arXiv subjects

A. T. Brown

Publications and source records attributed to A. T. Brown.

3 recordsLinked to original sources

arXiv2025

Oscillation in the SIRS model

We study the SIRS epidemic model, both analytically and on a square lattice. The analytic model has two stable solutions, post outbreak/epidemic (no infected, $I=0$) and the endemic state (constant number of infected: $I>0$). When the model is implemented with noise, or on a lattice, a third state is possible, featuring regular oscillations. This is understood as a cycle of boom and bust, where an epidemic sweeps through, and dies out leaving a small number of isolated infecteds. As immunity wanes, herd immunity is lost throughout the population and the epidemic repeats. The key result is that the oscillation is an intrinsic feature of the system itself, not driven by external factors such as seasonality or behavioural changes. The model shows that non-seasonal oscillations, such as those observed for the omicron COVID variant, need no additional explanation such as the appearance of more infectious variants at regular intervals or coupling to behaviour. We infer that the loss of immunity to the SARS-CoV-2 virus occurs on a timescale of about ten weeks.

physics.bio-ph↗

arXiv2015

Particle-Size Effects in the Formation of Bicontinuous Pickering Emulsions

We demonstrate that the formation of bicontinuous emulsions stabilized by interfacial particles (bijels) is more robust when nanoparticles rather than microparticles are used. Emulsification via spinodal demixing in the presence of nearly neutrally wetting particles is induced by rapid heating. Using confocal microscopy, we show that nanospheres allow successful bijel formation at heating rates two orders of magnitude slower than is possible with microspheres. In order to explain our results, we introduce the concept of mechanical leeway i.e. nanoparticles benefit from a smaller driving force towards disruptive curvature. Finally, we suggest that leeway mechanisms may benefit any formulation in which challenges arise due to tight restrictions on a pivotal parameter, but where the restrictions can be relaxed by rationally changing the value of a more accessible parameter.

cond-mat.soft↗

arXiv2014

Ionic effects in self-propelled Pt-coated Janus swimmers

Colloidal particles partially coated with platinum and dispersed in H2O2 solution are often used as model self-propelled colloids. Most current data suggest that neutral self-diffusiophoresis propels these particles. However, several studies have shown strong ionic effects in this and related systems, such as a reduction of propulsion speed by salt. We investigate these ionic effects in Pt-coated polystyrene colloids, and find here that the direction of propulsion can be reversed by addition of an ionic surfactant, and that although adding pH neutral salts reduces the propulsion speed, adding the strong base NaOH has little effect. We use these data, as well as measured reaction rates, to argue against propulsion by either neutral or ionic self-diffusiophoresis, and suggest instead that the particle's propulsion mechanism may in fact bear close resemblance to that operative in bimetallic swimmers.

cond-mat.soft↗