Seminar by Dr. Debjyoti Majumdar (Ben-Gurion University, Israel) on "Non-equilibrium dynamics of fractal networks with active forces."

18 Feb 2026
Seminar Room # 350, second floor annex

Speaker: Dr. Debjyoti Majumdar
Department of Solar Energy and Environmental Physics,
Jacob Blaustein Institutes for Desert Research
Ben-Gurion University, Israel

Title: "Non-equilibrium dynamics of fractal networks with
active forces."

Day and Date: Wednesday, February 18, 2026

Time: 11.00 am.

Venue: Room no. 350, Chemistry Department
Second floor, Annex
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Hosted by Prof. Rajarshi Chakrabarti

Talk Title : "Non-equilibrium dynamics of fractal networks with active forces."
Abstract
We investigate, using Langevin dynamics simulations, the Rouse-type dynamics of active fractal bead-spring networks constructed using the critical bond percolation cluster of the square lattice. Two types of active stochastic forces, modelled as random telegraph process with finite decorrelation time, are considered: force monopoles, acting on individual nodes in random directions, and force dipoles, where extensile or contractile forces act between pairs of connected nodes. For force monopoles, a dynamical steady state is reached where the network is dynamically swollen and the mean square displacement (MSD) shows sub-diffusive behavior determined solely by the spectral dimension of the underlying fractal network, in accord with a previously proposed general analytic theory [Singh and Granek, Chaos, 34(113107), 2024]. In contrast, dipolar forces require diverging times to reach the steady state and lead to network shrinkage. We find a continuous crossover to a collapsed state for the non-diluted square lattice, resulting from its marginal stability. The MSD is found to saturate at the same temporal regime, followed by ballistic-like and/or diffusive behaviors. We further extend our study of dipolar forces to diluted regimes above the “rigidity percolation” threshold for triangular lattices [D. Majumdar et al., J.Chem. Phys., 163(114902), 2025]. Here, weak dipolar forces do not shrink the network in the steady state. Moreover, for the triangular lattice, an incipient discontinuous collapse transition occurs above a critical force amplitude. Importantly, we find that the inclusion of active force dipoles rigidify the triangular network even below the regular rigidity percolation point, which can be exactly identified by including the extra constraints from the active dipole links in the Maxwell constraint counting method. Finally, we will show how spatial correlation in these active dipoles affect the dynamics of the active network.