Mechanics at soft interfaces

In this thesis the interaction of soft interfaces with liquid drops, solid particles and liquid flows is explored. ‘This led to some new, interesting phenomena,’ says Anupam Pandey. ‘We also brought a new perspective to the classical creasing instability of a compressed soft solid, where the interface folds onto itself and creates a self-contact.’

Anupam tells most of his research was curiosity driven. ‘I was inspired by everyday seemingly trivial phenomena, possessing some surprising aspects which are not yet fully understood,’ he says. ‘Also, I believe, my quite fundamental work on soft interfaces, has generic application potential since these solids may respond to a wide range of external stimuli. This makes them useful for: scaffolds in tissue engineering, microfluidics, or active components in metamaterials, to name just a few.’

In one of the last chapters, an experimental flaw revealed new insights into surface creases, a prominent feature of primate brains. When making soft gels, unintentionally some water droplets got incorporated in the gel, which evaporated slowly, leaving an axisymmetric furrow.

Anupam: ‘The furrow is an elastic analogue of a viscous sink where the liquid interface deforms into a sharp tip. Beyond a critical deformation, the furrow bifurcates to form a crease. We analysed the singular shape of these creases, which might shed new light on the mechanics of growth and morphogenesis.'

Anupam Pandey published several articles. ‘All six chapters resulted in publications in six different journals,’ Anupam says. ‘Physical Review Letters, Soft Matter, PNAS and Journal of Fluid Mechanics were among those.’


In the first chapters, the interaction between two liquid drops on a soft interface was studied. Capillary forces in a liquid drop can significantly deform a soft solid, creating an elasto-capillary meniscus around it.

‘When a second drop is placed nearby, it interacts with the first one through this meniscus, much like the floating paperclips that clump together on water,’ Anupam explains. ‘Droplets on a thick elastomer substrate attract each other and coalesce. Whereas for substrates with thicknesses much smaller than the drop size, the interaction is dominantly repulsive. We quantified these interactions by developing force-distance curves.’


In the next chapter the embedding of nanoparticles on a polymer surface was addressed. It was observed in experiments that below a critical size, silica nanoparticles are engulfed by a soft, polymer surface, while larger nanoparticles adhered to the interface.

‘We theoretically explained this phenomenon by the co-called line tension effect,’ says Anupam. ‘This effect has been previously observed for liquid interfaces. We were the first to demonstrate its dominance at the nanoscopic mechanics of soft solid interfaces.’


During his thesis work, Anupam collaborated with the Mesa+ Group: Materials Science and Technology of Polymers (MTP) on the above mentioned project of nanoparticle engulfment on polymer surfaces.

‘Our study could be very relevant for fabricating elastic foams in the near future, which act as highly efficient insulation materials. Since nanoparticles at a polymer interface act as nucleation sites for foaming, it is crucial to resist the engulfment,’ Anupam says.

In the last chapter the interaction of viscous flow with a soft boundary was studied. ‘We worked on formulating the lift forces particles experience, moving within a viscous liquid near a deformable boundary,’ says Anupam.

Anupam’s PhD work was part of a ERC-consolidator grant, studying soft wetting. He enjoyed the informal discussions with his promotor, Professor Jacco Snoeijer, which helped him proceeding his work and experiments.

Anupam: ‘Within Mesa+ I also enjoyed the Soft Matter colloquia which helped me in broadening my research interests. It definitely helped me to grow as a scientist and academic researcher. After these four years I am much more creative. Also I learned to explain my work better to a broader audience. Getting rid of all complicating concepts, I am now able to bring about my scientific work in an entertaining and informative way.’

After his PhD Defense Anupam will work for one more year at the Physics of Fluids Group. ‘After four years of doing theory I now would like to make my hands dirty and develop some experimental skills,’ he says.