RESEARCH AREAS
Deformation and flow of soft solids
Mechanical failure in solids is ubiquitous on length scales ranging from a few nanometers, for example in atomic or molecular systems, up to geological scales, as in earthquakes. Enhancing mechanical properties of materials using different approaches is an active area of research in many disciplines, from material science to biology, engineering, and geology. Apparently, the phenomenon of mechanical failure is quite generic, most materials ranging from atomic and molecular solids to soft solids (shaving foam, mayonnaise, etc.) exhibit similar characteristics, which can be evidenced from their stress-strain curves . So, we use colloidal solids to investigate the physics of deformation of crystalline and amorphous solids.
Colloids are micron sized particles that can be tracked in three dimensional space and time using advanced microscopic techniques such as confocal microscopy. Using index matching technique we can reconstruct and visualise motion of particles deep inside the sample. This gives unprecedented access to microscopic information, which is not possible in atomic systems. As a result, dense colloidal suspensions can provide considerable insight into elastic and plastic deformation of crystals and amorphous solids.
Active matter
Active matter systems are comprised of elementary constituents that have the ability to absorb and dissipate energy, and in the process perform a directed motion. The study of active matter was primarily motivated by living systems such as in-vitro mixtures of cell extracts of bio-filaments and related motor proteins, the whole cytoskeleton of living cells, bacterial suspensions, cell layers, and animals, fish, and bird flocks. Such systems are strongly out of equilibrium, and they display exotic properties including emergent structures with collective behavior, anomalous fluctuations, novel mechanical and rheological properties. Many such properties of active systems are universal, i.e., systems at different length scales with different microscopic details display broadly similar properties. This has motivated physicists to study them using the tools and techniques of condensed matter and statistical mechanics.
In recent years this area of research has attracted considerable attention due to well controlled experiments that have allowed quantitative analysis using theoretical modelling. Here again, colloidal models have emerged as popular experimental systems to investigate active matter. This field is still in its infancy and it offers many opportunities to discover new physics of systems far from equilibrium.
Optical tweezer experiments
The optical tweezers allow manipulation of colloids. Using a two-axis AOD, we create time shared traps to pin particles in different patterns on a plane. An example of this is shown in the image on the right, where the particles are trapped in the pattern of a football ground.
Using the same set-up with an SLM, we apply localized forces to measure the response of various soft and active matter systems.
Colloidal crystals, glasses and gels
Colloidal particles are popular as ”pseudo atoms” because they mimick various atomic phases. The investigation of colloidal models have brought new insight into various physical phenomena such as phase transition, crystal nucleation, glass formation and the local geometry of bulk matter, they have also provided important clues to understanding the process of folding or predicting the shape and structure of large biomolecules, including proteins.
We are using hard sphere like colloidal suspensions to investigate various aspects of crystallisations, gelation and glass formation.