Biomedical subjects

P-G de Gennes

Publications and source records attributed to P-G de Gennes.

7 recordsLinked to original sources

PubMed2006

Motions induced by asymmetric vibrations. The solid/solid case.

We discuss theoretically the motions of a coin on a horizontally vibrating plate, with dry friction between the coin and the plate. As first noticed by Daniel and Chaudhury in a different situation (droplets on a plate), when the periodic acceleration gamma(t) imposed by the plate is unsymmetrical, the coin can move macroscopically with a certain drift velocity V. We analyse here: (a) the vibration threshold below which V=0, and the generic behavior expected just above threshold; (b) the limiting behavior at very high amplitudes, where V should become independent of the amplitude; (c) the complications due to small, macroscopic inhomogeneities on the supporting plate.

Journal Article↗

PubMed2005

Wetting fibers with liposomes.

Giant unilamellar vesicles (GUVs) are deposited on glass microfibers. The vesicles adopt the classical "onduloidal" shape of liquid droplets on fibers. They spread by two simultaneous mechanisms: envelopment and emission of a precursor film. This film spreads faster than on a uniform plane surface and eventually stops, signaling the presence of defects on the rod. This fast spreading tenses the vesicles; transient pores open on the GUVs and the internal liquid leaks out. This process leads to a new technique for fiber coating.

Liposomes↗

PubMed2005

Vibration-actuated drop motion on surfaces for batch microfluidic processes.

When a liquid drop is subjected to an asymmetric lateral vibration on a nonwettable surface, a net inertial force acting on the drop causes it to move. The direction and velocity of the drop motion are related to the shape, frequency, and amplitude of vibration, as well as the natural harmonics of the drop oscillation. Aqueous drops can be propelled through fluidic networks connecting various unit operations in order to carry out batch processing at the miniature scale. We illustrate the integration of several unit operations on a chip: drop transport, mixing, and thermal cycling, which are precursor steps to carrying out advanced biological processes at microscale, including cell sorting, polymerase chain reaction, and DNA hybridization.

Journal Article↗

PubMed2005

Semiflexible polymers confined in soft tubes.

We discuss various conformations for a polymer (of persistent length l(p)) confined into a deformable tube (the wall being a lipid bilayer with a certain surface tension sigma and curvature energy K). Our study assumes that there is no adsorption of the chain on the wall. Three states are compared: (a) an unperturbed tube, plus a confined chain, (b) a tube swollen in all the region surrounding the chain (similar to a snake eating a sausage), (c) a globule, a roughly spherical coil surrounded by a strongly deformed tube. We construct a (qualitative) phase diagram for these systems with two variables: the surface tension sigma and the degree of polymerization N. Our main conclusion is that "globules" usually win over "snakes".

Adsorption↗

PubMed2004

Organization of a primitive memory: olfaction.

I present a tentative, and schematic, picture of the olfaction storage system in early mammalians: (i) how the excitation trains from the olfactory bulb excite selectively N types of cells in the piriform cortex and (ii) how these cells transfer their information to a storage area. Simple size and connectivity requirements impose that each odor is stored in a very small cluster of approximately 3 neurons. This surprising result holds even if the model parameters are strongly altered (e.g., if N = 10(3) rather than 10(2)).

Animals↗

PubMed2004

Chemotaxis: the role of internal delays.

When exposed to certain chemoattractants, bacteria like Escherichia coli move up the concentration gradient[Delta inverted]c with a velocity kappa[delta inverted]c. Microscopically, E. coli moves at constant speed when it's flagellum is rotating counter-clockwise (ccw) and tumbles when the rotation is clockwise (cw). The lifetime of a ccw interval, tau+, is a function of the concentration c( t') experienced at earlier times. The corresponding response function was measured long ago by Berg and co-workers. We present here a detailed description of the motion taking place during one ccw interval. This gives an explicit formula relating the chemotactic coefficient kappa to the response function; the formula has some surprising features.

Biological Clocks↗

PubMed2004

Frustrated rotations in nematic monolayers.

Tabe and Yokoyama found recently that the optical axis in a chiral monolayer of a ferronematic rotates when water evaporates from the bath: the chiral molecules act as propellers. When the axis is blocked at the lateral walls of the trough, the accumulated rotation inside creates huge splays and bends. We discuss the relaxation of these tensions, assuming that a single dust particle nucleates disclination pairs. For the simplest geometry, we then predict a long delay time followed by a non-periodic sequence of "bursts". These ideas are checked by numerical simulations.

Journal Article↗