Giovanni Del Monte and Emanuela Zaccarelli, Physical Review X 14, 041067 (2024).
We perform extensive molecular dynamics simulations of an ensemble of realistic microgel particles in swollen conditions in a wide range of packing fractions ζ.We compare neutral and charged microgels, where we consider charge distribution adherent to experimental conditions. Through a detailed analysis of singleparticle behavior, we are able to identify the different regimes occurring upon increasing concentration: from shrinking to deformation and interpenetration, always connecting our findings with available experimental observations. We then link these single-particle features with the collective behavior of the suspension, finding evidence of a structural reentrance that has no counterpart in the dynamics. Hence, while the maximum of the radial distribution function displays a nonmonotonic behavior with increasing ζ, the dynamics, quantified by the microgels’ mean-squared displacement, always slows down. This behavior, at odds with the simple Hertzian model, can be described by a phenomenological multi-Hertzian model, which takes into account the enhanced internal stiffness of the core. However, also this model fails when deformation enters into play, whereby more realistic many-body models are required. Thanks to our analysis, we are able to unveil the key physical mechanisms, shrinking and deformation, giving rise to the structural reentrance that holds up to very large packing fractions. We further identify key similarities and differences between neutral and charged microgels, for which we detect at high enough ζ the fusion of charged shells, previously invoked to explain key experimental findings, and responsible for the structural reentrance. Overall, our study establishes a powerful framework to uncover the physics of microgel suspensions, paving the way to tackle different regimes, e.g., high temperature, and internal architectures, such as for hollow and ultralow-cross-linked microgels, where experimental evidence is still limited.
(Fig.) Representative snapshots of neutral microgel suspensions at packing fractions ζ=0.39, 0.87, and 2.52, from left to right, respectively. Box dimensions are reproduced with the right proportions so that deswelling of the microgels can be visualized.
Sebastian Geier, Adrian Braemer, Eduard Braun, Maximilian Müllenbach, Titus Franz, Martin Gärttner, Gerhard Zürn, and Matthias Weidemüller, Physical Review Research 6, 033197 (2024).
Time reversal in a macroscopic system contradicts daily experience. It is practically impossible to restore a shattered cup to its original state by just time reversing the microscopic dynamics that led to its breakage. Yet, with the precise control capabilities provided by modern quantum technology, the unitary evolution of a quantum system can be reversed in time. Here, we implement a time-reversal protocol in a dipolar interacting, isolated many-body spin system represented by Rydberg states in an atomic gas. By changing the states encoding the spin, we flip the sign of the interaction Hamiltonian, and demonstrate the reversal of the relaxation dynamics of the magnetization by letting a demagnetized many-body state evolve back in time into a magnetized state. We elucidate the role of atomic motion using the concept of a Loschmidt echo. Finally, by combining the approach with Floquet engineering, we demonstrate time reversal for a large family of spin models with different symmetries. Our method of state transfer is applicable across a wide range of quantum simulation platforms and has applications far beyond quantum many-body physics, reaching from quantum-enhanced sensing to quantum information scrambling.
(Fig.) Time reversal on a Rydberg quantum many-body system; sketch of the protocol. The time reversal is based on transferring the state between two spin-1/2 encodings in the Rydberg manifold.
Farimani, Reyhaneh A.; Dehaghani, Zahra Ahmadian; Likos, Christos N.; Ejtehadi, Mohammad Reza, Physical Review Letters 132: 148101 (2024).
We perform computer simulations of mechanically linked (poly[2]catenanes, PC) and chemically bonded (bonded rings, BR) pairs of self-avoiding ring polymers in steady shear. We find that BRs develop a novel motif, termed gradient tumbling, rotating around the gradient axis. For the PCs the rings are stretched and display another new pattern, termed slip tumbling. The dynamics of BRs is continuous and oscillatory, whereas that of PCs is intermittent between slip-tumbling attempts. Our findings demonstrate the interplay between topology and hydrodynamics in dilute solutions of connected polymers.
(Fig.) Time averages of selected diagonal elements of the gyration tensor of the poly[2]catenane and the bonded rings system (entire molecules) in shear flow, as a function of the Weissenberg number, normalized over their equilibrium values, R_2^g (0)/3. The flow-direction element, G_xx; The dark blue-filled squares refer to PC (with hydrodynamic interactions: +HI), red-filled circles to BR (+HI), empty sky blue squares to PC (without hydrodynamic interactions: −HI), and empty orange circles to BR (−HI). The simulation snapshots show a PC molecule and a BR molecule at equilibrium (Wi = 0).
Schneck, Christoph; Smrek, Jan; Likos, Christos N.; Zöttl, Andreas, Nanoscale 16, 8880-8899 (2024)
We apply monomer-resolved computer simulations of supercoiled ring polymers under shear, taking full account of the hydrodynamic interactions, accompanied, in parallel, by simulations in which these are switched off. The combination of bending and torsional rigidities inherent in these polymers, in conjunction with hydrodynamics, has a profound impact on their flow properties. In contrast to their flexible counterparts, which dramatcially deform and inflate under shear [Liebetreu et al., Commun.Mater. 2020, 1, 4], supercoiled rings undergo only weak changes in their overall shape and they display both a reduced propensity to tumbling (at fixed Weissenberg number) and a much stronger orientational resistance with respect to their flexible counterparts. In the presence of hydrodynamic interactions, the coupling of the polymer to solvent flow is capable of bringing about a topological transformation of writhe to twist at strong shear upon conservation of the overall linking number.
(Fig.) Tumbling cross-correlation function C_xy at high shear rates, as indicated in the legend of panel (a), in presence of hydrodynamic interactions. (a) Relaxed ring without torsion (k_torsion = 0), (b) σ_sc = 0.00, (c) σ_sc = 0.01, (d) σ_sc = 0.02.
Esmaeel Moghimi, Iurii Chubak,Maria Kaliva, Parvin Kiany, Taihyun Chang, Junyoung Ahn, Nikolaos Patelis, Georgios Sakellariou, Sergei A. Egorov, Dimitris Vlassopoulos, and Christos N. Likos Physical Review Research 6, 013079 (2024).
We provide unambiguous experimental evidence that ring polymers are stronger depleting agents in colloidal suspensions than their linear counterparts. We use an intermediate volume fraction (φ_c = 0.44) colloidal gel based on the classic poly(methyl methacrylate) (PMMA) hard spheres, in which the polystyrene depletant is either linear or ring of the same molar mass or the same size. We systematically increase the depletant concentration from zero (no attraction) to well above the gelation point and find that in the presence of rings, gels are formed at smaller concentrations and possess a larger storage modulus in comparison to those induced by the linear chains. Consequently, the yield stress is enhanced; however, the yield strain (gel deformability) remains concomitantly unaffected. Our experimental findings agree with theoretical calculations based on effective interaction potentials. Hence, polymer architecture is a powerful entropic tool to tailor the strength of colloidal gels.
(Fig.) Representative theoretical phase for a colloid-ring mixture (black lines) and a colloid-linear mixture (red lines) of similar size ratio. The gray- (red-)shaded regions denote phase coexistence between the pure phases in the white area below (for the linear depletants, part of that white area appears gray as it is occupied by the phasecoexistence region of the colloid-ring mixtures). The pure phases are the colloidal fcc crystal (S), the colloidal liquid (L), and the colloidal vapor (V). The think black (red) lines denote the three-phase triangle between the V, L, and S at the triple points, and the black (red) square the critical point. The filled triangle and circle denote the location of the L and S phases at triple coexistence with the V phase, for which φ_c ∼= 0. The vertical blue line represents a path of fixed colloid packing fraction φ_c = 0.44, as in the experiments.
Nataša Adžic Clemens Jochum, Christos N. Likos, and Emmanuel Stiakakis, Small: 2308763 (2024).
A combined experimental and theoretical study of the structural correlations in moderately concentrated suspensions of all-DNA dendrimers of the second generation (G2) with controlled scaffold rigidity is reported here. Small-angle X-ray scattering experiments in concentrated aqueous saline solutions of stiff all-DNA G2 dendritic constructs reveal a novel anomalous liquid-like phase behavior which is reflected in the calculated structure factors as a two-step increase at low scattering wave vectors. By developing a new design strategy for adjusting the particle’s internal flexibility based on site-selective incorporation of single-stranded DNA linkers into the dendritic scaffold, it is shown that this unconventional type of self-organization is strongly contingent on the dendrimer’s stiffness. A comprehensive computer simulation study employing dendritic models with different levels of coarse-graining, and two theoretical approaches based on effective, pair-potential interactions, remarkably confirmed the origin of this unusual liquid-like behavior. The results demonstrate that the precise control of the internal structure of the dendritic scaffold conferred by the DNA can be potentially used to engineer a rich palette of novel ultrasoft interaction potentials that could offer a route for directed self-assembly of intriguing soft matter phases and experimental realizations of a host of unusual phenomena theoretically predicted for ultrasoft interacting systems.
(Fig.) Effective pair interactions between two G2-stiff (solid lines) and between two G2-flex (dashed lines), calculated using Widom insertion method within oxDNA model and shown as a function of center-of-mass-to-center-of-mass separation r, at two different salt concentrations c.
Maximilian M. Schmidt, José Ruiz-Franco, Steffen Bochenek, Fabrizio Camerin, Emanuela Zaccarelli and Andrea Scotti, Physical Review Letters 131, 258202 (2023).
In situ interfacial rheology and numerical simulations are used to investigate microgel monolayers in a wide range of packing fractions, ζ_2D. The heterogeneous particle compressibility determines two flow regimes characterized by distinct master curves. To mimic the microgel architecture and reproduce experiments, an interaction potential combining a soft shoulder with the Hertzian model is introduced. In contrast to bulk conditions, the elastic moduli vary nonmonotonically with ζ_2D at the interface, confirming long-sought predictions of reentrant behavior for Hertzian-like systems.
(Fig.) Compression isotherms reporting the surface pressure π (squares) and plateau of the elastic modulus G_p (circles), normalized by k_B T=ξ^2, as a function of generalized packing fraction ζ_2D. Lines are guides to the eye. Different colors identify different regimes of the compression isotherms.
