Please note that each talk is 35 +10 minutes. The last 10 minutes are for Q&A along with changeovers.

31 March 2023

02:15pm Welcome and Introduction

Session 1: Chair (Ashwin Joy, IIT Madras)

02:30pm Speaker: Vani Vemparala, IMSc, Chennai, India

Title: Extended to collapse transitions in polymers: role of counterions and crowders

Abstract: In this talk, we will discuss our work on the conformational landscape of neutral and charged polymers in the presence of crowders and counterions. In particular, we focus on non-intuitive extended-to-collapse transitions of (1) similarly charged polyelectrolytes in the presence of counterions and (2) neutral polymers in the presence of attractive crowders. We will also discuss the differences in the collapse kinetics of neutral and charged polymers.
03:15pm Speaker: Chandan Dasgupta, IISc and ICTS, Bangalore, India

Title: Unusual Properties of Athermal Persistent Active Matter

Abstract: In several biological systems, such as bacterial cytoplasm, cytoskeleton-motor complexes and epithelial sheets of cells, self-propulsion or activity is found to fluidize states that exhibit characteristic glassy behaviour and jamming in the absence of activity. I will discuss some of the results of our recent studies of jamming in athermal models of dense active matter. In these models, the self-propulsion force is characterized by two parameters: its magnitude and the persistence time associated with the decorrelation of its direction. In our studies, we consider the limit of infinite persistence time. In this limit, dense systems of athermal active particles exhibit a jamming transition as the strength of the active force is decreased. The homogeneous liquid state obtained at large values of the active force exhibits unusual properties: the average kinetic energy and the width of the distribution of the kinetic energy increase with increasing system size and a length scale extracted from spatial velocity correlations increases with system size without showing any sign of saturation. We also investigate how this active liquid approaches a force-balanced jammed state when the self-propulsion force is reduced to a small value. The jamming proceeds via a three-stage relaxation process whose timescale grows with the magnitude of the active force and the system size. We relate the dependence on the system size to the large correlation length observed in the liquid state. The properties of the jammed state obtained for small active force are substantially different from those of passive jammed systems. The distribution of contact forces in a jammed state depends strongly on the protocol used in its preparation. For a protocol designed to eliminate the presence of particles with two contacts in the jammed state, the distribution exhibits scaling with the magnitude of the active force with a scaling function that has a gap at small values of the contact force.
04:00pm High Tea
04:30pm Speaker: Rahul Mangal, IIT Kanpur, India

Title: Active Droplets in Polymer Solutions

Abstract: Typical bodily and environmental fluids encountered by biological swimmers consist of dissolved macromolecules such as proteins and polymers, rendering them even non-Newtonian at times. Active droplets mimic the essential propulsive characteristics of several biological swimmers and serve as ideal model systems to widen our understanding of their locomotive strategies. Here, we investigate the motion of a micellar solubilization-induced swimming oil droplet in an aqueous medium dispersed with macromolecular solutes. Through in-situ visualization of the self-generated chemical field around the droplet, we report unexpectedly high diffusivity of filled micelles in the presence of high molecular weight polymer solutes or macromolecules. This is attributed to the limitation of Stokes-Einstein relationship in accurately predicting micelle diffusivity due to significant size disparity between micelles and the macromolecular solute. With an increase in polymer concentration, particle image velocimetry reveals a mode-switching, from the conventional pusher mode to a puller mode of propulsion, characterized by a more persistent droplet motion. With a further increase in concentration, a secondary transition from a smooth to a jittery mode of propulsion occurs. A robust Peclet number framework is proposed that successfully captures the observed mode-switching of active droplets. Our experiments unveil a novel route to orchestrate complex transitions in active droplet propulsion by doping the ambient medium with the suitable choice of macromolecules.
On further increasing the polymer concentration, the surrounding medium was made viscoelastic in nature. The associated Deborah number (De) is tuned by varying the surfactant (fuel) and polymer concentration. At moderate De, the droplet exhibits a steady deformed shape, markedly different from the spherical shape observed in Newtonian media. A simple theoretical analysis based on the normal stress balance at the interface is shown to accurately predict the droplet shape. With a further increase in De, time-periodic deformations accompanied by oscillatory transitions in swimming modes are observed. The study unveils the rich complexity in the motion of active droplets in viscoelastic fluids, which has been hitherto unexplored.
05:15pm Speaker: Rajesh Ganapathy, JNCASR, Bangalore, India

Title: Glassy dynamics in persistent active matter

Abstract: There is mounting evidence that dynamics in dense living active matter assemblies are strikingly similar to equilibrium supercooled liquids. In fact, under suitable conditions, these systems can even undergo a transition to a glassy state. However, how activity influences dynamical slowing down and the glass transition remains debated. In my talk, I will describe findings from recent experiments on the synthetic active matter systems where we capture certain critical aspects of living systems allowing us to disentangle the role of achiral/chiral activity, shape, and deformability in glassy slowing down.

01 April 2023

Session 2: Chair (C V Krishnamurthy, IIT Madras)

09:00am Speaker: Kavita Jain, JNCASR, Bangalore, India

Title: Randomness in biological evolution

Abstract: Evolution is responsible for the great diversity in life on earth. While the role of natural selection in evolution is widely known, it is not well appreciated that evolution is, for the most part, a stochastic process. I will describe how ideas from physics can be used to understand evolutionary dynamics.
09:45am Speaker: Sreeram K. Kalpathy, IIT Madras, Chennai, India

Title: Mathematical modelling-assisted insights into fluid instabilities, colloidal particulate coatings and polymer patterning

Abstract: Soft condensed matter has emerged as an important sub-field of research in several disciplines in science and engineering. Its reach spans disciplines of chemical engineering, materials science, biotechnology, physics, chemistry, and even mathematics to name a few. The field broadly caters to suspensions, polymeric materials, colloids, liquid crystals, active matter, and complex fluids that exhibit peculiar flow behaviour. An exciting and challenging aspect of these materials is the understanding and controlling of interfacial phenomena, which can significantly impact how these materials respond to external stimuli, which can in turn shape product development.
In this presentation, I shall discuss a set of problems from my research group in which some exciting results were obtained, assisted by mathematical modelling. First, I will explain a fluid mechanical model on “solutal Marangoni effect” that shows how liquid films containing surfactant particles can change the effective wetting behaviour of the liquid film on an underlying solid surface. On a related note, I will explain how interfacial viscosity associated with these surfactant particles could result in a remarkably enhanced film stability, a mechanism potentially relevant to improving shelf life of certain food products, cosmetics, and industrial emulsions. Next, I shall illustrate light-assisted ordering and pattern formation in a photoactive polymer named “azo-polyurea”, which has applications in electronics, protein patterning etc.
Finally, I will discuss how coating flows and coating morphologies are affected when liquids (Newtonian/non-Newtonian), colloidal suspensions, or polymers are coated onto permeable / porous solids. “Viscous fingering” instabilities which generally arise during displacement of fluids in porous media will be shown. These instabilities have implications for improving the transportation of fluids in pipelines especially in the food processing and personal care products industries, like chocolates, lotions, sauces by tuning the viscosity and elasticity of fluids during processing.
10:30am Tea
11:00am Speaker: Rajesh Singh, IIT Madras, Chennai, India

Title: Emergent dissipative structures in active matter

Abstract: At thermal equilibrium, steady states follow from the free energy minimization. No such principle exists for dissipative systems operating far from thermodynamic equilibrium. Active matter describes non-equilibrium systems such as a suspension of active particles. Examples of active particles include microorganisms and synthetic microswimmers. This talk will describe a theory to understand the emergence of dissipative structures in many-body active matter systems by studying fluid-mediated forces and torques between active particles. We will then apply the framework to understand two recent experiments of active matter.
The first part of the talk will be about understanding the experimental phenomena of freezing colloids by heating one of them. An effective equilibrium description of this nonequilibrium problem in terms of an emergent optofluidic potential will be derived. The second part of the talk will be about emergent rigidity in a suspension of many active particles.
11:45m Speaker: Basudev Roy, IIT Madras, Chennai, India

Title: Study of some soft matter and biological systems using rotational optical tweezers

Abstract: A rigid body can have 6 degrees of freedom, namely the three translational degrees of freedom and the three rotational degrees of freedom. Of these, the translational degrees have been well explored in optical tweezers community. However, only the in-plane rotational degree of freedom has been explored. We call this in-plane degree of rotational freedom, the yaw motion in the nomenclature of the airlines. The pitch and roll degrees are only beginning to be explored recently.
In this talk, I will show you 4 ways of generating pitch rotation using the optical tweezers. I will also show you one way of detection of pitch rotation at high resolution using birefringent particles. Further, I will discuss some applications of this pitch rotation in soft matter systems and biology. I will also show you a way of generating roll rotation using optical tweezers in hexagonal shaped particles.​
12:30pm Panel discussion on teaching soft matter at UG and PG levels
01:00pm Lunch

Session 3: Chair (Manoj Gopalakrishnan, IIT Madras)

02:30pm Speaker: Mohamed Laradji, The University of Memphis, USA

Title: Collective Vortical Motion and Vorticity Reversals of Soft Self-Propelled Particles on Circularly Patterned Substrates and in Circular Confinement

Abstract: The collective motion of self-propelled particles is investigated through a coarse-grained model of disjoint semi-flexible ring polymers, with a motility force along their polarity, that depends on its historical kinetics. Cooperative motion emerges as the amplitude of the motility force or areal density is increased. Investigation of the model, under circular confinement and interaction with a circularly patterned substrate, shows interesting and rich collective behavior characterized by the emergence of vortical motion that is dominated by the patterned substrate at intermediate motility forces, and by the geometric confinement at high motility forces. The collective motion of the particles is also characterized by the emergence of an exclusion zone in the middle of the system and by intermittent vortical reversals that become quasi-periodic at high motility forces.
03:15pm Speaker: Vijayakumar Chikkadi, IISER Pune, India

Title: Identifying structural defects using softness in dense amorphous suspensions

Abstract: Defects are inherent in materials and affect their mechanical, electromagnetic, and thermal transport properties. In crystalline solids, defects such as vacancies, dislocations, and grain boundaries have been studied extensively. When external stresses are applied, the motion of these defects leads to plasticity or permanent deformation. However, most of the materials that surround us are amorphous. The nature of defects in amorphous materials and how to identify them remains unclear. Understanding the impact of the microscopic structure on their plastic deformation is a fundamental question in condensed matter physics and materials science.
In this talk, I will introduce the concept of "softness", a microstructural measure used to identify defect-like regions in dense colloidal suspensions. The softness of the system is inversely related to the depth of the mean-field caging potential experienced by particles. Our experimental results indicate that long time relaxation in quiescent systems and plastic events in sheared systems are strongly correlated with softness. In quiescent suspensions, an increase in density leads to an increase in the depth of the caging potential, resulting in lower softness and longer relaxation times. Furthermore, in sheared systems, plastic events occur preferentially in regions with lower softness. Therefore, "softness" offers a new perspective on relaxation in amorphous solids.
04:00pm Tea
04:30pm Speaker: Kabir Ramola, TIFR, Hyderabad, India

Title: Current fluctuations in interacting and non-interacting active particle systems

Abstract: We study the fluctuations of the integrated density current across the origin up to time T in one dimensional systems of non-interacting as well as interacting active particles. For non-interacting particles, we focus on the case of zero diffusion and study the differences between annealed and quenched initial conditions. We show that for the case of particles initiated with an initial bias in the positive direction, the fluctuations of the current at short times display a surprising difference: T versus T2 behaviours respectively, with a √T behaviour emerging at large times. For the interacting case, we explore a lattice model of active particles with hard-core interactions that is amenable to an exact description within a fluctuating hydrodynamics framework. For the case of uniform initial profiles, we show that the second cumulant of the integrated current displays three regimes: an initial √T rise with a coefficient given by the symmetric simple exclusion process, a cross-over regime where the effects of activity increase the fluctuations, and a large time √T behavior. In the limit of zero diffusion for the interacting system, we show that the fluctuations once again exhibit a T behavior at short times. Finally, we show that the results for non-interacting active particles are recovered for low densities
05:15pm Speaker: Richa Karmakar, IIT Madras, Chennai, India

Title: Quantitative experiments to understand Prokaryotic and Eukaryotic Chemotaxis

Abstract: Chemotaxis, the movement of cells guided by chemical gradients, plays an essential role in many biological processes, including tumour dissemination, wound healing, and embryogenesis. The first part will discuss quantitative experiments to understand prokaryotic chemotaxis. A suspension of motile cells exhibits complex rheological properties due to their collective motion. We measure the shear viscosity of a suspension of Escherichia coli strains varying in motor characteristics such as duration of run and tumble. At low cell densities, all strains, irrespective of their motor characteristics, exhibit a linear increase in viscosity with cell density, suggesting that the cells behave as a suspension of passive rods with an effective aspect ratio set by the motor characteristics of the bacteria. As the cell density is increased beyond a critical value, the viscosity drops sharply, signalling the presence of strongly coordinated motion among bacteria.
The second part will cover topics such as investigating the chemotactic behaviour of Dictyostelium discoideum (eukaryotic cells) in microfluidic devices and establishing cellular memory in the presence of an attractant. Cells of the social amoeba Dictyostelium discoideum migrate to a source of periodic travelling waves of chemoattractant as part of a self-organised aggregation process. An important part of this process is cellular memory, which enables cells to respond to the front of the wave and ignore the downward gradient in the back of the wave. During this aggregation, the background concentration of the chemoattractant gradually rises. In our microfluidic experiments, we exogenously applied periodic waves of chemoattractant with various background levels. We find that increasing background does not make detection of the wave more difficult, as would be naively expected. Instead, we see that the chemotactic efficiency significantly increases for intermediate values of the background concentration but decreases to almost zero for large values in a switch-like manner.
Finally, in the third part, I will discuss the current work of rapid detection for antibiotic susceptibility of bacteria using a Lab-on-a-chip design. Antibiotic resistance is a growing concern across the world, including India, and I will discuss how to develop a rapid lab-on-a-chip microfluidic device capable of detecting antibiotic susceptibility of bacteria.
06:00pm Closing remarks