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...

Adaptive materials with rapidly controllable and switchable energy-absorption and stiffness properties have a number of potential applications. We have developed, characterized and modeled a class of adaptive energy-absorbing systems consisting of nonlinear poroelastic composites wherein a field-responsive fluid, such as a magnetorheological fluid or a shear-thickening fluid, has been used to modulate the mechanical properties of a cellular solid. The mechanical properties and energy-absorbing capabilities of the composite are studied for variations in design parameters including imposed field strength, volume fraction of the field-responsive fluid within the composite and impact strain rates. The total energy absorbed by these materials can be modulated by a factor of 1- to 50-fold for small volume fractions of the fluid (∼15%) using moderate magnetic fields varying from 0 to 0.2 T. A scaling model is also proposed for the fluid–solid composite mechanical behavior that collapses experimental data onto a single master curve. The model allows optimization of the composite properties in tune with the application requirements. Potential application areas are discussed with emphasis on applicability in impact-absorbing headrests and cushioned assemblies for energy management.

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A novel, rapid, parallel, and high-throughput system for measuring viscosity of materials under different conditions of shear rate, temperature, time, etc., has been developed. This unique system utilizes the transient flow of a complex fluid through pipettes. This approach offers significant practical advantages over microfluidic-based devices for viscosity screening: no cleanup is required, the method is high throughput (<1 h for 100 samples), and only small sample volumes (<1 mL) are used. This paper details for the first time the experimental and modeling efforts to implement this mass- and pressure-based viscosity measurement concept as a robust viscosity estimation tool. This approach is very well-suited for viscosity measurements in high-throughput formulation workflows, as it is rapid and parallel and operates directly on samples in various microtiter plate formats. We present systematic experimental observations together with numerical and analytical modeling approaches to characterize instrument capabilities and limitations. The complex transient flow of fluids through these pipettes leads to data-rich pressure profiles. Numerical and analytical modeling is then used to extract viscosity and other rheological parameters from these pressure profiles. We have successfully utilized this viscosity screening tool for a multitude of complex fluids including oils, paints, solvents, and detergents.