Unlike in thermodynamic equilibrium where coexisting phases always have the same temperature, here we show that systems comprising "active" self-propelled particles can self-organize into two coexisting phases at different kinetic temperatures, which are separated from each other by a sharp and persistent temperature gradient. Contrasting previous studies which have focused on overdamped descriptions of active particles, we show that a "hot-cold-coexistence" occurs if and only if accounting for inertia, which is significant in a broad range of systems such as activated dusty plasmas, microflyers, whirling fruits or beetles at interfaces. Our results exemplify a route to use active particles to create a self-sustained temperature gradient across coexisting phases, a phenomenon, which is fundamentally beyond equilibrium physics. arXiv:1902.06116v2 [cond-mat.soft]

Glass forming liquids exhibit a rich phenomenology upon confinement. This is often related to the effects arising from wall-fluid interactions. Here we focus on the interesting limit where the separation of the confining walls becomes of the order of a few particle diameters. For a moderately polydisperse, densely packed hard-sphere fluid confined between two smooth hard walls, we show via event-driven molecular dynamics simulations the emergence of a multiple reentrant glass transition scenario upon a variation of the wall separation. Using thermodynamic relations, this reentrant phenomenon is shown to persist also under constant chemical potential. This allows straightforward experimental investigation and opens the way to a variety of applications in micro-and nanotechnology, where channel dimensions are comparable to the size of the contained particles. The results are in-line with theoretical predictions obtained by a combination of density functional theory and the mode-coupling theory of the glass transition. A thorough understanding of the slowing down of transport by orders of magnitude upon approaching the glass transition is one of the grand challenges of condensed matter theory [1][2][3][4][5]. A recent focus in the study of glasses has been to introduce competing mechanisms that lead to glass transition phase diagrams exhibiting non-monotonic behaviour. Reentrant scenarios have been uncovered, for example, upon adding a short-range attraction to colloidal particles [6][7][8], by competing near ordering in binary mixtures [9,10], or by inserting the liquid in a frozen disordered host structure [11][12][13]. However, instead of changing the structure of the liquid directly, one may also affect its properties by purely geometric means, via an increase of its confinement [14][15][16][17][18][19][20][21][22][23][24]. Depending on the ratio of the characteristic confinement length (e.g., the wall separation) to particle diameter, this can either lead to an increase or decrease of the first peak of the pair distribution function-the latter being a measure of the "stiffness" of the local packing structure [18]. As long as crystallization is kinetically hindered, this is expected to have a strong impact on the dynamics of the liquid and the glass transition. Earlier simulation studies and experiments of the confinement effects on the glass transition were mainly concerned with wall-to-wall separations of the order of several particle diameters or larger (see, e.g., [14][15][16][17][18][19] and references therein). Recently, however, the case of stronger confinement has received growing attention [20][21][22][23]. Here we focus on this latter regime of strong confinement, where only a few particle layers fit into the space between the walls. The problem of crystallization is circumvented by introducing size-dispersity [25] into our simulations, which leads to a geometric frustration. We evaluate the diffusion coefficient to assess the slowing-down of the dynamics and to establish a glass-transition state diag...

-Spatial correlations of microscopic fluctuations are investigated via real-space experiments and computer simulations of colloidal glasses under steady shear. It is shown that while the distribution of one-particle fluctuations is always isotropic regardless of the relative importance of shear as compared to thermal fluctuations, their spatial correlations show a marked sensitivity to the competition between shear-induced and thermally activated relaxation. Correlations are isotropic in the thermally dominated regime, but develop strong anisotropy as shear dominates the dynamics of microscopic fluctuations. We discuss the relevance of this observation for a better understanding of flow heterogeneity in sheared amorphous solids.Introduction. -The nature of microscopic fluctuations in the relaxation and flow of glasses is a topic of great current interest because of their importance to a wide range of materials including foams, emulsions, granulates, and colloidal and molecular glasses. Thermal glasses such as molecular and colloidal glasses exhibit structural relaxation due to thermally induced rearrangements [1-3], while athermal amorphous materials such as granulates and foams relax typically by external forces such as applied shear that drive their flow [4,5]. It has been suggested that the applied shear acts as an effective temperature that drives microscopic fluctuations in an isotropic way [6][7][8]. Such a concept is attractive since it opens a possibility of the generalized fluctuation dissipation relation under a nonequilibrium situation. This idea is also motivated by the jamming phase diagram [9] that depicts applied shear stress as an alternative way to fluidize the glass, in an attempt to unify amorphous materials in a common framework. Supported by simulations of sheared glasses [6,8], the current models of plasticity of amorphous materials [10,11] assume that the rate of activated events is controlled by an isotropic effective temperature that is dependent on the shear rate. While these models have been quite successful in describing many aspects regarding the plastic response of amorphous solids to external load such as simple shear, many other issues such as flow

Within a recently developed mode-coupling theory for fluids confined to a slit we elaborate numerical results for the long-time limits of suitably generalized intermediate scattering functions. The theory requires as input the density profile perpendicular to the plates, which we obtain from density functional theory within the fundamental-measure framework, as well as symmetry-adapted static structure factors, which can be calculated relying on the inhomogeneous Percus-Yevick closure. Our calculations for the nonergodicity parameters for both the collective as well as for the self motion are in qualitative agreement with our extensive event-driven molecular dynamics simulations for the intermediate scattering functions for slightly polydisperse hard-sphere systems at high packing fraction. We show that the variation of the nonergodicity parameters as a function of the wavenumber correlates with the in-plane static structure factors, while subtle effects become apparent in the structure factors and relaxation times of higher mode indices. A criterion to predict the multiple reentrant from the variation of the in-plane static structure is presented.

Efficient navigation through disordered, porous environments poses a major challenge for swimming microorganisms and future synthetic cargo-carriers. We perform Brownian dynamics simulations of active stiff polymers undergoing run-reverse dynamics, and so mimic bacterial swimming, in porous media. In accord with experiments of Escherichia coli, the polymer dynamics are characterized by trapping phases interrupted by directed hopping motion through the pores. Our findings show that the spreading of active agents in porous media can be optimized by tuning their run lengths, which we rationalize using a coarse-grained model. More significantly, we discover a geometric criterion for the optimal spreading, which emerges when their run lengths are comparable to the longest straight path available in the porous medium. Our criterion unifies results for porous media with disparate pore sizes and shapes and for run-and-tumble polymers. It thus provides a fundamental principle for optimal transport of active agents in densely-packed biological and environmental settings.

In a recent paper [Mandal et al., Phys. Rev. E 88, 022129 (2013)], the nature of spatial correlations of plasticity in hard-sphere glasses was addressed both via computer simulations and in experiments. It was found that the experimentally obtained correlations obey a power law, whereas the correlations from simulations are better fitted by an exponential decay. We here provide direct evidence-via simulations of a hard-sphere glass in two dimensions (2D)-that this discrepancy is a consequence of the finite system size in the 3D simulations. By extending the study to a 2D soft disk model at zero temperature [Durian, Phys. Rev. Lett. 75, 4780 (1995)], the robustness of the power-law decay in sheared amorphous solids is underlined. Deviations from a power law occur when either reducing the packing fraction towards the supercooled regime in the case of hard spheres or changing the dissipation mechanism from contact dissipation to a mean-field-type drag in the case of soft disks.

Single-particle fluctuations and directional correlations in driven hard-sphere glassesMandal, S.; Chikkadi, V.; Nienhuis, B.; Raabe, R.; Schall, P.; Varnik, F. Published in:Physical Review E DOI:10.1103/PhysRevE.88.022129 Link to publication Citation for published version (APA):Mandal, S., Chikkadi, V., Nienhuis, B., Raabe, R., Schall, P., & Varnik, F. (2013). Single-particle fluctuations and directional correlations in driven hard-sphere glasses. Physical Review E, 88, 022129. DOI: 10.1103/PhysRevE.88.022129 General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Via event-driven molecular dynamics simulations and experiments, we study the packing-fraction and shearrate dependence of single-particle fluctuations and dynamic correlations in hard-sphere glasses under shear. At packing fractions above the glass transition, correlations increase as shear rate decreases: the exponential tail in the distribution of single-particle jumps broadens and dynamic four-point correlations increase. Interestingly, however, upon decreasing the packing fraction, a broadening of the exponential tail is also observed, while dynamic heterogeneity is shown to decrease. An explanation for this behavior is proposed in terms of a competition between shear and thermal fluctuations. Building upon our previous studies [Chikkadi et al., Europhys. Lett. 100, 56001 (2012)], we further address the issue of anisotropy of the dynamic correlations.

There is growing evidence that the flow of driven amorphous solids is not homogeneous, even if the macroscopic stress is constant across the system. Via event driven molecular dynamics simulations of a hard sphere glass, we provide the first direct evidence for a correlation between the fluctuations of the local volume-fraction and the fluctuations of the local shear rate. Higher shear rates do preferentially occur at regions of lower density and vice versa. The temporal behavior of fluctuations is governed by a characteristic time scale, which, when measured in units of strain, is independent of shear rate in the investigated range. Interestingly, the correlation volume is also roughly constant for the same range of shear rates. A possible connection between these two observations is discussed. PACS numbers:Introduction.-Heterogeneous flow and shear banding are central to the rheology of complex fluids and are widely observed in many industrial and natural materials, such as foams, emulsions, pastes, or even rocks [1,2]. Despite its ubiquitous appearance, many aspects of this phenomenon are still not well-understood. In the simplest case, shear banding can be captured by a nonmonotonic dependence of the shear stress, σ, on the shear rate, γ [3,4]. For certain complex fluids, like colloidal gels, shear banding can be associated to the competition between a structural phase transition and a shear [5]. However, in other systems, such as dense hard sphere (HS) colloidal suspensions [6] and granular materials [7], flow heterogeneity is often observed without such accompanying structural changes. The rheological response of these systems is essentially determined by the competition between an inherent slow dynamics and the acceleration caused by the external drive [8,9]. This may lead to a spatially and temporally heterogeneous flow if the system is close to the yielding threshold [10][11][12].