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Biomedical subjects

D Bonn

Publications and source records attributed to D Bonn.

At least 19 recordsLinked to original sources
PubMed2001

Crossover from first-order to critical wetting: short-range tricritical wetting.

We study wetting in liquid mixtures of methanol and the n-alkanes. Mixing alkanes of different chain lengths, we can examine the crossover between critical (continuous) and first-order (discontinuous) wetting transitions. Measurements of the film thickness and surface specific heat exponent indicate that for carbon number n between 11 (undecane) and 9 (nonane), there is a crossover from first-order to critical wetting with a tricritical wetting point between an effective alkane carbon number of 9.6 and 10. The observed variation of the specific heat exponent in the tricritical region agrees fairly well with the predictions of a simple mean-field model with only short-range interactions.

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PubMed2001

Aging dynamics in a colloidal glass.

The aging dynamics of colloidal suspensions of Laponite, a synthetic clay, is investigated using dynamic light scattering (DLS) and viscometry after a quench into the glassy phase. DLS allows to follow the diffusion of Laponite particles and reveals that there are two modes of relaxation. The fast mode corresponds to a rapid diffusion of particles within "cages" formed by the neighboring particles. The slow mode corresponds to escape from the cages: its average relaxation time increases exponentially fast with the age of the glass. In addition, the slow mode has a broad distribution of relaxation times, its distribution becoming larger as the system ages. Measuring the concomitant increase of viscosity as the system ages, we can relate the slowing down of the particle dynamics to the viscosity.

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PubMed2001

Wetting of biological lipids on aqueous substrates.

We study the dynamics and final wetting state of skin lipids on water and brine by fluorescence microscopy and ellipsometry. When a lipid droplet is brought into contact with the water surface, a lipid wetting film spreads out rapidly by a Marangoni effect. Subsequently, this film undergoes a dewetting instability. However, the final equilibrium is not partial wetting. The film breaks up into droplets with a mesoscopic (approximately 50 A) film in between. These observations result from a subtle interplay between short- and long-range forces: surfactants naturally present in the lipids favor wetting, while the van der Waals forces oppose it. In addition, this reveals the likely organization of the hydrolipid film that covers and protects the skin.

Lipids↗
PubMed2001

Inhibition of the finite-time singularity during droplet fission of a polymeric fluid.

When a drop of fluid detaches from a capillary, singular behavior ensues. We show that the addition of very small amounts of polymer inhibits this singularity in an abrupt way and gives rise, after a period of self-similar dynamics as for simple liquids, to long-lived cylindrical necks or filaments which thin exponentially in time. This abrupt change occurs when the elongation rate epsilon* becomes comparable to the inverse of the polymer relaxation time leading to a large elongational viscosity eta(E) of the dilute polymer solution.

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PubMed2001

Shear-induced first-order sponge-to-lamellar transition in a lyotropic surfactant system.

We report a shear-induced sponge (L3) to lamellar (L(alpha)) transition in a surfactant system. Under a constant shear rate, after a delay time t(n) we observe random nucleation and subsequent growth of the L(alpha) phase, demonstrating that the shear-induced transition is first order. A simple argument for the energy of a two-dimensional nucleus accounts for the observed delay and its shear-rate dependence.

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PubMed2000

First-order and critical wetting of alkanes on water

Ellipsometry measurements of the wetting behavior of different alkanes on water show a sequence of two wetting transitions: a first-order (discontinuous) transition followed by a critical (continuous) one. We report temperature-induced wetting transitions for different alkanes and a novel pressure-induced wetting transition for an alkane mixture. The experiments enable us to determine the global wetting phase diagram as a function of chain length and temperature which we subsequently calculate theoretically. The two transition lines are found to be approximately parallel, in accordance with basic theoretical arguments.

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PubMed2000

Viscous fingering in a yield stress fluid

We study the Saffman-Taylor or viscous fingering instability in yield stress fluids. The theory for yield stress fluids shows that the dispersion equation of the instability is similar to that for Newtonian fluids; however, the capillary number governing the instability now contains the yield stress. Experiments using gels and foams reveal very branched fingers in the gel. The results are in excellent agreement with theory for the gel, with, in addition, a crossover from yield stress dominated to viscous behavior. The results for foams are very different due to the existence of wall slip.

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PubMed2000

Controlling droplet deposition with polymer additives

Controlling the impact of drops onto solid surfaces is important for a wide variey of coating and deposition processes--for example, the treatment of plants with herbicides and pesticides requires precise targeting in order to meet stringent toxicological regulations. However, the outer wax-like layer of the leaves is a non-wetting substrate that causes sprayed droplets to rebound; often less than 50% of the initial spray is retained by the plant. Although the impact and subsequent retraction of non-wetting aqueous drops on a hydrophobic surface have been the subjects of extensive experimental and theoretical work, non-newtonian rheological effects have not been considered in any detail. Here we report that, by adding very small amounts of a flexible polymer to the aqueous phase, we can inhibit droplet rebound on a hydrophobic surface and markedly improve deposition without significantly altering the shear viscosity of the solutions. Our results can be understood by taking into account the non-newtonian elongational viscosity, which provides a large resistance to drop retraction after impact, thereby suppressing droplet rebound.

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