Soft and Active Matter Talks

Chemotaxis, the movement of cells guided by chemical gradients, plays an essential role in many biological processes, including tumour dissemination, wound healing, and embryogenesis. Bacteria travel towards or away from a chemical during chemotaxis in search of food and to ensure their survival while the chemical concentration in the immediate surroundings continually changes (temporal). We are focused on understanding the chemotactic movement of eukaryotes and prokaryotes in a controlled chemical environment using microfluidics. We used Dictyostelium discoideum (eukaryotic cells) and Bacteria like Escherichia coli, Bacillus subtilis, and Pseudomonas aeruginosa as model organisms. 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. Using a novel microfluidic device, we try to understand the aggregation process of Dicty.

Also, we are working on the rapid detection of antibiotic susceptibility of bacteria using a Lab-on-a-chip design. Antibiotic resistance is a growing concern across the world, including India. Because of the increasing usage of antibiotics, these bacteria are getting resistant to the given antibiotics, which means there is no effect of the antibiotics on the bacteria. Much research is going on these antibiotics and the bacteria. Rapid Antibiotic Susceptibility Tests can tell us whether the bacteria are resistant to the particular antibiotic, and we can wisely administer the drug. We will test the bacterial movement towards an attractant/ repellent by chemotaxis method. Using our understanding, we developed a rapid lab-on-a-chip microfluidic device capable of detecting antibiotic susceptibility of bacteria.

A droplet of pure water placed on a clean glass surface will spread axisymmetrically, and a droplet of mercury will bead up into a spherical droplet. In both cases, the droplet is minimizing its surface energy- creating an object with a minimized surface area- and there is nothing to break the symmetry. Remarkably, droplets of the room-temperature liquid gallium-indium (EGaIn), which like all metals have an enormous surface tension, can nonetheless undergo fingering instabilities in the presence of an oxidizing voltage. I will describe how this oxide acts like a reversible  surfactant which accumulates in a membrane-like layer at the surface, such that EGaIn placed in an electrolyte under an applied voltage can achieve near-zero surface tension. Remarkably, this microscopic layer is responsible for generating a huge diversity of morphologies: fingering instabilities, tip-splitting, rippled droplets, coiling, and even fractals. Remarkably, we find that the effect of the surfactant can even be used to suppress the Rayleigh-Plateau instability in falling streams. Quantitative control of these effects provides a new route for the development of reconfigurable electronic, electromagnetic, and optical devices that take advantage of the metallic properties of liquid metals.

Microswimmers are self-propelled particles that move in an environment where viscous forces are dominant over inertia. We investigate the  dynamics of microswimmers driven by two different mechanisms. The first part of the presentation covers the rotational dynamics of spherical microswimmers trapped at fluid-fluid interfaces modeled using a slip-driven squirmer model, which accounts for the force dipole and source dipole components. Using numerical simulations, we show that the force dipole exerts a torque, orienting pushers parallel to the interface and pullers in the normal direction. The source dipole results in particle rotation only for a finite viscosity contrast between the two fluids. The superposition of these two contributions leads to rotational dynamics with a steady-state orientation that depends on the relative magnitudes of the force and source dipoles. In the second part, I will talk about flagellated microswimmers (e.g. Bacteria). Bacteria are ubiquitous in nature and the basis for spreading several diseases. Bacterial cells exhibit various swimming modes using their flagella to explore complex surroundings such as soil and porous polymer networks. Some single-flagellated bacteria swim with two distinct modes, one with its flagellum extended away from its body and another with its flagellum wrapped around it. The wrapped mode has been observed when the bacteria swim under tight confinements or in highly viscous polymeric melts. In this study, we investigate the hydrodynamics of these two modes inside a circular pipe. We find that the wrap mode is slower than the extended mode in bulk but more efficient under strong confinement due to a hydrodynamic increase of its flagellum translation-rotation coupling.

Dense particulate suspensions exhibit a dramatic increase in average viscosity above a critical, material-dependent shear stress. This non-intuitive behavior is not well understood, and many questions remain unanswered, particularly the distribution of stresses within the suspension. By developing a technique Boundary Stress microscopy, we have gained insights into several crucial questions. In this talk, first I will discuss role of localized stresses during shear thickening of colloidal suspensions. We find that the suspension spontaneously and intermittently separates into two different fluid phases experience dynamics high and low stresses with substantially different properties. In the second part of my talk, I will discuss that the shear thickening in corn-starch is driven by proliferation of stable propagating fronts experiencing non-affine particle velocity, varying localized particle volume fraction. By directly measuring the relative flow between the particle phase and the suspending fluid (fluid migration), we find the migration is highly localized to the fronts and changes direction across the front, indicating that the fronts are composed of a localized region of high dilatant pressure and low particle concentration.

In confined environments, swimming microorganisms interact with boundaries for various physical and biological imperatives. These boundaries can either be rigid such as cracks in rocks, metal surfaces, or deformable surfaces and interfaces such as blood capillaries, membrane boundaries and fluid interfaces. Understanding the fluid-mediated hydrodynamics of such systems is important to gain insights into the dynamics, which crucially depends on the type of boundary. In this talk, I will present a mathematical description involving the coupled hydrodynamics of an orientable microscopic swimmer with a deformable interface that separates two fluid regions. Modelling the swimmer as a slender body, I will characterize the modification to its rotation and translation relative to the interface. Our analysis considers the role of the swimmer type (pusher or puller) and interface deformation due to surface tension and bending resistance, for a swimmer that can execute random changes to its orientation. I will highlight how an inter-play between the intrinsic swimmer reorientations, the fluid and interface properties can enable the swimmer to migrate both towards and away from the interface.

Elasticity is a fundamental macroscopic property that emerges in any collection of interacting particles. Yet, amorphous materials display seemingly random behavior at short-length scales, with elasticity emerging at larger length scales. On the other hand, crystals, with a periodic spatial profile, also display similar elastic behavior at large length scales. Gradually introducing disorder into athermal crystalline packings can therefore be used to build a relation between the well-established physics of crystals and that of amorphous solids. In this talk I will present a universal characterization of stress correlations in athermal systems, across crystalline as well as amorphous packings. I will present numerical as well as theoretical results for stress correlations in energy-minimized configurations of soft particles, in two and three dimensions. I will also discuss how such a universality emerges due to the constraints of mechanical equilibrium at the local scale, and is therefore independent of other structural correlations, or correlations in orientational order.

Evaporation of sessile drops on solid surfaces is a widely investigated area of research. In a different configuration, we study the evaporation of microdroplets produced using the microfluidic water-in-oil emulsion method. We place a microfluidic water-in-oil emulsion drop on a solid substrate to evaporate. In this configuration, unlike that of a sessile drop, aqueous microdroplets form a cluster surrounded by the oil medium due to the capillary interaction. Distributed evaporation rates occur within the cluster manifested by a size gradient of microdroplets (see the image). We demonstrate that this system can be used for different applications such as (i) mimicking evaporation of aerosols and (ii) studying the liquid-liquid phase (LLLPS) in confinement.

(i) Aerosols and microdroplets are known to act as carriers for pathogens or vessels for chemical reactions. The natural occurrence of evaporation of these droplets has implications for the viability of pathogens or chemical processes. In this context, we propose evaporation of aqueous microdroplets in emulsion can mimic the evaporation of aerosols. Distributed evaporation allows comparing the consequences of evaporation and its rate for processes occurring in droplets. As a proof of concept, we study Escherichia coli and Bacillus subtilis as model organisms inside the microdroplets. Our experiments indicate that the rate of evaporation of microdroplets is an important parameter in deciding the viability of contained microorganisms. 

(ii) Liquid-liquid phase separation (LLPS) is the segregation and condensation of polymers and biomolecules triggered by physiochemical environments such as, concentration change, charge, ionic changes, or crowding by confinement. It is a process that can occur in bulk fluids to inside biological cells. We demonstrate that emulsion evaporation can be used to study the confinement effects on LLPS.

When a liquid is quenched rapidly such that freezing is bypassed, particle dynamics rapidly slow down and the system eventually becomes kinetically arrested to form a glass. The first part of my talk will describe recent experimental findings my group wherein we dissect how a colloidal glass crystallizes despite negligible motion of particles [1]. We will show that there is structure hidden in a glass that governs the manner in which it crystallizes. The second part of my will discuss how activity affects the nature of the glass transition. In our system of granular active ellipsoids, we will show that mobile +1/2 disclination defects act as conduits for structure relaxation even at very deep supercooling and helps suppress the glassy state [2].

[1] Structure Determines Where Crystallization Occurs in a Soft Colloidal Glass, Divya Ganapathi, Dibyashree Chakraborti, A K Sood and Rajesh Ganapathy, Nature Phys. 17, 114 (2021)

[2] Motile Topological Defects Hinder Dynamical Arrest in Dense Liquids of Active Ellipsoids, Pragya Arora, A K Sood and Rajesh Ganapathy, (at revision, 2022)

The coil–stretch transition is the complete unravelling of a polymer that occurs when the polymer is immersed in a non-uniform flow field and the magnitude of the velocity gradient surpasses a critical value. It was first predicted and observed experimentally in a laminar extensional flow. Later it was discovered also in random flows, even though with partially different features. In this seminar, I will review the main results on polymer stretching and the coil–stretch transition in random and turbulent flows.

Bacterial swarms display intriguing dynamical states like active turbulence. But how is this emergent hydrodynamics actually advantageous for the swarm? We show that a detailed study of the Lagrangian aspects of such suspensions reveal how the emergent turbulent and streaky hydrodynamics leads to a self-selection of more persistent Lévy walks trajectories over the diffusive ones. This dynamical heterogeneity manifests itself not only in the anomalous diffusion in such suspensions, but its biological advantages are tied up with an optimal first passage distribution. This anomalous Lagrangian behaviour of animate, low Reynolds flows are distinct from inanimate, high Reynolds turbulence and show up in other measures such as the pair-dispersion problem.

Liquid crystals (LCs), mesogenic phases found universally in soft and living systems, combine material fluidity with topological facets characteristic of crystalline solids. Topological Microfluidics [1] harnesses the material anisotropy, along with the topological constraints and intrinsic couplings in such systems, offering a novel yet highly versatile and generic counterpart to conventional microfluidics based on isotropic fluids. For instance, programmed generation of topological defects, tuned by the nematodynamic parameters (confinement, flow and surface anchoring), can be used as self-assembled system of flexible rails, along which micro-cargo (droplets and colloids containing the materials of interest) could be transported over large length scales in a targeted manner. More recently, Topological Microfluidics served as a unique testbed to uncover interactions between singularities in disparate fields: hydrodynamic and molecular fields [2]. Such cross-field coupling is characteristic to a range of soft and living materials, as evidenced recently in active microbial systems [3, 4]. Emerging concepts in Topological Microfluidics provide mechanistic frameworks for analysing active micro-scale transport in cellular systems, thus enabling theoretical foundations for the next generation of bio- medical, sensing and diagnostic applications. I will conclude the talk with a note on the footprints of Topological Microfluidics in future–from designing novel materials to exploring open questions in the physics of living matter–spanning diverse multi-field topological systems.

1. Topological Microfluidics: A. Sengupta, Springer International Publishing, Switzerland, 2013

2. Cross-talk between topological defects in different fields revealed by nematic microfluidics: L. Giomi, Ž. Kos, M. Ravnik, & A. Sengupta, PNAS (USA) 114, E5771, 2017

3. Microbial Active Matter: A Topological Framework: A. Sengupta, Frontiers in Physics 8, 184, 2020

4. Trade-offs in phenotypic noise synchronize emergent topology to actively enhance transport in microbial environments: J. Dhar, …, & A. Sengupta (under revision), preprint: arXiv:2105.00465, 2021

Cement is the main binding agent in concrete, literally gluing together rocks and sand into the most-used synthetic material on Earth. However, cement production is responsible for significant amounts of man- made greenhouse gases—in fact if the cement industry were a country, it would be the third largest emitter in the world. Alternatives to the current, environmentally harmful cement production process are essentially not available also because the gaps in fundamental understanding hamper the development of smarter and more sustainable solutions. The ultimate challenge is to link the chemical composition of cement grains to the nanoscale physics of the cohesive forces that emerge when mixing cement with water. Cement nanoscale cohesion originates from the electrostatics of ions accumulated in a water-based solution between like-charged surfaces but it is not captured by existing theories because of the nature of the ions involved and the high surface charges. Surprisingly enough, this is also the case for unexplained cohesion in a range of colloidal and biological matter. About one century after the early studies of cement hydration, we have gained new insight into how cement cohesion develops during hydration and cement hydrate gels grow. I will discuss how 3D numerical simulations that feature a simple but molecular description of ions and water, together with an analytical theory that goes beyond the traditional continuum approximations, helped us demonstrate that the optimized interlocking of ion-water structures determine the net cohesive forces and their evolution. These findings open the path to scientifically grounded strategies of material design for cements and have implications for a much wider range of materials and systems where ionic water-based solutions feature both strong Coulombic and confinement effects, ranging from biological membranes to soils. Construction materials are central to our society and to our life as humans on this planet, but usually far removed from fundamental science. We can now start to understand how cement physical-chemistry determines performance, durability and sustainability.

I shall describe some table-top experiments that we have recently performed in our laboratory. In the first experiment, millimeter-sized steel balls are dropped in aqueous clay suspensions. We observe that balls of larger diameters fail to achieve terminal velocities over the entire duration of the experiment. We propose a toy model that correctly predicts the time-dependence of the ball velocity for a range of ball sizes and clay concentrations. In another experiment, we record and analyze the interfacial fingering patterns that emerge when glycerol-water mixtures displace aqueous cornstarch suspensions in a radial Hele-Shaw cell. Increasing the viscosity of the displacing Newtonian fluid and the concentration-dependent elasticity of the outer cornstarch suspension both lead to significant suppression of interfacial instabilities. By performing a linear stability analysis of the interface, we predict a dominant wavelength of interfacial perturbation that closely matches with the spacing between fingers measured experimentally at the onset of instability.

Nonlinear shear rheology is a valid tool for understanding molecular dynamics of polymers in fast flows. On the one hand, it provides the necessary background for the development of predictive models in nonlinear regime. On the other hand, it allows to test nonlinear constitutive equations. However, rheological measurements of polymers in fast shear flows are challenging because of the occurrence of flow instabilities. Such instabilities include wall slip, shear banding and edge fracture. The latter has been widely documented in the literature since the early work of Tanner and Keentok.

The most efficient method to reduce the effects of edge fracture in rheological shear measurements is the use of a cone-partitioned-plate (CPP) geometry. The key idea lies in the increase of sample volume beyond that confined by the measurement border so that, when fracture occurs at the edge of the sample, it does not affect the torque measurements. However, this is not a complete solution to the problem, but it delays the phenomenon, until the unavoidable fracture that starts at the outer non-measured surface of the sample propagates inward, reaching the measuring tool. Besides shear viscosity, a complete characterization of the nonlinear shear flow behaviour of polymers requires the evaluation of first and second normal stress differences. However, measuring normal stresses of polymers is difficult, especially at high shear rates, and it requires ad-hoc methods. On the other hand, a straightforward approach can be implemented for extracting both first and second normal stress differences, by using CPP tools having different diameters.

By using CPP rheometry, we explore the features of the nonlinear shear flow behaviour of polymer melts, in both entangled and unentangled regime. As far as the entangled regime is concerned, we prove that the differences between polymer melts and concentrated solutions with the same number of entanglements observed in uniaxial extension are not detectable in shear, due to molecular tumbling. As far as the unentangled state is concerned, we elucidate the scaling laws of both viscosity and normal stress differences in nonlinear shear, and we remark the difference with entangled polymers. Finally, we discuss the perspectives of the nonlinear shear flow properties of complex molecular architectures.

Foam films and soap bubbles typically consist of fluid sandwiched between two surfactant-laden surfaces that are ~ 5 nm - 10 microns apart, and the drainage in films occurs under the influence of viscous, interfacial and intermolecular forces, including disjoining pressure. Ultrathin foam films of soft matter containing supramolecular structures like micelles, nanoparticles, smectic liquid crystals, lipid bilayers, and polyelectrolytes undergo drainage via stratification, manifested as step-wise thinning in interferometry-based measurements of average thickness. In this study, we focus exclusively on stratification in micellar foam films formed with aqueous solution of sodium dodecyl sulfate (SDS) above the critical micelle concentration (CMC). In reflected light microscopy, stratifying films (thickness < 100nm) display regions with distinct shades of grey implying that domains and nanostructures with varied thickness coexist in the thinning film. Understanding and analyzing such nanoscopic thickness transitions and variations have been long-standing experimental challenge due to the lack of technique with the requisite spatio-temporal resolution, and theoretical challenge due to the absence of models for describing hydrodynamics and thermodynamics in stratified thin films. Using IDIOM (interferometry digital imaging optical microscopy) protocols we developed recently, we show that the nanoscopic thickness variations in stratifying films can be visualized and analyzed with an unprecedented spatial (thickness ~ 1nm, lateral ~500 nm) and temporal resolution (< 1 ms). Stratification proceeds by formation of thinner domains that grow at the expense of surrounding films. Using the exquisite thickness maps created using IDIOM protocols, we provide the first visualization of nanoridges as well as mesas that form at the moving front around expanding domains. We contrast the step size measured in stratification studies with intermicellar distance obtained from scattering measurements, and explicitly measure the non-DLVO supramolecular oscillatory surface force contribution to disjoining pressure. Most significantly, we develop a self-consistent theoretical framework, a nonlinear thin film equation model that explicitly accounts for the influence of supramolecular oscillatory surface forces (using expressions we developed as a part of this study), as well as the physicochemical properties of surfactants. We show the complex spatio-temporal evolution of domains, nanoridges, nanoridge-to-mesa instability and mesas in stratifying foam films can be modeled quantitatively, and we elucidate how surfactant type and concentration can be manipulated and controlled for molecular engineering of micellar foams.