Mechanical Characterization of Ultralow Interfacial Tension Oil-in-Water Droplets by Thermal Capillary Wave Analysis in a Microfluidic Device

Air/Water Interface Rheology Probed by Thermal Capillary Waves

Atomic force microscopy (AFM) was used to study the interfacial rheology of air/water interfaces by investigating the thermal capillary fluctuations of surfactant-loaded interfaces. These interfaces are formed by depositing an air bubble on a solid substrate immersed in a surfactant (Triton X-100) solution. An AFM cantilever, in contact with the north pole of the bubble, probes its thermal fluctuations (amplitude of the vibration versus the frequency). The measured power spectral density of the nanoscale thermal fluctuations presents several resonance peaks corresponding to the different vibration modes of the bubble. The measured damping versus the surfactant concentration of each mode presents a maximum and then decreases to a saturation value. The measurements are in good agreement with the model developed by Levich for the damping of capillary waves in the presence of surfactants. Our results show that the AFM cantilever in contact with a bubble is a powerful tool to probe the rheological properties of air/water interfaces.

Flow pumping by external periodic shear applied to a soft interface

Flow pumping in viscous fluids is of prime importance in micro-fluidic applications. Here we show that a single colloidal particle in front of a soft wall, manipulated by external means like an optical tweezer, can pump the ambient viscous fluid. The particle, moving back and forth parallel to the soft wall, can produce an averaged net flow in a direction perpendicular to the wall. Using a perturbative scheme, we present the results. Analysis show that this flow in terms of capillary number, scales as [Formula: see text].

Near-Field Probe of Thermal Fluctuations of a Hemispherical Bubble Surface

We report measurements of resonant thermal capillary oscillations of a hemispherical liquid gas interface obtained using a half bubble deposited on a solid substrate. The thermal motion of the hemispherical interface is investigated using an atomic force microscope cantilever that probes the amplitude of vibrations of this interface versus frequency. 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 pinned on the substrate. The analysis of these peaks allows us to measure the surface viscosity of the bubble interface. Minute amounts of impurities are responsible for altering the rheology of the pure water surface.

Using Faraday Waves to Measure Interfacial Tension

We use Faraday waves to measure interfacial tension σ between two immiscible fluids, with an interest in (ultra)low values of σ. The waves are excited by vertically oscillating the container in which the fluids reside. Using linear stability theory, we map out the accessible range of interfacial tensions. The smallest value (σ_min ≈ 5 × 10^-4 N/m) is limited by the joint influence of gravity and viscous dissipation. A further limitation is posed by the greatest accelerations that can be realized in a laboratory. We perform experiments on a water-dodecane interface with an increasing concentration of a surfactant in the water layer that decreases the interfacial tension into the ultralow domain [σ = [Formula: see text](10^-6 N/m)]. Surprisingly, the smallest measured wavelength is larger by a factor of 2 than that predicted for vanishing σ. We hypothesize the effect of transport of the surfactant in the fluid flow associated with the waves.