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gentle_boy铜虫 (小有名气)
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CSC博士-法国-物理
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法国波尔多大学与CSC合作协议 招收CSC攻读博士学位博士生 可以直接联系导生或者给我发邮件： 我的邮件：zhangzaicheng8911@163.com 谢谢 Electro-Hydrodynamic Lift Force Supervisors: Alois Wurger alois.wurger@u-bordeaux.fr Nanoscale flows of electrolytes in confined geometry are of primary importance for active matter, energy storage devices, harvesting of waste energy, desalinization, and actuation and signal detection in micromechanical systems. Surface properties are predominant for the flow behavior and the transport properties in these systems . The coupling of the diffuse layer of counter-ions to liquid flow is at the origin of various electro-kinetic and visco-electric effects, leading to several applications ranging from microfluidics to medical analysis. Electro-hydrodynamic coupling has been suggested as an alternative approach to lubrication, which depends on the non-equilibrium properties of the electric double layer. Indeed, advection of the diffuse layer results in an additional dynamic electric disjoining pressure, which may by far exceed the electrostatic repulsion and induce a normal force on a sphere moving along a surface. The velocity profile consists of a linear shear flow and a backward Poiseuille flow, as sketched in the figure for pure water. Since the velocity diminishes with radial distance, advection in the Poiseuille flow tends to accumulate counter-ions at the back side of the sphere, thus generating an electric field parallel to the surface and thus a lift force perpendicular to the velocity. Colloidal Probe Atomic Force Microscope (AFM) have shown their ability to probe accurately the electrostatic properties of surfaces, such as surface charge and potential, by measuring the equilibrium interaction force between confined Electrostatic Double Layers (EDL). They have also shown their unique power to unravel the hydrodynamics at solid/liquid interfaces. The main objective is a comprehensive study, both experimentally and theoretically, of the dynamic coupling between the electric double layer to fluid flow. More precisely, we will investigate the visco-elastic and electro-dynamic response to lateral oscillations V of the substrate, in particular we will measure the lift force perpendicular to the velocity V. Since the force depends on the square value of the velocity, it comprises a static component and one oscillating at twice the driving frequency. The former is obtained by measuring the time-averaged force. We will study both of these two components and their dependence versus distance, viscosity and salinity of the solutions and frequency of the lateral oscillations. |
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gentle_boy
铜虫 (小有名气)
- 应助: 0 (幼儿园)
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- 专业: 流体力学
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法国波尔多大学与CSC合作协议 招收CSC攻读博士学位博士生 可以直接联系导生或者给我发邮件： 我的邮件：zhangzaicheng8911@163.com 谢谢 Supervisors: Abdelhamid Maali abdelhamid.maali@u-bordeaux.fr It is well known that the presence of impurities (surfactants) affects the flow properties at liquid interfaces. Thus surfactant molecules adsorbed at an interface give rise to a surface shear viscosity; a concentration gradient that modifies the surface tension and leads to visco-elastic behavior [1]. On the other hand, contamination with surfactants alters the properties of superhydrophobic surfaces and in particular reduces the slip slength [2-4]. The dynamic force microscope AFM is a powerful tool to investigate structure and rheology of confined liquids, such as molecular ordering and slip, and its dynamic mode allows to separate viscous and elastic forces. Our preliminary work [5] is the first dedicated measurement on surfacted interfaces, giving clear evidence for a strong elastic component besides the viscous drag. For molecules which adsorb at the interface but are non-soluble in water, we found that diffusion along the interface is of little relevance, and that the elastic out-of-phase response is entirely due to advection. A) Sketch of the experimental apparatus. The liquid-gas interface is prepared by deposing a spherical bubble on a PS surface. A vibrating glass sphere glued at the end of the AFM cantilever is used to induce the hydrodynamic flow and measure the visco-elastic responses. B) Example of our preliminary measurements of the viscoelastic component of the force versus the frequency of the cantilever vibration. From these data we extract the impurity concentration of one molecule per 20 nm 2 . We intend to investigate whether our approach could be used for the detection of contamination of interfaces by impurities, in particular non-fluorescent ones, or as a novel low-volume tensiometry device. Regarding impurity detection, in our preliminary study [5] we obtained a strong signal at a surfactant coverage of one molecule per 20 nm 2 . By optimizing the parameters and modifying the frequency range, we hope to push the sensitivity of our apparatus to much lower concentrations. At low impurity coverage, the tension change is proportional to the surfactant concentration. Regarding tensiometry, the AFM setup operates locally on the micro-scale, thus requiring much smaller liquid volumes than usual Wilhelmy plate devices (in the range of milliliters). As two possible routes we will investigate the effect of impurities on the visco-elastic response, and on the thermal-noise power spectrum of an AFM cantilever in contact with the interface. As main result, we will determine the resolution of the method and the range of parameters where it works efficiently, and compare with existing techniques. Our preliminary results suggest that the solubility could be an important parameter distinguishing among amphiphilic molecules, soap molecules from phospholipids or steroid alcohols. For molecules with low critical micelle concentration (cmc) which adsorb at the interface, we expect a signal quite different from that of highly soluble molecules which also diffuse in the water film. [1] Y. Amarouchene, G. Cristobal, and H. Kellay, Phys. Rev. Lett. 87, 206104 (2001). [2] G. Bolognesi, C. Cottin-Bizonne, C. Pirat, Physics of Fluids 26, 082004 (2014). [3] D. Schäffel, K. Koynov, D. Vollmer, H-J. Butt, C. Schönecker, PRL 116, 134501, (2016) [4] O. Manor et al., Phys. Rev. Lett. 101, 024501 (2008); [5] A. Maali, R. Boisgard, H. Chraibi, Z. Zhang, H. Kellay, A. Würger, PRL 118, 084501 (2017). |
2楼2020-01-04 03:35:04
gentle_boy
铜虫 (小有名气)
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- 专业: 流体力学
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法国波尔多大学与CSC合作协议 招收CSC攻读博士学位博士生 可以直接联系导生或者给我发邮件： 我的邮件：zhangzaicheng8911@163.com 谢谢 Thomas Salez Supervisors: Thomas Salez thomas.salez@u-bordeaux.fr@u-bordeaux.fr What determines phenomena as varied as: the shape of a soap bubble, the spreading of a drop of dew on a leaf, the dewetting of a Teflon pan, the “tears” of wine, the meniscus around a fishing line, or the difficulty in extracting certain oils from the soil? Capillarity. This force which tends to minimize the interfaces and whose origin is microscopic. Indeed, the molecules of a liquid have a certain affinity between them, and attract each other thus ensuring the cohesion of the whole. But now, add a boundary an interface - and the molecules become less surrounded by their peers and thus frustrated! The net result is a macroscopic force, surface tension, tending to reduce the interfaces. If capillarity seems to be the prerogative of fluid interfaces at small scales (where it outweighs other forces such as gravity), the interfaces between solids and fluids also have an energy cost, which controls for instance the wettability of a given solid material. However, there is an essential difference between a solid and a liquid: the latter can rearrange under stress unlike a rigid crystal. Consequently, if a crystalline solid is stretched, its constituents can be “diluted” and the surface tension modified, which is impossible with an incompressible liquid. This effect, predicted by Shuttleworth in the 1950s, was extensively studied later. There is however another class of solids. Non-crystalline solids, called amorphous solids. These are more and more studied since the rise of soft matter. They can be so soft (cosmetic and food gels or vesicles for example) that they sometimes behave like liquids and their surface tension outweighs their bulk elasticity. In addition, they are the source of a lively controversy following a pioneering experiment and an international conference in Leiden, the Netherlands, in 2015. In this emerging community, dedicated to the capillarity of soft solids, the existence of a Shuttleworth effect for amorphous materials is extensively debated due to the physico-chemical complexity and dual nature of soft matter. It therefore seems crucial to test these ideas by independent experiments and models. Ultimately, one could perhaps imagine controlling the wetting properties of a plastic material by simply stretching it! From fog nets, or intelligent textiles, to surface treatments, the number of potential applications is extremely large. But besides this, and the possible resolution of a lively controversy, this project could give us further fundamental insights about the fascinating properties of amorphous and soft matter. Within this context, our group at LOMA, University of Bordeaux, has developed a growing expertise in precise atomic force microscope (AFM) measurements, combined with applied mathematics and physical modeling of those intriguing questions. Our goal in the context of this controversy is to shed new light on the Shuttleworth enigma in soft matter, by providing a combination of direct AFM observations and theoretical modelling at the nanoscale. Specifically, the student will use the colloidal-probe techniques to measure the interaction forces between soft samples and a colloidal sphere glued at the tip of the AFM cantilever. We will investigate different substrate’s rigidities (Young’s moduli), and also different sizes of colloids, in order to make the contribution of the surface tension larger than bulk elasticity. The AFM will be operated either in static mode to measure directly the force, and thus the response of the samples, or in dynamic mode where the interaction force is deduced from the measurements of the amplitude and phase of the cantilever’s vibrations. The theoretical modelling will involve a modification of the soft-lubrication framework to take strain-dependent capillarity into account. |
3楼2020-01-04 03:37:22
Aaronswf1t
新虫 (正式写手)
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4楼2020-01-04 05:35:58
gentle_boy
铜虫 (小有名气)
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5楼2020-01-17 04:49:08
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