arXiv subjects

Pierre-Gilles de Gennes

Publications and source records attributed to Pierre-Gilles de Gennes.

17 recordsLinked to original sources

arXiv2008

A spatial interpretation of emerging superconductivity in lightly doped cuprates

The formation of domains comprising alternating 'hole rich' and 'hole poor' ladders recently observed by Scanning Tunneling Microscopy by Kohsaka et al., on lightly hole doped cuprates, is interpreted in terms of an attractive mechanism which favors the presence of doped holes on Cu sites located each on one side of an oxygen atom. This mechanism leads to a geometrical pattern of alternating hole-rich and hole-poor ladders with a periodicity equal to 4 times the lattice spacing in the CuO plane, as observed experimentally. To cite this article: G. Deutscher, P.-G. de Gennes, C. R. Physique 8 (2007).

cond-mat.supr-con↗

arXiv2004

Damping of acoustic waves in dilute polymer solutions

The shear viscosity eta_s(w) of dilute polymer solutions (of flexible coils) has been classically measured and interpreted in terms of the Zimm modes. We point out that the longitudinal viscosity eta_l(w) is the sum of two components eta_l=4*eta_s/3+eta_p, where eta_p is due to an effect of pressure on the coil size. We discuss eta_p for a crude model involving a dependence of both the Kuhn length a and the Flory parameter chi on pressure. We find that eta_p is comparable to eta_s for good solvent conditions, but that eta_p should dominate over eta_s near the theta point.

cond-mat.soft↗

arXiv2004

Self Trapping of a Single Bacterium in its Own Chemoattractant

Bacteria (e.g. E. Coli) are very sensitive to certain chemoattractants (e.g. asparate) which they themselves produce. This leads to chemical instabilities in a uniform population. We discuss here the different case of a single bacterium, following the general scheme of Brenner, Levitov and Budrene. We show that in one and two dimensions (in a capillary or in a thin film) the bacterium can become self-trapped in its cloud of attractant. This should occur if a certain coupling constant $g$ is larger than unity. We then estimate the reduced diffusion D_eff of the bacterium in the strong coupling limit, and find D_eff ~ 1/g.

q-bio.PE↗

arXiv2003

Phase transitions of nematic rubbers

Single crystal nematic elastomers undergo a transition from a strongly ordered phase N to an "isotropic" phase I. We show that: (a) samples produced under tension by the Finkelmann procedure are intrinsically anisotropic and should show a small (temperature dependent) birefringence in the high temperature I phase. (b) for the I->Ntransition via cooling there is a spinodal limit but for the N->I transition via heating there is no soft mode at the standard spinodal temperature. (c) the N->I transition is reminiscent of a martensitic transformation: nucleation of the I phase should occur in the form of platelets, making a well defined angle with the director.

cond-mat.soft↗

arXiv2002

Surface flows of granular materials: A short introduction to some recent models

We present a short review of recent theoretical descriptions of flows occuring at the surface of granular piles, and focus mainly on two models: the phenomenological ``BCRE'' model and the hydrodynamic model, based on Saint-Venant equations. Both models distinguish a ``static phase'' and a ``rolling'' phase inside the granular packing and write coupled equations for the evolutions of the height of each of these phases, which prove similar in both approaches. The BCRE description provides a very intuitive picture of the flow, whereas the Saint-Venant hydrodynamic description establishes a general and rigorous framework for granular flow studies.

cond-mat↗

arXiv2002

A Scaling Theory of the Competition between Interdiffusion and Cross-Linking at Polymer Interfaces

We study theoretically situations where competition arises between an interdiffusion process and a cross-linking chemical reaction at interfaces between pieces of the same polymer material. An example of such a situation is observable in the formation of latex films, where, in the presence of a cross-linking additive, colloidal polymer particles initially in suspension come at contact as the solvent evaporates, and, optimally, coalesce into a continuous coating. We considered the low cross-link density situation in a previous paper (A. Aradian, E. Raphael, P.-G. de Gennes, Macromolecules 33, 9444 (2000)), and presented a simple control parameter that determines the final state of the interface. In the present article, with the help of simple scaling arguments, we extend our description to higher cross-link densities. We provide predictions for the strength of the interface in different favorable and unfavorable regimes, and discuss how it can be optimized.

cond-mat.soft↗

arXiv2001

On fluid/wall slippage

Certain (non polymeric) fluids show an anomalously low friction when flowing against well chosen solid walls. We discuss here one possible explanation, postulating that a gaseous film of small thickness h is present between fluid and wall. When h is smaller than the mean free path l of the gas (Knudsen regime) the Navier length b is expected to be independent of h and very large (microns).

cond-mat.soft↗

arXiv2001

Instabilities during the evaporation of a film: non glassy polymer + volatile solvent

We consider solutions where the surface tension of the solvent is smaller than the surface tension of the polymer. In an evaporating film, a plume of solvent rich fluid, then induces a local depression in surface tension, and the surface forces tend to strengthen the plume. We give an estimate (at the level of scaling laws) for the minimum thickness required to obtain this instability. We predict that the thickness a) is a decreasing function of the solvent vapor pressure b) should be very small (<1 micron) provided that the initial solution is rather dilute. The overall evaporation time for the film should be much longer than the growth time of the instability. The instability should lead to distortions of the free surface and may be optically observable. It should dominate over the classical Bernard-Marangoni instability induced by cooling.

cond-mat.soft↗

arXiv2001

Solvent evaporation of spin cast films: crust effects

When a glassy polymer film is formed by evaporation, the region near the free surface is polymer rich and becomes glassy first, as noticed long ago by Scriven et al. We discuss the thickness of this "crust" and the time interval where it is present -before freezing of the whole film. We argue that the crust is under mechanical tension, nd should form some cracks. This may be the source of the roughness observed on the final, dry films, when the solvent vapor pressure is high (and leads to thin crusts).

cond-mat.soft↗

arXiv2001

Maximum pull out force on DNA hybrids

We discuss theoretically the force F between two colloidal particles, each of them carrying one single strand DNA. The two strands are complementary only on a finite sequence of consecutive base pairs. We define an adjustment length (a few base pairs): in the adjustment regions near both ends of the paired region, the tension is still mainly on one single strand. But in the central part, the two backbones are equally loaded. This leads to a rupture force, increasing linearly with length, and saturating for large lengths.

physics.bio-ph↗

arXiv2001

"Marginal pinching" in soap films

We discuss the behaviour of a thin soap film facing a frame element: the pressure in the Plateau border around the frame is lower than the film pressure, and the film thins out over a certain distance lambda(t), due to the formation of a well-localized pinched region of thickness h(t) and extension w(t). We construct a hydrodynamic theory for this thinning process, assuming a constant surface tension: Marangoni effects are probably important only at late stages, where instabilitites set in. We find lambda(t) ~ t^{1/4}, and for the pinch dimensions h(t) ~ t^{-1/2}$ and w(t) ~ t^{-1/4}. These results may play a useful role for the discussion of later instabilitites leading to a global film thinning and drainage, as first discussed by K. Mysels under the name ``marginal regeneration''.

cond-mat↗

arXiv2000

"Young" soap films

If we pull out rapidly a metallic frame out of a surfactant solution, we arrive at a "young" soap film with relatively simple features, as noticed first by Lucassen. The weight of the film is equilibrated by a vertical gradient of surface tension. At each level, the local solution concentration equilibrates with the local monolayers. A detailed analysis of the young films was started by us in 1987. We present here an approach which is more illuminating a) the concentration profiles decay exponentially at large heights, with a characteristic length in the range of meters b) the surface is protected up to a different (larger) height c) we also review the dynamic requirements. The surfactant must reach the surface in a time shorter than the free fall time of a pure water film.

cond-mat.soft↗

arXiv2000

Two remarks on wetting and emulsions

This paper is extracted from an opening address given at the workshop "Wetting: from microscopic origins to industrial applications" (Giens, May 6-12, 2000). It discusses two special points a) the nature of line energies for a contact line b) the aging of emulsions.

cond-mat.soft↗

arXiv2000

Glass transitions of freely suspended polymer films

We present a modified version of our "sliding model", where chain arcs, between two contacts at the surface, may move if all the barriers along the arc are weaker than a certain threshold.An important advance of the revised model is that the high limiting chain lengths N* observed in the experiments are naturally accoundted for. In this model, a film of thickness h, smaller than the coil size, can show either a "sandwich" structure with two mobile sublayers (at low temperatures T T(h). But the accident occurring at T=T(h), does not necessarily coincide with the apparent glass transition, determined by the intersection of two tangents in a plot of thickness versus temperature.

cond-mat.soft↗

arXiv2000

Towards artificial muscles

Solid state actuators (piezoelectric, ferroelectric, ...) deform under an external field, and have found many applications. They respond fast, but their mechanical deformations are very small. There is a need for soft actuators, giving larger responses, but necessarily less fast. This paper describes the early attempts by Katchalsky and others, and the problems which showed up -related to time constants, and, most importantly, to fatigue. Two current attempts are reviewed.

cond-mat.soft↗

arXiv1999

Thick surface flows of granular materials: The effect of the velocity profile on the avalanche amplitude

A few years ago, Bouchaud al. introduced a phenomenological model to describe surface flows of granular materials [J. Phys. Fr. I, 4, 1383 (1994)]. According to this model, one can distinguish between a static phase and a rolling phase that are able to exchange grains through an erosion/accretion mechanism. Boutreux et al. [Phys. Rev. E, 58, 4692 (1998)] proposed a modification of the exchange term in order to describe thicker flows where saturation effects are present. However, these approaches assumed that the downhill convection velocity of the grains is constant inside the rolling phase, a hypothesis that is not verified experimentally. In this article, we therefore modify the above models by introducing a velocity profile in the flow, and study the physical consequences of this modification in the simple situation of an avalanche in an open cell. We present a complete analytical description of the avalanche in the case of a linear velocity profile, and generalize the results for a power-law dependency. We show, in particular, that the amplitude of the avalanche is strongly affected by the velocity profile.

cond-mat↗

arXiv1998

Surface Flows of Granular Mixtures: III. Canonical Model

We present the generalization of the minimal model for surface flows of granular mixtures, proposed by Boutreux and de Gennes [J. Phys. I France 6, 1295 (1996)]. The minimal model was valid for grains differing only in their surface properties. The present model also takes into account differences in the size of the grains. We apply the model to study segregation in two-dimensional silos of mixtures of grains differing in size and/or surface properties. When the difference in size is small, the model predicts that a continuous segregation appears in the static phase during the filling of a silo. When the difference in size is wide, we take into account the segregation of the grains in the rolling phase, and the model predicts complete segregation and stratification in agreement with experimental observations.

cond-mat.stat-mech↗