In this thesis the influence of nearby walls is studied for understanding flow dynamics of colloid suspensions. These effects are of relevance for practical problems where colloids occur inside capillaries. ‘My PhD work can be of interest for various processes, for example in food industry,’ Somnath Gosh says. ‘Shear and slip phenomena are also present in micro channels where additional parameters are influential. Experimental work and theoretical modelling techniques both were vital in my PhD work.’

When starting his project Somnath aimed at performing experiments and measurements regarding inelastic properties in concentrated suspensions. Direct particle-wall interactions (e.g. slip effects, when flows pass a wall) and shear phenomena became increasingly important when work progressed.

‘By performing various experiments, I was able to observe diverse shear induced collision phenomena, some of them leading to surprising results,’ Somnath says. ‘For example: shear-induced enhancement of diffusion is strongly counteracted by the contribution from the wall. As a result the diffusion coefficients become very small near the wall even though the shear rate is highest there.’

Good description of experiments

Using particle tracking techniques in wall slip measurements in flows, Somnath was able to design a simple model that assumed a local depletion of particles very close to the wall, resulting in a remarkably good description of the experiments.

‘My supervisors guided me, while also giving me enormous freedom to come up with open research questions and to use a wide variation of experimental techniques,’ he says. ‘As a result almost every day some exciting observations occurred. Also I explored different fields, getting to know a lot of experimental techniques and gaining knowledge on fluid dynamics involving colloid hard speres.’

Writing abilities

Performing experiments is familiar to Somnath. He was especially content with his work on the concluding chapter of his thesis. Here the diffusive dynamics of colloids confined by cylinders was addressed. Varying the cylinder radius from large to small allowed Somnath to examine the local effects of a single curved wall, as well as the global effect of the entire cylinder.

‘I made serious progress on my writing abilities during my PhD working period,’ he says. ‘The paper on my concluding chapter was accepted with only a few corrections, and the reviewer came up with some very good comments. This was very rewarding for me.’

Helping to strengthen his academic and writing skills, Somnath wishes to thank co-promotor Dr. Michel Duits. 'Without his tremendous help and support my work wouldn't be completed,' he says. ‘From my promoter Professor Frieder Mugele I learned to interpret my results and derive from them exciting follow-up research questions. From Dirk van den Ende I learned to mathematically model and explain my experimental observations. So, my supervisors contributed in developing my skills in a direct way.’


Now confident in performing scientific research, Somnath favors to stay into academics, starting a post-doc job at Lund University, Sweden after his defense, joining the Physical Chemistry Group. Somnath: ‘Here I will work on 3D imaging techniques in complex fluids. This is a nice continuation of the imaging expertise I have built up here in Twente. Thinking about ambitions in the future, I would like to establish a Group of my own in India. Who knows. I’m certainly planning to go back to India one day, as I miss my family and friends a lot.’