Abstract
Complex interfaces stabilized by proteins, polymers or nanoparticles, have a much richer dynamics than those stabilized by simple surfactants. By subjecting fluid-fluid interfaces to step extension-compression deformations, we show that in general these complex interfaces have dynamic heterogeneity in their relaxation response that is well described by a Kohlrausch-Williams-Watts function, with stretch exponent β between 0.4–0.6 for extension, and 0.6–1.0 for compression. The difference in β between expansion and compression points to an asymmetry in the dynamics. Using atomic force microscopy and simulations we prove that the dynamic heterogeneity is intimately related to interfacial structural heterogeneity and show that the dominant mode for stretched exponential relaxation is momentum transfer between bulk and interface, a mechanism which has so far largely been ignored in experimental surface rheology. We describe how its rate constant can be determined using molecular dynamics simulations. These interfaces clearly behave like disordered viscoelastic solids and need to be described substantially different from the 2d homogeneous viscoelastic fluids typically formed by simple surfactants.
Highlights
The behavior of complex interfaces, such as those found in living cells, is not well understood, unlike fluid-fluid interfaces stabilised with synthetic low molecular weight components, of which the dynamics are understood in great detail
We show, using step dilatational experiments, imaging with atomic force microscopy (AFM), and computer simulations that the structure of interfaces in soft interface-dominated materials (SIDMs) is far more complex, and in general highly heterogeneous
We identify the dominant relaxation mode involved in this behavior and show how its rate constant can be determined using molecular dynamics simulations
Summary
The behavior of complex interfaces, such as those found in living cells, is not well understood, unlike fluid-fluid interfaces stabilised with synthetic low molecular weight components, of which the dynamics are understood in great detail. The growing interest in bio-nanotechnology and biomimetic systems (e.g. artificial cells), has spurred substantial research on interface stabilization with nanoparticles (Pickering stabilization), protein (-complexes), or polymers When stabilized by such materials, the macroscopic flow behavior of multiphase systems is markedly different from those stabilized by low molecular weight (LMW) surfactants. A proper understanding of how these parameters affect macroscopic dynamics of SIDMs is often still lacking, and this has inspired a vast number of studies using a wide range of stabilizers, at both oil-water and air-water interfaces Most of these studies do not use constitutive models to analyse mechanical data, and those that do, tend to use 2d generalizations of existing 3d phenomenological models (e.g., Maxwell, Burgers, or Jeffreys model)[1], or variations of the Lucassen van den Tempel (LvdT) model[13,14] (which link dilatational responses to adsorption-desorption processes). Our results imply that the general behavior of these interfaces is fundamentally different from that of a 2d viscoelastic fluid
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