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Cambridge University Science Magazine
Coatings, solar cells, sensors and medical implants all exploit properties of thin film polymers including their viscous and elastic nature, morphology and optical behaviour. Despite their widespread use, questions still remain about the stability of polymers, particularly in thin films, which often exhibit properties differing substantially from those of the bulk. Pola Goldberg-Oppenheimer is trying to answer some of these questions during her PhD in the department of physics.

Thin films can crack, develop holes or even destabilise to form droplets. Most of the time smooth even surfaces are required, so those developments are unwelcome. However, in some cases, by exploiting the destabilisation process, precise control can be gained over the instabilities that form and novel structures produced.

Pola has been looking at what happens when electrostatic forces – the forces between particles resulting from their electric charge – are applied to thin films. These forces are able to destabilise smooth films. The resulting instabilities, known as electrohydrodynamic instabilities, have a characteristic spacing that depends on film thickness, air gap, polymer properties and the applied electric field. By adjusting these parameters, the dimensions of the structure can be predetermined.

The experiment is conducted by sandwiching the polymer film in a capacitor-like device between two electrodes with a small air-gap. The sample is heated close to its transition temperature where it changes from having a ‘glassy’ or solid-like nature to a liquid-like one. A small voltage is applied across the two electrodes resulting in a high electric field and a buildup in the electrostatic pressure at the air surface. This destabilises the film and generates an undulating surface. Over time, these undulations grow until they span the air gap and reach the upper electrode. The final morphology consists of a hexagonal array of pillars with well defined spacing.

Unfortunately, things don’t always go according to plan and the image on the front cover shows what can happen when the capacitor device is not quite correctly assembled. Here, finger like instabilities have formed, due to the upper electrode and polymer film coming into contact and trapping air. These are known as Saffman-Taylor instabilities and occur when one liquid is replaced with another of lower viscosity.

Saffman and Taylor originally observed these instabilities when they were looking at water-oil mixtures between two parallel plates. During the 1930s ‘water flooding’ was assumed to be an efficient method of extracting oil from wells. However, Saffman and Taylor observed destabilisation of the normally well-defined water-oil interface. In closely spaced parallel plates, instabilities such as fingers, fractal trees and dendrite or branched patterns were observed. For the oil companies, this meant that after a certain time only water would be recovered from the well.

Pola has also been looking at electrohydrodynamic instabilities that form when the upper electrode is replaced by one which is no longer flat and smooth, but contains some sort of patterning. The instabilities that form are drawn towards the protrusions, resulting in the formation of a positive replica of the imposed structure.

These methods can produce and replicate patterns on sub-micron, possibly sub-tenth of a micron, length scales and are a promising low-cost alternative to conventional optical-patterning techniques. Pola is particularly interested in using electrohydrodynamic instabilities to pattern novel systems and materials, in an attempt to make the pattern formation process faster and the resulting structures smaller, thus making this technique more practical, reliable and robust.

Katherine Thomas is a PhD student in the Department of Physics.