A moving contact line as a rheometer for nanometric interfacial layers

International audienceHow a liquid drop sits or moves depends on the physical and mechanical properties of the underlying substrate. This can be seen in the hysteresis of the contact angle made by a drop on a solid, which is known to originate from surface heterogeneities, and in the slowing of droplet motion on deformable solids. Here, we show how a moving contact line can be used to characterize a molecularly thin polymer layer on a solid. We find that the hysteresis depends on the polymerization index and can be optimized to be vanishingly small (o0.07°). The mechanical properties are quantitatively deduced from the microscopic contact angle, which is proportional to the speed of the contact line and the Rouse relaxation time divided by the layer thickness, in agreement with theory. Our work opens the prospect of measuring the properties of functionalized interfaces in microfluidic and biomedical applications that are otherwise inaccessible

Water at the cavitation limit: Density of the metastable liquid and size of the critical bubble

The ability of a liquid to sustain mechanical tension is a spectacular manifestation of the cohesion of matter. Water is a paradigmatic example, because of its high cohesion due to hydrogen bonds. The knowledge of its limit of rupture by cavitation can bring valuable information about its structure. Up to now, this limit has been obscured by the diversity of experimental results based on different physical measures of the degree of metastability of the liquid. We have built a fiber optic probe hydrophone to provide the missing data on the density of the liquid at the acoustic cavitation limit. Our measurements between 0 and 50 °C allow a clear-cut comparison with another successful method where tension is produced in micron-sized inclusions of water in quartz. We also extend previous acoustic measurements of the limiting pressure to 190 °C, and we consider a simple modification of classical nucleation theory to describe our data. Applying the nucleation theorem gives the first experimental value for the size of the critical bubble, which lies in the nanometer range. The results suggest the existence of either a stabilizing impurity in the inclusion experiments, or an ubiquitous impurity essential to the physics of water

Stability of angular confinement and rotational acceleration of a diatomic molecule in an optical centrifuge

Modern femtosecond technology can be used to create laser pulses that induce controlled spinning of anisotropic molecules to very high angular momentum states ("optical centrifuge"). In this paper we extend our previous study [M. Spanner and M. Ivanov, J. Chem. Phys. 114, 3456 (2001)] and focus on the stability of angular trapping and forced rotational acceleration of a diatomic molecule in an optical centrifuge. The effects of laser intensity modulations and rovibrational coupling are analyzed in detail, classically and quantum mechanically. The numerical simulations show excellent qualitative agreement between the quantum and classical systems. Forced rotations of the classical system can exhibit chaotic behavior, which becomes rather unique when the accelerating rotation of the angular trapping potential combines with efficient rovibrational coupling. In this regime the Lyapunov exponent becomes time-dependent and the trajectories separate as exp(lambdaF(t)).Peer reviewed: YesNRC publication: Ye

Exploring water and other liquids at negative pressure

International audienceWater is famous for its anomalies, most of which become dramatic in the supercooled region, where the liquid is metastable with respect to the solid. Another metastable region has been hitherto less studied: the region where the pressure is negative. Here we review the work on the liquid in the stretched state. Characterization of the properties of the metastable liquid before it breaks by nucleation of a vapour bubble (cavitation) is a challenging task. The recent measurement of the equation of state of the liquid at room temperature down to 26 MPa opens the way to more detailed information on water at low density. The threshold for cavitation in stretched water has also been studied by several methods. A puzzling discrepancy between experiments and theory remains unexplained. To evaluate how specific this behaviour is to water, we discuss the cavitation data on other liquids. We conclude with a description of the ongoing work in our groups