Stable oil/water emulsions are usually obtained by using mixtures of different surfactants. Such systems display synergistic interface stabilizing effects, which have not been fully elucidated yet. Moreover, in many applications surfactants are added at concentrations well above their critical micellar concentration (CMC), and this regime has not been thoroughly explored in the literature as well. Here, we investigate oil/water emulsions through oil/water interfacial tension using two common non-ionic surfactants, Tween 80 and Span 20, in the concentration range C (0.3–1 wt%) well above their respective CMCs. Mesoscale molecular simulations coupled interfacial tensiometry experiments to characterise these interfaces at a molecular level. Interfacial tension γ was measured by a pendant drop technique. Coarse-grained calculations provided a microscopic view of the interface at the molecular level (i.e. surfactant arrangement, interface thickness), and were employed to extend the study to those surfactant concentrations where experiments could hardly provide reliable data, if any. We found a significant synergistic effect between Tween 80 and Span 20, with low molecular weight Span molecules occupying free spaces between the much larger, bulky Tween compounds. The surfactant intermolecular interactions could be associated to a striking decrease of interfacial tension in going from pure surfactants to a mixture at the same total weight concentration. Furthermore, the interface was found to exhibit a spatial inhomogeneity with a “patch-like” organisation, reminiscent of microphase separation. Our results show that the proposed, combined experimental/in silico approach provides relevant insights for several industrial applications, such as emulsion stability and oil spill remediation.

In this work, a microfluidic system to investigate the flow behavior of red blood cells in a microcirculation-mimicking network of PDMS microchannels with thickness comparable to cell size is presented. We provide the first quantitative description of cell velocity and shape as a function of the applied pressure drop in such devices. Based on these results, a novel methodology to measure cell membrane viscoelastic properties in converging/diverging flow is developed, and the results are in good agreement with data from the literature. In particular, in the diverging channel the effect of RBC surface viscosity is dominant with respect to shear elasticity. Possible applications include measurements of cell deformability in pathological samples, where reliable methods are still lacking.

Recently, optical tweezing has been used to provide a method for microrheology addressed to measure the rheological properties of small volumes of samples. In this work, we corroborate this emerging field of microrheology by using these optical methods for the characterization of polyelectrolyte solutions with very low viscoelasticity. The influence of polyelectrolyte (i.e., polyacrylamide, PAM) concentration, specifically its aging, of the salt concentration is shown. The close agreement of the technique with classical bulk rheological measurements is demonstrated, illustrating the advantages of the technique.