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Work by the 2011 Soft Matter Lectureship winner, Dr Michael Solomon, is showcased on the back cover of Issue 1 of Soft Matter 2013. The cover highlights the research collaboration between Dr John Younger of the Department of Emergency Medicine and Dr Michael Solomon of the Department of Chemical Engineering at the University of Michigan, USA. Their paper ‘In situ rheology of Staphylococcus epidermidis bacterial biofilms’ is free to read for a short time.

Nominations for the 2013 Soft Matter Lectureship will be opening early next year. Stay tuned for more information.

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When a droplet impacts on a solid surface it deforms. Depending on the properties of the drop and the surface (velocity, viscosity, surface tension, hydrophobicity etc.) this deformation can be temporary, the droplet spreads out before retracting and bouncing back, or permanent, the drop breaks apart on impact making a splash.

Air also plays an important role in determining the behaviour of the impacting drop. Detlef Lohse, University of Twente, is interested in understanding how and why the air layer profile under a drop influences its deformation as it falls and subsequently hits a surface. His group has developed an ultra-high speed colour interferometry imaging method, allowing them to resolve the dynamics of an impacting drop on short timescales. Using this method, Lohse has been able to measure the time evolution of the drop before and during impact on a solid surface. As the drop falls the air between the drop and surface is strongly squeezed. A region of high pressure builds up, which leads to the formation of a dimple on the underside of the drop. At high velocities this can result in splashing as the air is compressed on impact. The presence of an air bubble can also stop the droplet from touching the surface at all leading to some interesting effects.

When a liquid droplet impacts a surface heated above the liquid’s boiling point three impact behaviour regimes are observed. In the first, ‘contact boiling’, the droplet immediately boils as it comes into contact with the surface. The droplet contacts the surface and spreads out. Bubbles then form and the liquid evaporates. For ‘gentle film boiling’ the droplet appears to hit the surface before bouncing back. In this regime a Leidenfrost vapour layer forms under the drop before it hits, preventing it from coming into direct contact with the surface. The final behavioural regime is ’spray film boiling’. In this case, although the droplet does not contact the surface, breakup does occur. A Leidenfrost vapour layer forms below the drop. As the vapour tries to escape it drags fluid out with it and the droplet forms a thinner pancake shape. Tiny drops are the ejected upwards in what can be quite a violent spray event. The impact conditions under which each regime can be observed were recently published in Physical Review Letters.

Lohse has also looked at how the structure of the surface can influence the splashing dynamics for high velocity impacting drops. In this case the surfaces are all at room temperature. This is discussed in detail for Newtonian and non-Newtonian liquids in two recent Soft Matter papers. Directional splashing can be tuned and suppressed by varying the periodicity of the lattice and, or the air pressure. A number of videos of the impacting drops can be found in the supplementary information accompanying the Soft Matter article and are well worth a look.

A Soft Matter paper by groups from the Centre of Molecular Materials for Photonics and Electronics and the Inkjet Research Centre at the University of Camrbidge has appeared in a BBC News story. The paper describes the inkjet printing of liquid crystals on to a wet, solution-based polymer with precise control over droplet size.

Printed photonic arrays from self-organized chiral nematic liquid crystals
D. J. Gardiner, W.-K. Hsiao, S. M. Morris, P. J. W. Hands, T. D. Wilkinson, I. M. Hutchings and H. J. Coles, Soft Matter, 2012, 8, 9977.

The BBC news story can be found here.

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A Soft Matter paper has been highlighted on the science news website Science Daily. In the Soft Matter paper (Designing maneuverable micro-swimmers actuated by responsive gel) Alexander Alexeev and co-workers at Georgia Institute of Technology, USA, design a simple maneuverable micro-swimmer that can self-propel and navigate in microfluidic channels. The micro-swimmer is designed using computational modelling.

You can read the write up in Science Daily here:
Microswimmers: Micron-Scale Swimming Robots Could Deliver Drugs and Carry Cargo Using Simple Motion

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