Imaging the effects of peptide bio-surfactants on droplet deformation in a Taylor–Couette shear cell

Abstract

Controlling the properties of fluid–fluid interfaces is important in many fields including oil recovery, waste water treatment, food processing and pharmaceutical formulation. A fascinating new group of peptide bio-surfactants (AM1 and AFD4) have recently received attention because of their ability to provide a tunable change between a mechanically strong cohesive film and a mobile detergent-like non-film state via various stimuli. We investigate the effect of these peptide bio-surfactants coupled with a ZnSO₄ solution on the deformation of a single immiscible droplet (36 wt% toluene, 64 wt% chloroform) suspended in glycerol inside a wide-gap Taylor–Couette system rotated from 0 to 2 rev s⁻¹ (〈〉 = 0 to 9.7 s⁻¹). This deformation is studied using a novel rapid nuclear magnetic resonance imaging method (ROTACOR) which compensates for image blurring due to the rotation of the droplet inside the Taylor–Couette system. The peptide bio-surfactants are observed to reduce the deformation of the droplets under these sheared conditions, whereas by comparison a droplet with a non-ionic surfactant (Tween 60) present shows increased deformation. Static interfacial tension measurements confirmed that all surfactants reduced the interfacial tension of the system. An addition of EDTA to the AM1 peptide bio-surfactant system resulted in an increase in droplet deformation whereas static interfacial tension measurements were unaffected. This is consistent with the bio-surfactants forming a mechanically strong cohesive interfacial layer which reduces droplet deformation in the presence of shear despite also reducing the interfacial tension as measured conventionally using static droplet shape analysis.