Surface shear inviscidity of soluble surfactants

Abstract

Foam and emulsion stability has long been believed to correlate with the surface shear viscosity of the surfactant used to stabilize them. Many subtleties arise in interpreting surface shear viscosity measurements, however, and correlations do not necessarily indicate causation. Using a sensitive technique designed to excite purely surface shear deformations, we make the most sensitive and precise measurements to date of the surface shear viscosity of a variety of soluble surfactants, focusing on SDS in particular. Our measurements reveal the surface shear viscosity of SDS to be below the sensitivity limit of our technique, giving an upper bound of order 0.01 μN·s/m. This conflicts directly with almost all previous studies, which reported values up to 10 3 -10 4 times higher. Multiple control and complementary measurements confirm this result, including direct visualization of monolayer deformation, for SDS and a wide variety of soluble polymeric, ionic, and nonionic surfactants of high-and low-foaming character. No soluble, small-molecule surfactant was found to have a measurable surface shear viscosity, which seriously undermines most support for any correlation between foam stability and surface shear rheology of soluble surfactants.S urfactants facilitate the formation of foams and emulsions by reducing surface tension, thereby lowering the energy required to create excess surface area (1-3). These multiphase materials, however, are thermodynamically unstable, and coarsen through bubble or drop coalescence, as well as diffusive exchange between bubbles or drops (1, 4-6). Surfactants can additionally be used to control this coarsening rate, with effective foaming surfactants retarding coalescence, and defoamers speeding it. For example, coalescence may be slowed by repulsive forces between the surfactant monolayers adsorbed to either side of the (continuous) phase separating bubbles or drops. Ionic surfactants, for example, introduce electrostatic repulsions (1, 2, 5), whereas nonionic surfactants (e.g., polymers, proteins, or particles) provide steric barriers against coalescence (7-9). Moreover, Marangoni stresses arise when compressional or dilatational deformations drive gradients in surfactant concentration (and thus surface tension). The resulting dilatational surface elasticity resists surface area changes, slowing drainage and rupture of the thin fluid films between adjacent bubbles (4, 5, 10-13).Additionally, surfactant monolayers may exhibit nontrivial rheological responses. For example, the surface shear viscosity η S gives the excess viscosity associated with shearing deformations within the 2D surfactant monolayer. Because surfactant interfaces are inherently compressible, they may exhibit a surface dilatational viscoelasticity η D *, in addition to η S *, even under small-amplitude deformations. This contrasts with incompressible Newtonian liquids, which are well-described by a single scalar viscosity. Moreover, surface shear and dilatational viscosities need not have equal (14), or even compara...

Influence of surface viscosity on droplets in shear flow

“…In other soft matter particles with surface viscosity, such as protein covered droplets, vesicles and red blood cells, shear Boussinesq numbers Bq s = O(1) − O(10) are quite reasonable (Chang & Olbricht 1993;Erni 2011). The recent work of Zell et al (2014) on shear surface viscosity of surfactants shows that µ s < 10 −2 µN · s m , in contrast to previous studies that proposed values several orders of magnitude larger (Dickinson et al 1988;Harvey et al 2005). Even for relatively small droplets and a low bulk viscosity, this suggests that Bq s < 1.…”

“…Mun & McClements (2006) measured a high dilational modulus of SDS-chitosan interface for example. Second, recent results by Zell et al (2014) cast doubt on previous experimental measurements of shear surface viscosity and then on conclusions drawn from these measurements on emulsion stability. As briefly presented, the question of the measurement of surface viscosities is still under progress and debate.…”

Surface viscosity and Marangoni stresses at surfactant laden interfaces

“…These factors make it extremely difficult to determine quantitatively the origin of the force on the probe, and make it extremely difficult, if not impossible, to infer values of the surface viscosities from experimental measurements of the force. Clearly, a translating probe with the shape of a disk is not at all optimal as a probe for interfacial viscosities, and one may infer that the same would be true for any probe that produces a mixed-type flow at the interface and therefore that methods that produce either pure shear (Zell et al 2014) or dilatation (Kotula & Anna 2015) should be preferred. Admittedly, we have only considered specialized cases where the surfactant concentration is nearly uniform, and numerical methods would be needed to solve for more general cases.…”

“…One resolution to this issue, following practices developed in rheology, is to probe the interface in a way that produces a pure shear deformation. In fact, recent experimental studies using a rotating microdisk at an interface (Zell et al 2014) show that this device produces a pure shear flow, and does not generate surfactant gradients that might otherwise complicate the interpretation via the presence of Marangoni contributions to the torque. Measurements with the soluble surfactant SDS showed that it has a surface viscosity that is immeasurably small (corresponding to η s 10 −8 N s m −1 ) with ≈10 µm probes, even though other protocols reported values 10 3 -10 4 times higher (Zell et al 2014).…”

Pressure-dependent surface viscosity and its surprising consequences in interfacial lubrication flows

“…DPPC forms a liquid condensed phase above a critical surface pressure (∼8 mN/m at room temperature), whose surface viscosity grows exponentially with [26][27][28], with surface pressure scale c measured to be ∼6-8 mN/m. By contrast, eicosanol monolayers pressure-thin, as their tail groups undergo a tilt-untilt transition (from an L2 phase to an LS I phase [29]) as surface pressure increases, and η s drops 10-fold as increases from ∼8-15 mN/m for c ∼ 3 mN/m, above which the surface viscosity plateaus [11,25].…”