CyberSec Research

A particle moving through a worm-like micellar fluid (WLM) shows instability and large fluctuations beyond a threshold. Despite many detailed studies, a direct measurement of the time-dependent stress on the probe particle remains unexplored. To address this, we have designed a measuring geometry coupled with a commercial rheometer to study the dynamics of a cylindrical probe through a WLM system of 2 wt.\% cetyltrimethyl ammonium tosylate(CTAT) + 100 mM sodium chloride(NaCl) for a wide range of velocity and stress scales. We map out the in-situ velocity distribution using particle imaging velocimetry. Beyond a certain velocity threshold, we observe large stress fluctuation events with gradual stress build-up followed by sudden stress drop indicating the storage and release of elastic energy. The length scale constructed from the stress build-up time scale and the probe's velocity match the length scale of extensile deformation in the sample just before the stress drop, further confirming the strong correlation of such storage and release of energy with the unstable motion of the probe. Interestingly, despite their significant difference in magnitudes, the Weissenberg number ($Wi$) for the onset of flow instability obtained from the shear and extensile components remains almost the same. We also find that the turbulent motion of the probe at higher $Wi$ results from the complex mixing of the stick-slip events originating from the partial release of the stored elastic energy. Further, we show that the magnitude of the stick-slip events depends on the detailed micellar structure and dynamics controlled by salt concentration and temperature.

The self-assembly of complex structures from engineered subunits is a major goal of nanotechnology, but controlling their size becomes increasingly difficult in larger assemblies. Existing strategies present significant challenges, among which are the use of multiple subunit types or the precise control of their shape and mechanics. Here we introduce an alternative approach based on identical subunits whose interactions promote crystals, but also favor crystalline defects. We theoretically show that topological restrictions on the scope of these defects in large assemblies imply that the assembly size is controlled by the magnitude of the defect-inducing interaction. Using DNA origami, we experimentally demonstrate both size and shape control in two-dimensional disk- and fiber-like assemblies. Our basic concept of defect engineering could be generalized well beyond these simple examples, and thus provide a broadly applicable scheme to control self-assembly.

This study employs modified Poisson-Boltzmann theory to systematically investigate the influence of zwitterionic osmolyte additives to an electrolyte solution on disjoining pressure and electric differential capacitance within charged slit-like nanopores with conductive walls. We demonstrate that increasing concentrations of zwitterionic osmolytes result in a marked synergistic enhancement of both disjoining pressure and differential capacitance, highlighting their dual role in improving supercapacitor performance. The insights gained underscore the unique capabilities of zwitterionic osmolytes as multifunctional additives for fine-tuning the properties of electric double layers, thereby bridging the gap between capacitive efficiency and electrode longevity.

The kinetic theory of dilute granular gases with hard-core and inverse power-law potentials is developed. The scattering process is studied theoretically, which yields the relative speed and the impact parameter dependence of the scattering angle. The viscosity is derived from the Boltzmann equation and its temperature dependence is plotted. We also perform the direct simulation Monte Carlo to check the validity of the theory.

Simulations of knotting and unknotting in polymers or other filaments rely on random processes to facilitate topological changes. Here we introduce a method of \textit{topological steering} to determine the optimal pathway by which a filament may knot or unknot while subject to a given set of physics. The method involves measuring the knotoid spectrum of a space curve projected onto many surfaces and computing the mean unravelling number of those projections. Several perturbations of a curve can be generated stochastically, e.g. using the Langevin equation or crankshaft moves, and a gradient can be followed that maximises or minimises the topological complexity. We apply this method to a polymer model based on a growing self-avoiding tangent-sphere chain, which can be made to model proteins by imposing a constraint that the bending and twisting angles between successive spheres must maintain the distribution found in naturally occurring protein structures. We show that without these protein-like geometric constraints, topologically optimised polymers typically form alternating torus knots and composites thereof, similar to the stochastic knots predicted for long DNA. However, when the geometric constraints are imposed on the system, the frequency of twist knots increases, similar to the observed abundance of twist knots in protein structures.

Ergodicity breaking and aging effects are fundamental challenges in out-of-equilibrium systems. Various mechanisms have been proposed to understand the non-ergodic and aging phenomena, possibly related to observations in systems ranging from structural glass and Anderson glasses to biological systems and mechanical systems. While anomalous diffusion described by Levy statistics efficiently captures ergodicity breaking, the origin of aging and ergodicity breaking in systems with ultraslow dynamics remain unclear. Here, we report a novel mechanism of ergodicity breaking in systems exhibiting log-aging diffusion. This mechanism, characterized by increasingly infrequent rare events with aging, yields statistics deviating significantly from Levy distribution, breaking ergodicity as shown by unequal time- and ensemble-averaged mean squared displacements and two distinct asymptotic probability distribution functions. Notably, although these rare events contribute negligibly to statistical averages, they dramatically change the system's characteristic time. This work lays the groundwork for microscopic understanding of out-of-equilibrium systems and provides new perspectives on glasses and Griffiths-McCoy singularities.

We investigate the collapse kinetics of charged polymers (polyelectrolytes) induced by counterion condensation using coarse-grained molecular dynamics simulations. Under good solvent conditions, polyelectrolytes above the critical charge density ($A > A_c$) exhibit significantly faster collapse dynamics compared to neutral polymers, with dynamic scaling exponents ($\nu_c \approx 0.76-0.84$) distinctly smaller than those observed for neutral polymers ($\nu_c \approx 1.44$) . This accelerated collapse is driven primarily by three mechanisms: (1) local charge neutralization due to counterion condensation, which facilitates immediate local compaction, (2) screening of long-range electrostatic repulsions, reducing the conformational search space, and (3) bridging interactions mediated by multivalent counterions, enhancing efficient formation of intra-chain contacts. We systematically explore the effects of polymer length, charge density, and counterion valency (monovalent, divalent, and trivalent) on collapse dynamics, demonstrating that increased counterion valency significantly lowers the critical charge density required for collapse and accelerates the collapse process. Our findings highlight the limitations of modeling charged biopolymers using purely neutral coarse-grained models, underscoring the importance of electrostatic interactions and counterion dynamics in determining their kinetic pathways. These insights may aid in better understanding the folding, organization, and dynamics of inherently charged biomolecules, such as proteins and nucleic acids.

The construction of accurate interatomic potentials, and related fields of forces, from equilibrium conformational distributions of molecules is a crucial step in coarse-grained modeling. In this work we show that in order to develop accurate lab-frame force fields that preserve translational and rotational diffusion of a molecule, the observed body-fixed free energy landscape must be corrected for conformation-dependent rotational entropy to isolate the potential energy surface. We further demonstrate that even when the instantaneous effects of the correction are small, the resulting lagged correlations of the modeled force can be greatly altered and hence the correction is especially vital when parameterizing friction coefficients using modeled interatomic potentials.

This work explores spin-wave dynamics in magnetic nanotubes, focusing on the influence of the Dzyaloshinskii-Moriya interaction and curvature. The study uses analytical methods to examine how these factors influence the emergence of nonreciprocity and azimuthal standing waves in nanotubes with longitudinal magnetization along the axis or with a vortex-like magnetization. The interplay between exchange, Dzyaloshinskii-Moriya, and dipolar couplings in determining the chirality of spin waves is discussed. When the magnetization is saturated along an axis, the spin waves propagating along it are symmetric under the inversion of the wave vector. However, magnetochirality, mainly driven by exchange and Dzyaloshinskii-Moriya couplings, is observed in the azimuthal standing modes. In the vortex state, frequency nonreciprocity occurs for waves propagating along the tube, while the azimuthal modes remain reciprocal. For positive Dzyaloshinskii-Moriya interaction, and depending on the helicity of the vortex, the asymmetry induced by the dipolar interaction is reinforced, whereas a negative coupling opposes this asymmetry. The influence of radial anisotropy is also examined. It is found that radial anisotropy reduces the frequency of the modes and shifts the dispersion minimum to a finite wave vector in the vortex state. The properties of modes near zero frequency offer insight into the emergence of chiral magnetic textures.

Fibre-reinforced elastomers are lightweight and strong materials that can sustain large deformations. When filled with magnetic particles, their effective mechanical response can be modified by an external magnetic field. In the present study, we propose an effective theory of fibre-reinforced composite, based on a neo-Hookean elastic response and a linear magnetic law in each phase. The theory is shown suitable to describe the motion of composite cylinders. Furthermore, it is found appropriate for the modelling of fibre-reinforced composites subjected to a permanent magnetic field aligned with the fibres. To reach this result, we use the incremental theory ('small on large'), in combination with homogenisation theory and the Bloch-Floquet method. This way, we show that wave directivity is sensitive to the application of a permanent magnetic field, whereas the frequency range in which wave propagation is forbidden is not modified by such a load (the band gaps are invariant). In passing, we describe a method to deduce the total stress in the material based on the measurement of two wave speeds. Furthermore, we propose an effective energy function for the description of nonlinear composites made of Yeoh-type generalised neo-Hookean fibres within a neo-Hookean matrix.

This article deals with the existence and scaling of an energy cascade in steady granular liquid flows between the scale at which the system is forced and the scale at which it dissipates energy. In particular, we examine the possible origins of a breaking of the Kolmogorov Universality class that applies to Newtonian liquids under similar conditions. In order to answer these questions, we build a generic field theory of granular liquid flows and, through a study of its symmetries, show that indeed the Kolmogorov scaling can be broken, although most of the symmetries of the Newtonian flows are preserved.

Fluctuations play a central role in many fields of physics, from quantum electrodynamics to statistical mechanics. In active matter physics, most models focus on thermal fluctuations due to a surrounding solvent. An alternative but much less explored noise source can occur due to fluctuating external fields, which typically feature certain spatial correlations. In this work, we introduce a minimal model to explore the influence of spatially correlated but temporally uncorrelated noise on the collective behavior of active particles. We find that specifically in chiral active particles such fluctuations induce the formation of network patterns, which neither occur for spatially (uncorrelated) thermal noise, nor in the complete absence of fluctuations. These networks show (i) a percolated structure, (ii) local alignment of the contained particles, but no global alignment, and (iii) hardly coarsen. We perform a topological data analysis to systematically characterize the topology of the network patterns. Our work serves as a starting point to explore the role of spatially correlated fluctuations and presents a route towards noise-induced phenomena in active matter.

We explore the statistical nature of point defects in a two-dimensional hexagonal colloidal crystal from the perspective of stochastic dynamics. Starting from the experimentally recorded trajectories of time series, the underlying drifting forces along with the diffusion matrix from thermal fluctuations are extracted. We then employ a deposition in which the deterministic terms are split into diffusive and transverse components under a stochastic potential with the lattice periodicity to uncover the dynamic landscape as well as the transverse matrix, two key structures from limited ranges of measurements. The analysis elucidates some fundamental dichotomy between mono-point and di-point defects of paired vacancies or interstitials. Having large transverse magnitude, the second class of defects are likely to break the detailed balance, Such a scenario was attributed to the root cause of lattice melting by experimental observations. The constructed potential can in turn facilitate large-scale simulation for the ongoing research.

A novel quantum representation of lattice spin operators (LSOs) is achieved by mapping quantum spins onto their classical analogues for spin size $S=1/2$ and $S=1$. The "braket" representations of LSOs are attained thanks to a profound inspection into the binary/ternary distribution of classical bits/trits in non-negative integers. We claim the possility of getting the $j$th digit of a positive integer without performing any binary/ternary decomposition. Analytical formulas returning the $j$th bits/trits of an integer are presented. Impacts of our achievements in Physics are highlighted by revisiting the $1D$ spin-$1/2$ {XXZ} Heisenberg model with open boundaries in a magnetic field in both absence (uniform magnetic field) and presence of disorder (random magnetic field). In the absence of disorder (clean system), we demonstrate that the corresponding eigenvalues problem can be reduced to a tight-binding problem on a graph and solved without resorting to any spinless transformation nor the Bethe Anzath. In the presence of disorder, a convergent perturbation theory is elaborated. Our analytical results are compared with data from exact diagonalization for relatively large spin systems ($K\leq 18$ spins with $K$ denoting the total number of spins) obtained by implementing both the global $U(1)$ symmetry to block-diagonalize the Hamiltonian and the spin-inversion symmetry for two-fold block-diagonalization in the sector with total magnetization $\mathcal{J}^z=0$. We observe a good agreement between both results.

The twist-grain-boundary (TGB) phases, characterized by a periodic, helical arrangement of blocks made of polar smectic phases, SmAF and SmCF, have been discovered. They have been observed for rod-like molecules with a strong longitudinal dipole moment, featuring an (S)-2-methylbutyl end group having only weak twisting power, and emerge below the antiferroelectric SmAAF phase, where the lamellar structure is already well established. It is suggested that the structure is governed by electrostatic interactions amplified by weak chiral forces, in striking contrast to the mechanism of TGB phase formation found in non-polar materials. The TGB phases exhibit light selective reflection in the visible range, while the value of electric polarization confirms an almost perfectly ordered dipole alignment.

We consider two (off-lattice) varieties of out-of-equilibrium systems, viz., granular and active matter systems, that, in addition to displaying velocity ordering, exhibit fascinating pattern formation in the density field, similar to those during vapor-liquid phase transitions. In the granular system, velocity ordering occurs due to reduction in the normal components of velocities, arising from inelastic collisions. In the active matter case, on the other hand, velocity alignment occurs because of the inherent tendency of the active particles to follow each other. Inspite of this difference, the patterns, even during density-field evolutions, in these systems can be remarkably similar. This we have quantified via the calculations of the two-point equal time correlation functions and the structure factors. These results have been compared with the well studied case of kinetics of phase separation within the framework of the Ising model. Despite the order-parameter conservation constraint in all the cases, in the density field, the quantitative structural features in the Ising case is quite different from those for the granular and active matters. Interestingly, the correlation function for the latter varieties, particularly for an active matter model, quite accurately describes the structure in a real assembly of biologically active particles.

We study motility-induced phase separation~(MIPS) in active AB binary mixtures undergoing the chemical reaction $A \rightleftharpoons B$. Starting from the evolution equations for the density fields $\rho_i(\vec r, t)$ describing MIPS, we phenomenologically incorporate the effects of the reaction through the reaction rate $\Gamma$ into the equations. The steady-state domain morphologies depend on $\Gamma$ and the relative activity of the species, $\Delta$. For a sufficiently large $\Gamma$ and $\Delta\ne 1$, the more active component of the mixture forms a droplet morphology. We characterize the morphology of domains by calculating the equal-time correlation function $C(r, t)$ and the structure factor $S(k, t)$, exhibiting scaling violation. The average domain size, $L(t)$, follows a diffusive growth as $L(t)\sim t^{1/3}$ before reaching the steady state domain size, $L_{\rm ss}$. Additionally, $L_{\rm ss}$ shows the scaling relation $L_{\rm ss}\sim\Gamma^{-1/4}$, independent of $\Delta$.

The increasing demand for optical technologies with dynamic spectral control has driven interest in chromogenic materials, particularly for applications in tunable infrared metasurfaces. Phase-change materials such as vanadium dioxide and germanium-antimony-tellurium, for instance, have been widely used in the infrared regime. However, their reliance on thermal and electrical tuning introduces challenges such as high power consumption, limited emissivity tuning, and slow modulation speeds. Photochromic materials may offer an alternative approach to dynamic infrared metasurfaces, potentially overcoming these limitations through rapid, light-induced changes in optical properties. This manuscript explores the potential of thiazolothiazole-embedded polymers, known for their reversible photochromic transitions and strong infrared absorption changes, for tunable infrared metasurfaces. The material exhibits low absorption and a strong photochromic contrast in the spectral range from 1500 cm-1 to 1700 cm-1, making it suitable for dynamic infrared light control. This manuscript reports on infrared imaging experiments demonstrating photochromic contrast in thiazolothiazole-embedded polymer and thereby provides compelling evidence for their potential applications for dynamic infrared metasurfaces.