Located in: The Maths Observatory
Supervised by: Professor Dominic Vella
Poking something is a natural way of testing its material properties. In everyday life, fruit is poked to check how ripe it is, and bicycles tyres to test how inflated they are. Here, we poked a thin elastic sheet floating on a liquid bath and studied its resultant deformation. We mapped the transition from bending to stretching and investigated the influence that the surface tension of the liquid has on ultra-thin elastic membranes. The findings are applicable across a broad spectrum of length scales, from AFM measurements of graphene membranes and biological membranes like skin, through frogs sitting on lily pads to mountain ranges that are supported by the Earth’s tectonic plates.
The spreading of a fluid in between the layers of a composite material is a fluid-structure interaction applicable to the formation of blisters under the skin and magmatic intrusions in between rock strata. Here, we investigated what happens when one of the layers is permeable so that the spreading fluid also drains away. We injected fluid in between an elastic sheet and a rigid, permeable substrate and measured the interaction between fluid flow and elastic deformation. We found that the draining of injection fluid through the permeable base arrests the propagation of the intruding fluid beneath the sheet, a finding which is applicable in hydraulic fracturing and CO2 sequestration.
The motion of particles in a fluid is an ubiquitous process found throughout nature and industry. Many applications exist in which particles are dispersed in a fluid, from paints and polymer suspensions to turbidity currents which distribute and deposit sediment in the deep ocean. Investigating the motion of particle in a fluid without affecting the local environment is a challenge. In this research, we developed a novel technique to control the oscillatory motion of a sphere immersed in a fluid that is simple and cheap to implement experimentally. We then used this non-contact method to investigate the influence that nearby solid boundaries have on an oscillating particle in a very viscous fluid and observed how a driven sphere moves a second sphere in its vicinity. We also developed a swimming device comprised of three spheres linked by elastic struts which propelled itself through the fluid at low Reynolds number.
Squashing an object is another means of testing its structural properties. When you squash an object localised failure often causes the object to crumple in on itself. In this research we compressed cellular solids and observed a pattern switch which occurs globally throughout the materials. Under compression, an elastic instability causes a square array of circular holes to transform into orthogonal ellipses. This pattern switch was shown to be reversible in soft solids and attainable in plastic structures under dynamic compression. Examples of other low density cellular solids to which the findings are applicable include cancellous bone and photonic crystals.
Segregation of granular materials occurs in industrial silos and in everyday life when smaller pieces fall to the bottom of the cereal packet - a process commonly known as the ‘Brazil nut effect’. We explored the segregation of a binary mixture of particles in a thin rotating drum. Slow rotation of the drum caused the particles to segregate in a radial pattern that resembles petals. The number of petals which form depends on the rate at which the drum is rotated. By modulating the rotation rate we could induce a change in the number of granular petals compared to the number which form for steady rotation. Increases in the number of petals occurred through petal splitting, while decreases occurred through petal merging.