Biomedical subjects

R Holyst

Publications and source records attributed to R Holyst.

At least 19 recordsLinked to original sources

PubMed2005

Evaporation of a sub-micrometer droplet.

Evaporation of a spherically symmetric sub-micrometer size liquid droplet is studied using a diffuse interface hydrodynamic model supplemented by the van der Waals equation of state with parameters characteristic for argon. The droplet, surrounded by saturated vapor, is held in a container with the temperature of the walls kept fixed. The evaporation is triggered by a sudden rise of the temperature of the walls. Time and space evolution of the basic thermodynamic quantities is presented. The time and space scales studied range from picoseconds to microseconds and from nanometers to micrometers, respectively. We find that the temperature and chemical potential are both continuous at the interface on the scale larger than the interfacial width. We find that at long times the radius R of the droplet changes with time t as R(2)(t) = R(2)(0) - 2tkappa(v)(T(w) - T(l))/ln(l), where kappa(v) is the heat conductivity of the vapor, n(l) and T(l) are the density and the temperature of liquid inside the droplet, respectively, l is the latent heat of transition per molecule, and T(w) is the temperature of the ambient vapor.

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PubMed2005

Ordering in surfactant mixtures induced by polymers.

We studied ternary mixtures of nonionic surfactant (C12E6, n-dodecyl hexaoxyethylene glycol monoether), polymer (PEG, polyethylene glycol), and water. A small amount of PEG induces demixing into the polymer-rich and surfactant-rich phases in the ternary PEG/C12E6/water mixture. Above a certain concentration and/or molecular weight of PEG, the surfactant-rich phase orders, even in a solution consisting of a few percent of surfactant. The phase boundary acts as a semipermeable membrane, and the equilibrium is determined by the chemical potential of water in two phases. The explicit expression for the amount of PEG needed to order C12E6 water solution is given and verified experimentally. The analysis of the coexistence conditions leads to the conjecture that only two oxygen atoms in the outward part of the hydrophilic surfactant head strongly affect the chemical potential of water. Our methodology is generic, i.e., on the same basis one can design a similar experiment for any surfactant/polymer/water system and find the right proportions of polymer that induce order in a surfactant-rich phase.

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PubMed2000

Topological lifshitz line, off-specular scattering, and mesoporous materials

Ordered phases formed by surfactants in water solutions, and used in technological processes as templates for the synthesis of mesoporous materials, exhibit topological fluctuations. From the results of the Monte Carlo simulations of the lamellar phase we have established a relation between topological fluctuations and the behavior of the off-specular scattering intensity. We have defined the topological Lifshitz line. At this line the peak position in the off-specular scattering intensity moves from the zero (lamellar phase with fixed topology) to the nonzero value of the scattering wave vector (lamellar phase with fluctuating topology).

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PubMed2000

Coupling between meniscus and smectic-A films: circular and catenoid profiles, induced stress, and dislocation dynamics

In this paper we discuss the formation and shape of the meniscus between a free-standing film of a smectic-A phase and a wall (in practice the frame that supports the film). The wall may be flat or circular, and the system with or without a reservoir of particles. The formation of the meniscus is always an irreversible thermodynamic process, since it involves the creation of dislocations in the bulk (therefore it involves friction). The four basic shapes of meniscus discussed are the following: exponential, algebraic (x(3/2)), circular, and catenoid. Three principal regions of the whole meniscus must be distinguished: close to the wall with a high density of dislocations, away from the wall with medium density of dislocations, and far from the wall (i.e., close to the film) with a low density of dislocations (vicinal regime). The region with medium density of dislocations is observable using a microscope, and is determined by the competition between surface tension, energy of dislocations, and pressure difference set by the mass of the meniscus or by the reservoir. Its profile is circular as observed in recent experiments [J.-C. Geminard, R. Holyst, and P. Oswald, Phys. Rev. Lett. 78, 1924 (1997)]. By contrast, the vicinal regime with low density of dislocations is never observable with an optical microscope. In the regime with a high density of dislocations, the reasons why the dislocations tend to gather by forming giant dislocations and rows of focal conics are discussed. Finally, we discuss the stability of a smectic film with respect to the formation of a dislocation loop. We show experimentally that the critical radius of the loop is proportional to the curvature radius of the meniscus in its circular part, in agreement with the theory. In addition, we show that the mobility of edge dislocations measured in thick films is in agreement with that found in bulk samples from a creep experiment. This result confirms again our model of the meniscus.

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PubMed2000

Influence of the free-energy functional form on simulated morphology of spinodally decomposing blends

The spinodal decomposition of a binary mixture has been studied within several mesoscopic models. It has been found that the form of the equilibrium free energy has a crucial effect on the morphological development in asymmetric blends. We have shown that the principal quantity that determines the topology of the interface (and type of morphology) is the equilibrium minority phase volume fraction, while the transition from bicontinuous to droplet morphology can be treated as a percolation. The concentration dependence of the square gradient coefficient attributed for the Flory-Huggins-de Gennes free energy has no significant influences on the average domain growth, but can be distinguished experimentally from its constant-coefficient alternative by measuring the maximum wave vector of the scattering intensity as a function of the minority phase volume fraction for spinodally decomposing asymmetric blends. The concentration dependence of the Onsager coefficient has the weak, systematic effect of slowing down the morphological development. The local shape of the interface is not affected considerably by the concentration dependence of the square gradient and Onsager coefficient.

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PubMed1996