Adam Burbidge

Food structure and functionality: a soft matter perspective

The structure and functionality of foods are described from the perspective of recent advances in soft condensed matter physics. An overview is given of the structure and properties of food materials in terms of the physically relevant length scales. Recent developments in the understanding of the physics of gels, micelles, liquid crystals, biopolymer complexes and amorphous carbohydrates are presented.

Avalanches of coalescence events and local extensional flows – Stabilisation or destabilisation due to surfactant

From two-drop collision experiments, it is known that local extensional flow favors coalescence. Recently, Bremond et al. used microfluidic methods to evidence this point. Similarly, we used specific microfluidic geometries to impose sudden extensional flow, following drop collision under controlled conditions, and coalescence events were recorded with a high-speed camera. In this study we focus on the effect of surfactant on the coalescence, or stabilisation against it, between drops flowing apart due to either imposed external flow or capillary forces related to drop shape relaxation. Coalescence can be induced even when drops are initially separated by an intersticial lubricating film by far thicker than the critical thickness for rupturing under the action of Van der Waals forces. This is particularly relevant to avalanches of coalescence events, in flowing or even quiescent emulsions or foams. When non-ionic surfactant was used, it was observed that small concentrations apparently enhance coalescence in extension. But at higher concentrations it provides stabilisation through a specific mechanism of thread formation and rupture; the stabilisation mechanism can be complex.

Microfluidic preparation and self diffusion PFG-NMR analysis of monodisperse water-in-oil-in-water double emulsions

Monodisperse water-in-oil-in-water (WOW) double emulsions have been prepared using microfluidic glass devices designed and built primarily from off the shelf components. The systems were easy to assemble and use. They were capable of producing double emulsions with an outer droplet size from 100 to 40 μm. Depending on how the devices were operated, double emulsions containing either single or multiple water droplets could be produced. Pulsed-field gradient self-diffusion NMR experiments have been performed on the monodisperse water-in-oil-in-water double emulsions to obtain information on the inner water droplet diameter and the distribution of the water in the different phases of the double emulsion. This has been achieved by applying regularization methods to the self-diffusion data. Using these methods the stability of the double emulsions to osmotic pressure imbalance has been followed by observing the change in the size of the inner water droplets over time.

Investigating the dynamics of segregation of high-jetsam binary batch fluidised bed systems.

Abstract Batch fluidised bed systems of jetsam concentrations x =0.5 and 0.75 were fluidised over a range of velocities, causing segregation into a jetsam-rich defluidised layer and a flotsam-rich fluidised layer. The dynamics of segregation from an initial fully mixed condition were examined by measuring both the concentration within the fluidised layer and the position of the interface between the two layers over time. It was found that the dynamics of both these characteristics could be approximated by a first order equation approaching an equilibrium with a rate constant. Within the aspect ratio range 0.8–1.2, results showed that provided segregation occurred, the type of distributor plate and the aspect ratio of the bed did not affect the equilibrium concentration within the fluidised layer, although segregation with a perforated plate proceeded at a slower rate than with a porous plate. The relationship between the fluidising velocity and the rate constant was not clear. The interface dynamics were greatly affected by the presence of flotsam trapped within the defluidised layer at low fluidising velocities. Where this was not the case, both the equilibrium position of the interface and the rate constant ω h showed an inverse linear dependence on the excess gas velocity.

In vivo observations and in vitro experiments on the oral phase of swallowing of Newtonian and shear-thinning liquids

In this study, an in vitro device that mimics the oral phase of swallowing is calibrated using in vivo measurements. The oral flow behavior of different Newtonian and non-Newtonian solutions is then investigated in vitro, revealing that shear-thinning thickeners used in the treatment of dysphagia behave very similar to low-viscosity Newtonian liquids during active swallowing, but provide better control of the bolus before the swallow is initiated. A theoretical model is used to interpret the experimental results and enables the identification of two dynamical regimes for the flow of the bolus: first, an inertial regime of constant acceleration dependent on the applied force and system inertia, possibly followed by a viscous regime in which the viscosity governs the constant velocity of the bolus. This mechanistic understanding provides a plausible explanation for similarities and differences in swallowing performance of shear-thinning and Newtonian liquids. Finally, the physiological implications of the model and experimental results are discussed. In vitro and theoretical results suggest that individuals with poor tongue strength are more sensitive to overly thickened boluses. The model also suggests that while the effects of system inertia are significant, the density of the bolus itself plays a negligible role in its dynamics. This is confirmed by experiments on a high density contrast agent used for videofluoroscopy, revealing that rheologically matched contrast agents and thickener solutions flow very similarly. In vitro experiments and theoretical insights can help designing novel thickener formulations before clinical evaluations.

A study of extensional flow induced coalescence in microfluidic geometries with lateral channels

The coupled mechanisms of extensional coalescence and subsequent shape relaxation can lead to catastrophic destabilization of moderately concentrated emulsions. We demonstrate that application of local extensional flow through the use of small lateral channels allows controlled, systematic investigation of both single drop pair and propagating (avalanche) coalescence through a chain of drops. Drop–drop collisions and separations were controlled independently, and did not significantly disturb the primary flow. The probability of the first coalescence event was controlled by bulk flow parameters, allowing for systematic investigation of these phenomena. Simulations with COMSOL® were used in order to quantify and thus validate various assumptions relating to the flow characteristics of our setup. For the configurations tested, the droplet pair separation speed increased linearly with the lateral channel infusion rate. Flows were laminar and collision conditions remained stable until a first coalescence event between a pair of drops was triggered by the superposed local extensional flow field close to the lateral channels. Results are described in terms of coalescence probability versus separation capillary number (Casep). For all systems tested, an upper limit value Ca*sep was observed, above which coalescence did not occur. The probability and length of upstream coalescence propagation induced by the drop shape relaxation following the initial, triggered event are reported. Drop–drop contact times were varied by injecting fluid using different combinations of lateral channels. Ca*sep shifted to a higher value for a given system as the lubricating film drained for a longer time, which, in addition, increased the probability and length of an avalanche of events. The present results demonstrate how microfluidic tools can be used for systematically mapping the most probable behavior of complex systems with respect to coalescence under well controlled hydrodynamic conditions. In general we observe that larger drops, slower separation and higher surfactant concentration favour extensional coalescence and its propagation, in agreement with earlier published experimental studies.

Biophysics of food perception

In this article, we present food perception across a range of time and length scales as well as across the disciplines of physics, chemistry and biology. We achieve the objective of the article by presenting food from a material science angle as well as presenting the physiology of food perception that enables humans to probe materials in terms of aroma, taste and texture. We highlight that by using simple physical concepts, one can also decipher the mechanisms of transport that link food structure with perception physiology and define the regime in which physiology operates. Most importantly, we emphasise the notion that food/consumer interaction operates across the biological fluid interface grouped under the terminology of mucus, acting as a transfer fluid for taste, aroma and pressure between food and dedicated receptors.

Examining predictive correlations for equilibrium concentration profiles in jetsam-rich systems

Abstract A bi-component fluidized bed system of glass ballotini and beach sand ilmenite of d 50 values 124 and 179 μm and particle densities of 2500 and 4500 kg/m 3 , respectively, was examined, with the concentration of ilmenite above 50%. The results were compared with Rowe and Nienows empirical correlation for flotsam-rich systems to assess the potential difficulties in using existing correlations for the system examined. The empirical relationship between the mixing ratio and the normalized fluidizing velocity for this specific system was found to be a modified version of the existing correlation. It was also found that segregation also occurred within the jetsam component, with some fine jetsam particles behaving as flotsam whilst the jetsam at the bottom of the bed was coarser than the original sample.

Rheological behavior of low-viscous emulsions and interpretation with a theoretical model

Two high frequency resonant probes are implemented for the measurement of the rheological behavior of four oil-in-water model emulsions. The measurements are made in a frequency range, which includes the reciprocal relaxation time of the suspended droplets, where droplet deformation based elasticity is expected to occur. The measured rheological behavior is compared with values resulting from the monodisperse Palierne model. Experimental results show that provided that hydrodynamic and viscous forces prevail over Brownian, respectively, inertial forces for the deformation of the fluid droplets, i.e. measurements are done at high Peclet and low particle Reynolds numbers, the emulsion samples investigated in the present work behave in an appreciable extent in the manner predicted by the Palierne model.

An in vitro experiment to simulate how easy tablets are to swallow

The compliance of patients to solid oral dosage forms is strongly conditioned by the perceived ease of swallowing, especially in geriatric and pediatric populations. This study proposes a method, based on an in vitro model of the human oropharyngeal cavity, to study quantitatively the oral phase of human swallowing in presence of single or multiple tablets. The dynamics of swallowing was investigated varying the size and shape of model tablets and adjusting the force applied to the mechanical setup to simulate tongue pressure variations among individuals. The evolution of the velocity of the bolus, the oral transit time, and the relative position of the solid oral dosage form within the liquid bolus were measured quantitatively from high speed camera recordings. Whenever the solid dosage forms were big enough to interact with the walls of the in vitro oral cavity, a strong effect of the volume of the medication in respect of its swallowing velocity was observed, with elongated tablets flowing faster than spherical tablets. Conversely, the geometrical properties of the solid oral dosage forms did not significantly affect the bolus dynamics when the cross section of the tablet was lower than 40% of that of the bolus. The oral phase of swallowing multiple tablets was also considered in the study by comparing different sizes while maintaining a constant total mass. The predictive power of different theories was also evaluated against the experimental results, providing a mechanistic interpretation of the dynamics of the in vitro oral phase of swallowing. These findings and this approach could pave the way for a better design of solid oral medications to address the special needs of children or patients with swallowing disorders and could help designing more successful sensory evaluations and clinical studies.