Memory formation in matter Keim, Nathan C.; Paulsen, Joseph D.; Zeravcic, Zorana ...
Reviews of modern physics,
07/2019, Volume:
91, Issue:
3
Journal Article
Peer reviewed
Open access
Memory formation in matter is a theme of broad intellectual relevance; it sits at the interdisciplinary crossroads of physics, biology, chemistry, and computer science. Memory connotes the ability to ...encode, access, and erase signatures of past history in the state of a system. Once the system has completely relaxed to thermal equilibrium, it is no longer able to recall aspects of its evolution. The memory of initial conditions or previous training protocols will be lost. Thus many forms of memory are intrinsically tied to far-from-equilibrium behavior and to transient response to a perturbation. This general behavior arises in diverse contexts in condensed-matter physics and materials, including phase change memory, shape memory, echoes, memory effects in glasses, return-point memory in disordered magnets, as well as related contexts in computer science. Yet, as opposed to the situation in biology, there is currently no common categorization and description of the memory behavior that appears to be prevalent throughout condensed-matter systems. Here the focus is on material memories. The basic phenomenology of a few of the known behaviors that can be understood as constituting a memory will be described. The hope is that this will be a guide toward developing the unifying conceptual underpinnings for a broad understanding of memory effects that appear in materials.
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The invasion of one fluid into another of higher viscosity in a quasi-two dimensional geometry typically produces complex fingering patterns. Because interfacial tension suppresses short-wavelength ...fluctuations, its elimination by using pairs of miscible fluids would suggest an instability producing highly ramified singular structures. Previous studies focused on wavelength selection at the instability onset and overlooked the striking features appearing more globally. Here we investigate the non-linear growth that occurs after the instability has been fully established. We find a rich variety of patterns that are characterized by the viscosity ratio between the inner and the outer fluid, η(in)/η(out), as distinct from the most-unstable wavelength, which determines the onset of the instability. As η(in)/η(out) increases, a regime dominated by long highly-branched fractal fingers gives way to one dominated by blunt stable structures characteristic of proportionate growth. Simultaneously, a central region of complete outer-fluid displacement grows until it encompasses the entire pattern at η(in)/η(out)≈0.3.
We present an analysis of finite-size effects in jammed packings of N soft, frictionless spheres at zero temperature. There is a 1/N correction to the discrete jump in the contact number at the ...transition so that jammed packings exist only above isostaticity. As a result, the canonical power-law scalings of the contact number and elastic moduli break down at low pressure. These quantities exhibit scaling collapse with a nontrivial scaling function, demonstrating that the jamming transition can be considered a phase transition. Scaling is achieved as a function of N in both two and three dimensions, indicating an upper critical dimension of 2.
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At densities higher than the jamming transition for athermal, frictionless repulsive spheres we find two distinct length scales, both of which diverge as a power law as the transition is approached. ...The first, ξ_{Z}, is associated with the two-point correlation function for the number of contacts on two particles as a function of the particle separation. The second, ξ_{f}, is associated with contact-number fluctuations in subsystems of different sizes. On scales below ξ_{f}, the fluctuations are highly suppressed, similar to the phenomenon of hyperuniformity usually associated with density fluctuations. The exponents for the divergence of ξ_{Z} and ξ_{f} are different and appear to be different in two and three dimensions.
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Recent advances in designing metamaterials have demonstrated that global mechanical properties of disordered spring networks can be tuned by selectively modifying only a small subset of bonds. Here, ...using a computationally efficient approach, we extend this idea to tune more general properties of networks. With nearly complete success, we are able to produce a strain between any two target nodes in a network in response to an applied source strain on any other pair of nodes by removing only ∼1% of the bonds. We are also able to control multiple pairs of target nodes, each with a different individual response, from a single source, and to tune multiple independent source/target responses simultaneously into a network. We have fabricated physical networks in macroscopic 2D and 3D systems that exhibit these responses. This work is inspired by the long-range coupled conformational changes that constitute allosteric function in proteins. The fact that allostery is a common means for regulation in biological molecules suggests that it is a relatively easy property to develop through evolution. In analogy, our results show that long-range coupled mechanical responses are similarly easy to achieve in disordered networks.
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Periodic training of creeping solids Hexner, Daniel; Liu, Andrea J.; Nagel, Sidney R.
Proceedings of the National Academy of Sciences - PNAS,
12/2020, Volume:
117, Issue:
50
Journal Article
Peer reviewed
Open access
We consider disordered solids in which the microscopic elements can deform plastically in response to stresses on them. We show that by driving the system periodically, this plasticity can be ...exploited to train in desired elastic properties, both in the global moduli and in local “allosteric” interactions. Periodic driving can couple an applied “source” strain to a “target” strain over a path in the energy landscape. This coupling allows control of the system’s response, even at large strains well into the nonlinear regime, where it can be difficult to achieve control simply by design.
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A drop impacting a solid surface with sufficient velocity will emit many small droplets creating a splash. However, splashing is completely suppressed if the surrounding gas pressure is lowered. The ...mechanism by which the gas affects splashing remains unknown. We use high-speed interference imaging to measure the air beneath all regions of a spreading viscous drop as well as optical absorption to measure the drop thickness. Although an initial air bubble is created on impact, no significant air layer persists until the time a splash is created. This suggests that splashing in our experimentally accessible range of viscosities is initiated at the edge of the drop as it encroaches into the surrounding gas.
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We report the design of three-dimensional (3D) hierarchical wrinkle substrates that can maintain their superhydrophobicity even after being repeatedly stretched. Monolithic poly(dimethysiloxane) ...with multiscale features showed wetting properties characteristic of static superhydrophobicity with water contact angles (>160°) and very low contact angle hysteresis (<5°). To examine how superhydrophobicity was maintained as the substrate was stretched, we investigated the dynamic wetting behavior of bouncing and splashing upon droplet impact with the surface. On hierarchical wrinkles consisting of three different length scales, superhydrophobic bouncing was observed. The substrate remained superhydrophobic up to 100% stretching with no structural defects after 1000 cycles of stretching and releasing. Stretchable superhydrophobicity was possible because of the monolithic nature of the hierarchical wrinkles as well as partial preservation of nanoscale structures under stretching.
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For more than a century, physicists have described real solids in terms of perturbations about perfect crystalline order1. Such an approach takes us only so far: a glass, another ubiquitous form of ...rigid matter, cannot be described in any meaningful sense as a defected crystal2. Is there an opposite extreme to a crystal--a solid with complete disorder--that forms an alternative starting point for understanding real materials? Here, we argue that the solid comprising particles with finite-ranged interactions at the jamming transition3, 4, 5 constitutes such a limit. It has been shown that the physics associated with this transition can be extended to interactions that are long ranged6. We demonstrate that jamming physics is not restricted to amorphous systems, but dominates the behaviour of solids with surprisingly high order. Just as the free-electron and tight-binding models represent two idealized cases from which to understand electronic structure1, we identify two extreme limits of mechanical behaviour. Thus, the physics of jamming can be set side by side with the physics of crystals to provide an organizing structure for understanding the mechanical properties of solids over the entire spectrum of disorder.
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Using an electrical method and high-speed imaging, we probe drop coalescence down to 10 ns after the drops touch. By varying the liquid viscosity over two decades, we conclude that, at a sufficiently ...low approach velocity where deformation is not present, the drops coalesce with an unexpectedly late crossover time between a regime dominated by viscous and one dominated by inertial effects. We argue that the late crossover, not accounted for in the theory, can be explained by an appropriate choice of length scales present in the flow geometry.
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