a two-dimensional soap foam. Preliminary analyses have been made of the cell-side distribution and the average linear cell dimension as a function of time.

Quantitative studies of evanescent wave intensity profiles using optical fluorescence

The fluorescence of uniformly distributed chromophores in an organic solution has been used to probe the energy density profile of an optical evanescent wave, generated by total internal reflection at the solid-solution interface. The results obtained in the case of an aqueous fluorescein solution in contact with a highly polished silica surface are in good quantitative agreement with the expected exponential decay of the optical energy at the interface. It also justifies the use of the newly developed evanescent wave-induced fluorescence technique to study adsorption and depletion layers of polymer in solution close to solids walls. In such experiments the fluorescence intensity is assumed to be the Laplace transform of the concentration profile of the fluorescently labeled polymer chains. The present data validate the above assumption.

Spreading of “heavy” droplets: II. Experiments

Abstract We present systematic measurements of the spreading of PDMS oils on flat oxidized silicon wafers. We have determined the complete drop profile for droplet radii larger than the capillary length, κ−1, defined by κ −1 = ( γ ρg ) 1 2 , where γ is the surface tension of the liquid, ρ its density, and g the gravitational acceleration; The droplet shape is found to be quasi-static, up to a critical radius Rc, which is found to be one order of magnitude higher than the capillary length. The droplet is essentially flat except on the edges and its thickness at center, h0, is related to the dynamical contact angle, θd, by h0 ≈ κ−1θd. This measured profile is in perfect agreement with the numerical profile which has been calculated in the first paper of this series (J. Colloid Interface Sci. 142, 518, 1991). The time dependence of the drop radius and thickness obeys apparent power laws, with the exponents 0.126 ± 0.002 and −0.27 ± 0.01, respectively. In addition, we have determined the logarithmic prefactor L which characterizes the liquid-solid system in the calculation of the viscous dissipation, L = 2.6 ± 0.1. Thickness measurements are based on two new methods, one mechanical and one optical, which are described in detail.

X-ray evanescent wave-induced fluorescence study of polymer adsorption below the theta-point

X-ray evanescent wave-induced fluorescence (XEWIF) has been used to measure the monomer segment profile of poly(dimethylsiloxane) adsorbed from a dilute solution in bromocyclohexane to the solution/vapour interface. Rather than employment of a fluorescent label on the polymer chain, the bromine in the solvent provided the fluorescent label and sufficient intensity was observed to perform the experiments using a conventional X-ray source. The theta-point for this system has been reported as 29 °C and our measurements were performed at (23 ÷ 24) °C with no indication of bulk phase separation. Our results cannot be interpreted by scaling or mean-field models for polymer adsorption from a theta solvent. However, a step profile, in which a polymer-rich phase with polymer = 0.32 ± 0.02 extends (14 ± 3) nm into the bulk phase fits the data well. The step profile is consistent with the interpretation of prewetting near the solution critical point.