基礎講座 電気泳動(Electrophoresis)

The review deals with drop impacts on thin liquid layers and dry surfaces. The impacts resulting in crown formation are referred to as splashing. Crowns and their propagation are discussed in detail, as well as some additional kindred, albeit nonsplashing, phenomena like drop spreading and deposition, receding (recoil), jetting, fingering, and rebound. The review begins with an explanation of various practical motivations feeding the interest in the fascinating phenomena of drop impact, and the above-mentioned topics are then considered in their experimental, theoretical, and computational aspects.

Drop Impact Dynamics: Splashing, Spreading, Receding, Bouncing ...

Nanofluidics has emerged recently in the footsteps of microfluidics, following the quest for scale reduction inherent to nanotechnologies. By definition, nanofluidics explores transport phenomena of fluids at nanometer scales. Why is the nanometer scale specific? What fluid properties are probed at nanometric scales? In other words, why does ‘nanofluidics’ deserve its own brand name? In this critical review, we will explore the vast manifold of length scales emerging for fluid behavior at the nanoscale, as well as the associated mechanisms and corresponding applications. We will in particular explore the interplay between bulk and interface phenomena. The limit of validity of the continuum approaches will be discussed, as well as the numerous surface induced effects occurring at these scales, from hydrodynamic slippage to the various electro-kinetic phenomena originating from the couplings between hydrodynamics and electrostatics. An enlightening analogy between ion transport in nanochannels and transport in doped semi-conductors will be discussed (156 references).

http://ilm-perso.univ-lyon1.fr/~lbocquet/ChemSocRev-Bocquet-2010.pdf

Nanofluidics, from bulk to interfaces

A drop hitting a solid surface can deposit, bounce, or splash. Splashing arises from the breakup of a fine liquid sheet that is ejected radially along the substrate. Bouncing and deposition depend crucially on the wetting properties of the substrate. In this review, we focus on recent experimental and theoretical studies, which aim at unraveling the underlying physics, characterized by the delicate interplay of not only liquid inertia, viscosity, and surface tension, but also the surrounding gas. The gas cushions the initial contact; it is entrapped in a central microbubble on the substrate; and it promotes the so-called corona splash, by lifting the lamella away from the solid. Particular attention is paid to the influence of surface roughness, natural or engineered to enhance repellency, relevant in many applications.

https://hal.archives-ouvertes.fr/hal-01398138/document

Drop Impact on a Solid Surface

a time as one year after leaving the Rehabilitation Unit, and about one quarter were not in competitive jobs but were in sheltered employment. Only just over one quarter were still working in ordinary jobs. Their average wage was £8 I is. 6d. Furthermore if one compares the type of work these patients were able to perform there was a definite decline from their premorbid position. The social class grouping before and after rehabilitation was Class II, i-i, Class III, 9-3, Class IV, 4-6, and Class V, IO-I4. Indeed the authors report that 'even the least handicapped of these patients presented continuous problems . . .' and it is noted that such a programme requires special personnel and a great deal of work. Comparing these results with the extra cost in personnel, time, and effort (which could be directed elsewhere) a Doctor Beeching of the psychiatric services would probably scrap such a rehabilitation service before it even started. But is this the right way of looking at it? The authors point out that such a programme, if applied throughout the country, would affect about 6,ooo patients. If the failure rate were the same as in this experiment, about I,500 would be rescued from a disabled life in a mental hospital and once more returned to an at least partially useful and, one assumes, more satisfying life. Obviously more is involved here than mere economics. We were interested to read that as regards behaviour at the Rehabilitation Centre and during the follow-up year 'There were no outstanding differences' between the schizophrenic and the non-schizophrenic rehabilitees. It appears that the group of schizophrenics had difficulties in social adjustment which were even greater than those of work adjustment. '. . . the men concerned had greater difficulty in living outside hospital, than in working outside hospital. If, however, adequate arrangements are made to cater for these various needs, there seems to be every reason to expect that a small selected group of long stay schizophrenic patients can be successfully resettled in work.' The experiment and the report show the high standards we have come so confidently to expect from Dr. Wing and his colleagues, and the publication will be read with interest, not only by psychiatrists, but by all those concemed with rehabilitation problems of chronically disabled patients. J. HOENIG

https://oem.bmj.com/content/oemed/23/2/161.2.full.pdf

The Mechanics of Aerosols

The impact of a drop on a rough solid surface is studied experimentally using a shadowgraph method with a rapid CCD camera system. Depending upon the physical parameters of impact a drop either spreads or splashes. For water drops colliding with aluminum surfaces, our experimental results for the onset of splashing on rough surfaces agree with those of Stow and Hadfield (1). The experimental results obtained here for various liquids and surfaces are correlated by an empirical formula relating the minimum “critical” Weber number for splashing to the ratio of the drop radius to the surface roughness coefficient. This formula was proposed by Wu (2) under the assumption that viscous forces can be neglected. By using various mixtures of water and glycerin, the critical Weber number for splashing is found to be indeed independent of viscosity. The formula is however not universal since two coefficients have to be adjusted for each liquid/surface combination. The real shape of the surface also introduces some discrepancy. Perturbations have been observed on the rim of the spreading film. These perturbations appear for a minimum impact velocity and their number depends on surface roughness.

Influence of Surface Roughness on Liquid Drop Impact

Superhydrophobic surfaces reduce drag by combining hydrophobicity and roughness to trap gas bubbles in a microscopic texture. Recent work has focused on specific cases, such as arrays of pillars or grooves, with limited theoretical guidance. Here, we consider the experimentally relevant limit of thin channels and obtain rigorous bounds on the effective slip length for any two-component (e.g., low-slip and high-slip) texture with given area fractions. Among all anisotropic textures, parallel stripes attain the largest (or smallest) possible slip in a straight, thin channel for parallel (or perpendicular) orientation with respect to the mean flow. Tighter bounds for isotropic textures further constrain the effective slip. These results provide a framework for the rational design of superhydrophobic surfaces.

/pdf/effective-slip-over-superhydrophobic-surfaces-in-thin-tu6mtidu42.pdf

Effective slip over superhydrophobic surfaces in thin channels.

We report the results of direct measurements of velocity profiles in a microchannel with hydrophobic and hydrophilic walls, using a new high-precision method of double-focus spatial fluorescence cross correlation under a confocal microscope. In the vicinity of both walls the measured velocity profiles do not go to zero by supplying a plateau of constant velocity. This apparent slip is proven to be due to a Taylor dispersion, an augmentation by shear diffusion of nanotracers in the direction of flow. Comparing the velocity profiles near the hydrophobic and hydrophilic walls for various conditions shows that there is a true slip length due to hydrophobicity. This length, of the order of several tens of nanometers, is independent of the electrolyte concentration and shear rate.

/pdf/direct-measurements-of-hydrophobic-slippage-using-double-280o7aib78.pdf

Direct measurements of hydrophobic slippage using double-focus fluorescence cross-correlation.

The creeping flow around a sphere close to a wall is calculated by revisiting the bipolar coordinates solution technique The coefficients in the expansions of the spherical harmonics are easily calculated from two recurrence relationships, following the idea of O'Neill and Bhatt That is, there is no need to solve successively larger truncated linear systems to obtain the coefficients, as was done in earlier papers for a sphere translating along a wall by (O'Neill) or rotating by (Dean and O'Neill) and for a shear flow around a fixed sphere by (Tozeren and Skalak) Consequently, results can be obtained even for small gaps between the sphere and the wall without excessive computer resources Thus, the present technique provides a reference to obtain all the friction coefficients and flow fields with excellent precision Results for the friction coefficients are given with up to 17 significant digits for later reference

François Feuillebois

Papers

Influence of Surface Roughness on Liquid Drop Impact

Effective slip over superhydrophobic surfaces in thin channels.

Direct measurements of hydrophobic slippage using double-focus fluorescence cross-correlation.

Creeping flow around a sphere in a shear flow close to a wall

Freezing of a subcooled liquid droplet