30 Octobre – Thesis defense - Zaicheng Zhang

14 h Room 215 - LOMA / Building 4N (Talence campus)

Nano-rheology at soft interfaces probed by atomic force microscope.

Recent progresses in experimental and theoretical studies have shown that the liquid flow at micro/nano scale behaves differently from that at macroscale. At microscale, surface properties are predominant for the flow behavior at the boundaries. For high confinement, not only the physico-chemistry of the confining surfaces are important, their elastic behavior should also be taken into account.  In this thesis, we used the dynamic colloidal AFM to probe the confined flow at soft surfaces (Air bubbles and PDMS samples) and we have shown that:
• At the air-water interface, the presence of surfactant impurities modifies the flow near the interfaces in a drastic manner, which leads to the viscoelastic responses. The viscous and elastic forces acting on the sphere are extracted from the measurement of the sphere motion. Due to the surfactant contamination, the viscous force shows a crossover from non-slip to full slip boundary conditions and the elastic force also appears with a comparable value to viscous force.
• At small distance, the viscous pressure induced by the colloidal probe vibration deforms the bubble surface and gives rise to the visco-capillary interaction. Thermal noise excitation or external acoustic excitation are used to drive the AFM probe. To explain our measurements, we have developed a simplified model based on a spring-dashpot in series and we have also performed numerical solution of the Navier-Stokes equation combined with Laplace-Young equation.  Fitting our experimental results allow us to measure the surface tension of bubble interface without contact.
• The AFM cantilever is a powerful tool to probe the thermal motion of the hemispherical bubble interface. The spectrum of such nanoscale thermal oscillations of the bubble surface presents several resonance peaks and reveals that the contact line of the hemispherical bubble is fixed on the substrate. The surface viscosity of the bubble interface due to the surfactant contamination is obtained from the analysis of these peaks.
• An elastohydrodynamic lift force is acting on a sphere moving near and along a soft substrate within a viscous liquid. The lift force is probed as a function of the gap size, for various driving velocities, liquid viscosities, and sample stiffnesses.  At large distance, the experimental results are in excellent agreement with a model developed from the soft lubrication theory. At small gap distance, a saturation of the lift force is observed and a scaling law for this saturation is given and discussed.

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