Study of the visco-elastic properties of the air-water interfaces and detection of contamination
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.
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 nm2. 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).
导师：Abdelhamid Maali， DR2
https://www.loma.cnrs.fr/abdelhamid-maali/
联系方式：abdelhamid.maali@u-bordeaux.fr
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