Ian Hopkinson

Measuring the distribution of interdroplet forces in a compressed emulsion system

The micromechanics of a variety of systems experiencing a structural arrest due to their high density could be unified by a thermodynamic framework governing their approach to ‘jammed’ configurations. The mechanism of supporting an applied stress through the microstructure of these highly packed materials is important in inferring the features responsible for the inhomo- geneous stress transmission and testing the universality for all jammed matter. In this paper, we present a novel method for measuring the force distribution within the bulk of a compressed emulsion system using confocal microscopy and explain our results with a simple theoretical model and computer simulations. We obtain an exponential distribution at large forces and a small peak at small forces, in agreement with previous experimental and simulation data for other particulate systems.

Studies of a weak polyampholyte at the air–buffer interface: The effect of varying pH and ionic strength

We have carried out experiments to probe the static and dynamic interfacial properties of β-casein monolayers spread at the air–buffer interface, and analyzed these results in the context of models of weak polyampholytes. Measurements have been made systematically over a wide range of ionic strength and pH. In the semidilute regime of surface concentration a scaling exponent, which can be linked to the degree of chain swelling, is found. This shows that at pH close to the isoelectric point, the protein is compact. At pH away from the isoelectric pH the protein is extended. The transition between compact and extended states is continuous. As a function of increasing ionic strength, we observe swelling of the protein at the isoelectric pH but contraction of the protein at pH values away from it. These behaviors are typical of a those predicted theoretically for a weak polyampholyte. Dilational moduli measurements, made as a function of surface concentration exhibit maxima that are linked to the collapse of ...

Scaling of dynamics in 2d semi-dilute polymer solutions

We consider the dynamics of thermal concentration fluctuations in polymer Langmuir monolayers, probed with surface light scattering. We present data on the proteins β-lactoglobulin and β-casein and on the synthetic polymers Poly(vinyl acetate) and Poly(4-hydroxystyrene). We show that for all these systems, in the semi-dilute concentration regime, there is a power law dependency of both the elastic and viscous components of the dilational modulus on the concentration. We find that for all the systems considered, the viscosity scales with a power that is double that of the elasticity. A simple explanation for this universal dynamical behavior in semidilute polymer monolayers is suggested.

Water ingress into starch and sucrose:starch systems

The ingress of water from various solutions into dense starch and sucrose:starch plaques has been observed over a range of temperatures using stray field nuclear magnetic resonance imaging. Profiles of water content as a function of depth from the surface and sorption time have been analysed to obtain mutual diffusion coefficients. This has been done using the Boltzmann transform and by measuring the front position as a function of time which produces an average mutual diffusion coefficient. This average diffusion coefficient can be fit with an Arrenhius expression with an activation energy of 56 ^ 4 kJ mol 21 . It was found that the average diffusion coefficients for 30:70 and 50:50 sucrose:starch mixtures could be calculated using a weighted mean of the average diffusion coefficients for pure sucrose and pure starch. Although the diffusion coefficients for amorphous and crystalline sucrose are quite different, this has a minor effect on the calculated diffusion coefficients for the range of crystallinities observed in the sucrose:starch mixtures. q 2001 Elsevier Science Ltd. All rights reserved.

Photocontrol of protein conformation in a Langmuir monolayer

We report a method to control the conformation of a weak polyampholyte (the protein β-casein) in Langmuir monolayers by light, even though the protein is not photosensitive. Our approach is to couple the monolayer state to a photochemical reaction excited in the liquid subphase. The conformational transition of the protein molecule is triggered through its sensitivity to a subphase bulk field (pH in this study), changing in the course of the photochemical process. Thus, reaction of photoaquation of the ferrocyanide ion, which increases the subphase pH from 7.0 to about 8.3, produces a change in the surface monolayer pressure, ΔΠ, between −0.5 and +1.5 mN/m (depending on the surface concentration), signalling a conformational switch. The approach proposed here can be used to selectively target and influence different interfacial properties by light, without embedding photosensitizers in the matrix.