Shearing or Compressing a Soft Glass in 2D: Time-Concentration Superposition

Abstract

We report surface shear rheological measurements on dense insoluble monolayers of micron sized colloidal spheres at the oil/water interface and of the protein beta-lactoglobulin at the air/water surface. As expected, the elastic modulus shows a changing character in the response, from a viscous liquid towards an elastic solid as the concentration is increased, and a change from elastic to viscous as the shear frequency is increased. Surprisingly, above a critical packing fraction, the complex elastic modulus curves measured at different concentrations can be superposed to form a master curve. This provides a powerful tool for the extrapolation of the material response function outside the experimentally accessible frequency range. The results are discussed in relation to recent experiments on bulk systems, and indicate that these two-dimensional monolayers should be regarded as being close to a soft glass state.

Importance of physical vs. chemical interactions in surface shear rheology

“…A good description of such a model based on packing density rather than covalent interactions can be found in the work of Cicuta and Terentjev and Cicuta et al [23,24], and Edwards and Wasan [49]. In these models, the shear behaviour is the result of a decreased mobility of the particles due to the close packing.…”

“…An alternative view, shared by several authors, states that the surface shear behaviour of adsorbed protein layers is the result of the dense packing of loose proteins [17,23,24]. This concept seems to be able to account for observations that the shear elasticity only increases once a certain concentration of adsorbed particles is reached [6,11], and that proteins are displaced from the interface relatively quickly by LMW surfactants [25 -27].…”

Forced Desorption of Nanoparticles from an Oil–Water Interface

“…Experiments have revealed a solid-like behavior [14][15][16][17][18][19][20][21][22] and in some instances, particle expulsion into one of the fluid phases [23][24][25] . This range of behavior is reminiscent of that of spread monolayers of lipids under strong compression: condensed monolayers exhibit solid-like response and collapse by folding, while monolayers that remain liquid expel material in the fluid subphase 26 .…”

Dynamics of Surfactant-Driven Fracture of Particle Rafts

“…Recent experiments [7,8] show that a densely packed monolayer of particles at an interface (a 'particle raft') has many of the characteristics of a two-dimensional linear elastic solid in certain regimes. For example, under compressive loading, a particle raft statically buckles out of the plane demonstrating that collectively its constituent particles possess a non-zero shear modulus G ∼ γ/d [8], with γ the surface tension coefficient of the pure liquid-gas interface and d the particle diameter.…”

“…For instance, liquid drops coated with a fine hydrophobic powder become non-wetting [1], forming an artificial analog of a much older solution stumbled upon by insects [2]. Similarly, the addition of particles to the surface of liquid drops prior to coalescence stabilises the coalesced drops to the common pinch-off instability and can lead to reversible morphological instabilities such as buckling when subject to pressure Recent experiments [7,8] show that a densely packed monolayer of particles at an interface (a 'particle raft') has many of the characteristics of a two-dimensional linear elastic solid in certain regimes. For example, under compressive loading, a particle raft statically buckles out of the plane demonstrating that collectively its constituent particles possess a non-zero shear modulus…”