77 research outputs found
Non-affine response: jammed packings versus spring networks
We compare the elastic response of spring networks whose contact geometry is
derived from real packings of frictionless discs, to networks obtained by
randomly cutting bonds in a highly connected network derived from a
well-compressed packing. We find that the shear response of packing-derived
networks, and both the shear and compression response of randomly cut networks,
are all similar: the elastic moduli vanish linearly near jamming, and
distributions characterizing the local geometry of the response scale with
distance to jamming. Compression of packing-derived networks is exceptional:
the elastic modulus remains constant and the geometrical distributions do not
exhibit simple scaling. We conclude that the compression response of jammed
packings is anomalous, rather than the shear response.Comment: 6 pages, 6 figures, submitted to ep
Memory formation in matter
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. 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: 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 we focus on
material memories. We will describe the basic phenomenology of a few of the
known behaviors that can be understood as constituting a memory. We hope that
this will be a guide towards developing the unifying conceptual underpinnings
for a broad understanding of memory effects that appear in materials
Catalysis from the bottom-up
Catalysis, the acceleration of chemical reactions by molecules that are not
consumed in the process, is essential to living organisms but currently absent
in physical systems that aspire to emulate biological functionalities with
artificial components. Here we demonstrate how to design a catalyst using
spherical building blocks interacting via programmable potentials, and show
that a minimal catalyst design, a rigid dimer, can accelerate a ubiquitous
elementary reaction, the cleaving of a bond. By combining coarse-grained
molecular dynamics simulations and theory, and by comparing the mean reaction
time in the presence and absence of the catalyst, we derive geometrical and
physical constraints for its design and determine the reaction conditions under
which catalysis emerges in the system. The framework and design rules that we
introduce are general and can be applied to experimental systems on a wide
range of scales, from micron size DNA-coated colloids to centimeter size
magnetic handshake materials, opening the door to the realization of
self-regulated artificial systems with bio-inspired functionalities.Comment: 8 pages, 4 figures. Submitte
Vibrations in materials with granularity
This thesis concerns the vibrational properties of different classical disordered condensed matter systems. In the first part we focus on materials that exhibit a rigidity transition as their density is increased. By introducing a new method into the field, we were able to look into the localization behavior of vibrational modes of jammed packings of soft spherical particles, both in the localized regime where the localization length is much less and in the regime where it is grater than the linear system size. We also analyze the nature of vibrational modes of jammed packings of soft elliptical particles, where we uncover the change of the structure of the spectrum, compared to the simplest model of sphere packings, due to the rotational degrees of freedom of the particles. In the second part of this thesis we explore the localization properties of collective modes and response to uniform driving of bubble clouds. We find that the response is often very different from that of a typical mode because the frequency response of each mode is sufficiently wide that many modes are excited when the cloud is driven by an ultrasound.UBL - phd migration 201
Contact Changes near Jamming
We probe the onset and effect of contact changes in soft harmonic particle
packings which are sheared quasistatically. We find that the first contact
changes are the creation or breaking of contacts on a single particle. We
characterize the critical strain, statistics of breaking versus making a
contact, and ratio of shear modulus before and after such events, and explain
their finite size scaling relations. For large systems at finite pressure, the
critical strain vanishes but the ratio of shear modulus before and after a
contact change approaches one: linear response remains relevant in large
systems. For finite systems close to jamming the critical strain also vanishes,
but here linear response already breaks down after a single contact change.Comment: 5 pages, 4 figure
Memory formation in Matter
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
Critical jamming of frictional grains in the generalized isostaticity picture
While frictionless spheres at jamming are isostatic, frictional spheres at
jamming are not. As a result, frictional spheres near jamming do not
necessarily exhibit an excess of soft modes. However, a generalized form of
isostaticity can be introduced if fully mobilized contacts at the Coulomb
friction threshold are considered as slipping contacts. We show here that, in
this framework, the vibrational density of states (DOS) of frictional discs
exhibits a plateau when the generalized isostaticity line is approached. The
crossover frequency to elastic behavior scales linearly with the distance from
this line. Moreover, we show that the frictionless limit, which appears
singular when fully mobilized contacts are treated elastically, becomes smooth
when fully mobilized contacts are allowed to slip.Comment: 4 pages, 4 figures, submitted to PR
Excitations of Ellipsoid Packings near Jamming
We study the vibrational modes of three-dimensional jammed packings of soft
ellipsoids of revolution as a function of particle aspect ratio and
packing fraction. At the jamming transition for ellipsoids, as distinct from
the idealized case using spheres where , there are many
unconstrained and non-trivial rotational degrees of freedom. These constitute a
set of zero-frequency modes that are gradually mobilized into a new rotational
band as increases. Quite surprisingly, as this new band is
separated from zero frequency by a gap, and lies below the onset frequency for
translational vibrations, , the presence of these new degrees of
freedom leaves unaltered the basic scenario that the translational spectrum is
determined only by the average contact number. Indeed, depends
solely on coordination as it does for compressed packings of spheres. We also
discuss the regime of large , where the two bands merge.Comment: 6 pages, 4 figure
Effects of particle-size ratio on jamming of binary mixtures
We perform a systematic numerical study of the effects of the particle-size
ratio on the properties of jammed binary mixtures. We find that
changing does not qualitatively affect the critical scaling of the pressure
and coordination number with the compression near the jamming transition, but
the critical volume fraction at the jamming transition varies with .
Moreover, the static structure factor (density correlation) strongly
depends on and shows distinct long wave-length behaviors between large and
small particles. Thus the previously reported behavior of in the
long wave-length limit is only a special case in the limit, and cannot
be simply generalized to jammed systems with .Comment: 5 pages and 4 figures, submitted to Soft Matter, special issue on
Granular and Jammed Material
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