Stanford Advisors


All Publications


  • Influence of Attractive Functional Groups on the Segmental Dynamics and Glass Transition in Associating Polymers Macromolecules Ghosh, A., Samanta, S., Ge, S., Sokolov, A. P., Schweizer, K. S. 2022; 55 (6): 2345–2357
  • Statistical behavior of nonequilibrium and living biological systems subjected to active and thermal fluctuations. Physical review. E Ghosh, A., Spakowitz, A. J. 2022; 105 (1-1): 014415

    Abstract

    We present a path-integral formulation of the motion of a particle subjected to fluctuating active and thermal forces. This general framework predicts the statistical behavior associated with the stochastic trajectories of the particle, accounting for all possible realizations of Brownian and active forces, over an arbitrary potential landscape. Temporal correlations in the active forces result in non-Markovian statistics, necessitating the inclusion of a fixed active-force value at specified times within the statistical treatment. We specialize our theory to that of exponentially correlated active forces for a particle in a harmonic potential. We find the exact results for the statistical distributions for the initial position of the particle, accounting for the impact of the correlated active forces at all times prior to the initial time. Our theory is then used to find the two-point distribution for the active Brownian particle, which governs the joint probability that a particle begins and ends at specified locations. Analyses of the active Brownian statistics demonstrate that the impact of active forces can be interpreted through a time-dependent temperature whose influence depends on the competition of timescales of the active-force correlation and the relaxation time of the particle in the harmonic potential. The general results presented in this work are transferable to a broad range of nonequilibrium systems with active and Brownian motion, and the time-dependent temperature serves as a governing principle to describe the competition of timescales associated with active forces and internal relaxation processes.

    View details for DOI 10.1103/PhysRevE.105.014415

    View details for PubMedID 35193230

  • Linear and nonlinear viscoelasticity of concentrated thermoresponsive microgel suspensions JOURNAL OF COLLOID AND INTERFACE SCIENCE Chaudhary, G., Ghosh, A., Kang, J., Braun, P., Ewoldt, R. H., Schweizer, K. S. 2021; 601: 886-898

    Abstract

    We present an integrated experimental and theoretical study of the dynamics and rheology of self-crosslinked, slightly charged, temperature responsive soft poly(N-isopropylacrylamide) (pNIPAM) microgels over a wide range of concentration and temperature spanning the sharp change in particle size and intermolecular interactions across the lower critical solution temperature (LCST). Dramatic, non-monotonic changes in viscoelasticity are observed as a function of temperature, with distinct concentration dependence in the dense fluid, glassy, and soft-jammed regimes. Motivated by our experimental observations, we formulate a minimalistic model for the size dependence of a single microgel particle and the change of the interparticle interaction from purely repulsive to attractive upon heating. Using microscopic equilibrium and time-dependent statistical mechanical theories, theoretical predictions are quantitatively compared with experimental measurements of the shear modulus. Good agreement is found for the nonmonotonic temperature behavior that originates as a consequence of the competition between reduced microgel packing fraction and increasing interparticle attractions. Testable predictions are made for nonlinear rheological properties such as the yield stress and strain. To our knowledge, this is the first attempt to quantitatively understand in a unified manner the viscoelasticity of dense, temperature-responsive microgel suspensions spanning a wide range of temperatures and concentrations.

    View details for DOI 10.1016/j.jcis.2021.05.111

    View details for Web of Science ID 000687288600006

    View details for PubMedID 34186277

  • Physical Bond Breaking in Associating Copolymer Liquids ACS MACRO LETTERS Ghosh, A., Schweizer, K. S. 2021; 10 (1): 122-128
  • The role of collective elasticity on activated structural relaxation, yielding, and steady state flow in hard sphere fluids and colloidal suspensions under strong deformation JOURNAL OF CHEMICAL PHYSICS Ghosh, A., Schweizer, K. S. 2020; 153 (19): 194502

    Abstract

    We theoretically study the effect of external deformation on activated structural relaxation and aspects of the nonlinear mechanical response of glassy hard sphere fluids in the context of elastically collective nonlinear Langevin equation theory. This microscopic force-based approach describes activated relaxation as a coupled local-nonlocal event involving caging and longer range collective elasticity, with the latter becoming more important and ultimately dominant with increasing packing fraction under equilibrium conditions. The central new question we address is how this physical picture of activated relaxation, and the relative importance of local caging vs collective elasticity physics, depends on external deformation. Theoretical predictions are presented for deformation-induced enhancement of mobility, the onset of relaxation speed up at remarkably low values of stress, strain, or shear rate, apparent power law thinning of the steady state structural relaxation time and viscosity, a non-vanishing activation barrier in the shear thinning regime, an apparent Herschel-Bulkley form of the rate dependence of the steady state shear stress, exponential growth of different measures of a dynamic yield or flow stress with the packing fraction, and reduced fragility and dynamic heterogeneity under deformation. The results are contrasted with experiments and simulations, and qualitative or better agreement is found. An overarching conclusion is that deformation strongly reduces the importance of longer range collective elastic effects relative to the local caging aspect for most, but not all, physical questions, with deformation-dependent fragility and dynamic heterogeneity phenomena being qualitatively sensitive to collective elasticity. Overall, nonlinear rheology is predicted to be a more local problem than quiescent structural relaxation, albeit with deformation-modified activated processes still important.

    View details for DOI 10.1063/5.0026258

    View details for Web of Science ID 000595279100001

    View details for PubMedID 33218226

  • Microscopic Theory of the Effect of Caging and Physical Bonding on Segmental Relaxation in Associating Copolymer Liquids MACROMOLECULES Ghosh, A., Schweizer, K. S. 2020; 53 (11): 4366-4380
  • Microscopic theory of onset of decaging and bond-breaking activated dynamics in ultradense fluids with strong short-range attractions Physical Review E Ghosh, A., Schweizer, K. S. 2020; 101: 060601(R)
  • Microscopic theory of the influence of strong attractive forces on the activated dynamics of dense glass and gel forming fluids JOURNAL OF CHEMICAL PHYSICS Ghosh, A., Schweizer, K. S. 2019; 151 (24): 244502

    Abstract

    We theoretically study the nonmonotonic (re-entrant) activated dynamics associated with a finite time scale kinetically defined repulsive glass to fluid to attractive glass transition in high volume fraction particle suspensions interacting via strong short range attractive forces. The classic theoretical "projection" approximation that replaces all microscopic forces by a single effective force determined solely by equilibrium pair correlations is revisited based on the "projectionless dynamic theory" (PDT). A hybrid-PDT approximation is formulated that explicitly quantifies how attractive forces induce dynamical constraints, while singular hard core interactions are treated based on the projection approach. Both the effects of interference between repulsive and attractive forces, and structural changes due to attraction-induced bond formation that competes with caging, are included. Combined with the microscopic Elastically Collective Nonlinear Langevin Equation (ECNLE) theory of activated relaxation, the resultant approach appears to properly capture both the re-entrant dynamic crossover behavior and the strong nonmonotonic variation of the activated structural relaxation time with attraction strength and range at very high volume fractions as observed experimentally and in simulations. Testable predictions are made. Major differences compared to both ideal mode coupling theory and ECNLE theory based on the full force projection approximation are identified. Calculations are also performed for smaller time and length scale intracage dynamics relevant to the non-Gaussian parameter based on analyzing the dynamic free energy that controls particle trajectories. Implications of the new theory for thermal glass forming liquids with relatively long range attractive forces are briefly analyzed.

    View details for DOI 10.1063/1.5129941

    View details for Web of Science ID 000513160200043

    View details for PubMedID 31893898

  • Thermoresponsive Stiffening with Microgel Particles in a Semiflexible Fibrin Network MACROMOLECULES Chaudhary, G., Ghosh, A., Bharadwaj, N., Kang, J., Braun, P. V., Schweizer, K. S., Ewoldt, R. H. 2019; 52 (8): 3029-3041
  • Linear and nonlinear rheology and structural relaxation in dense glassy and jammed soft repulsive pNIPAM microgel suspensions SOFT MATTER Ghosh, A., Chaudhary, G., Kang, J., Braun, P. V., Ewoldt, R. H., Schweizer, K. S. 2019; 15 (5): 1038-1052

    Abstract

    We present an integrated experimental and quantitative theoretical study of the mechanics of self-crosslinked, slightly charged, repulsive pNIPAM microgel suspensions over a very wide range of concentrations (c) that span the fluid, glassy and putative "soft jammed" regimes. In the glassy regime we measure a linear elastic dynamic shear modulus over 3 decades which follows an apparent power law concentration dependence G' ∼ c5.64, a variation that appears distinct from prior studies of crosslinked ionic microgel suspensions. At very high concentrations there is a sharp crossover to a nearly linear growth of the modulus. To theoretically understand these observations, we formulate an approach to address all three regimes within a single conceptual Brownian dynamics framework. A minimalist single particle description is constructed that allows microgel size to vary with concentration due to steric de-swelling effects. Using a Hertzian repulsion interparticle potential and a suite of statistical mechanical theories, quantitative predictions under quiescent conditions of microgel collective structure, dynamic localization length, elastic modulus, and the structural relaxation time are made. Based on a constant inter-particle repulsion strength parameter which is determined by requiring the theory to reproduce the linear elastic shear modulus over the entire concentration regime, we demonstrate good agreement between theory and experiment. Testable predictions are then made. We also measured nonlinear rheological properties with a focus on the yield stress and strain. A theoretical analysis with no adjustable parameters predicts how the quiescent structural relaxation time changes under deformation, and how the yield stress and strain change as a function of concentration. Reasonable agreement with our observations is obtained. To the best of our knowledge, this is the first attempt to quantitatively understand structure, quiescent relaxation and shear elasticity, and nonlinear yielding of dense microgel suspensions using microscopic force based theoretical methods that include activated hopping processes. We expect our approach will be useful for other soft polymeric particle suspensions in the core-shell family.

    View details for DOI 10.1039/c8sm02014k

    View details for Web of Science ID 000457540000019

    View details for PubMedID 30657517

  • Variational aspects of the Klein-Gordon equation INDIAN JOURNAL OF PHYSICS Datta, S. N., Ghosh, A., Chakraborty, R. 2015; 89 (2): 181-187