Coalescence of particle-laden fluid interfaces

Colloidal particles are capable of stabilizing emulsions and, thus, slowing or preventing their complete breakdown into phase-separated systems. Direct observations of the dynamics of such particles on both water and oil droplets are reported as two colloid-laden interfaces are brought into contact with each other. As coalescence proceeds, the complementary systems, representing oil-in-water and water-in-oil emulsions, exhibit contrasting mechanisms for the formation of ring and disk structures by the particles as they serve to temporarily stabilize the approaching surfaces. An explanation of such behavior leads to a better understanding of the stabilization and breaking mechanisms of so-called Pickering emulsions.     

Connect the drops: using solids as adhesives for liquids

Colloidal particles are shown to be capable of developing adhesion between liquid phases through a bridging mechanism by which intervening, micrometer-scaled, fluid films are stabilized. Particle dynamics leading to the assembly of the stabilizing structure are discussed. Models for the resulting adhesive force are developed from considerations of both interface shape perturbation and the force applied by surface tension on an individual particle. Finally, predictions from these models are compared to direct measurements of the forces that arise during the separation of adhering interfaces. Such comparisons lead to a novel method for determining the three-phase contact angle inherent to particles residing at fluid interfaces.     

A simple asymmetric lineshape for fitting infrared absorption spectra

Almost exclusively, lineshape functions used to model infrared (IR) absorption peaks are based on symmetric frequency distributions. However, in complex systems such as large biological macromolecules in aqueous solutions, the distribution of vibrational frequencies may in fact be asymmetric. In this communication, we show that asymmetry can be introduced to the standard symmetric Lorentzian and Gaussian lineshapes using a simple, easy to implement method. Our technique involves replacing the static width parameter of the symmetric profiles with a smoothly varying function that is wave number dependent. In this way, the width varies across the IR band resulting in an asymmetric peak. In our model, the width varies sigmoidally with wave number. As a demonstration, we fit experimental spectra of adenosine 5′-monophosphate (AMP).     

A neutron scattering study of the structure and water partitioning of selectively deuterated copolymer micelles

We present a scattering study of a selectively deuterated micelle-forming diblock copolymer. The copolymer comprises a partially deuterated polystyrene (d,h-PS) block and an imidazolium-functionalized PS (IL) block. In toluene solutions, the copolymers assemble into elongated micelles where the IL block forms the micelle core. Through dynamic light scattering (DLS) measurements, we obtain the overall size of the micelles. In our small-angle neutron scattering (SANS) studies, we use contrast matching to characterize the IL core and the PS shell of the micelles independently. The PS block forming the micelle shell exhibits either a starlike or brushlike conformation depending upon the size of the core to which it is tethered. We find the IL block to be in an extended conformation, driving the formation of slightly elongated and relatively stiff micelle cores. The elongated micelle core cross-sectional radius and length depend linearly on the length of the IL block. We find that the micelles can sequester a few water molecules for each IL repeat unit; the addition of water slightly increases the cross section of the elongated micelles.     

Shearing or compressing a soft glass in 2D: time-concentration superposition

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.