123 research outputs found
Grand Challenges in Soft Matter Physics
As its name implies, soft matter science deals with materials that are easily deformed. These materials, which include polymers, gels, colloids, emulsions, foams, surfactant assemblies, liquid crystals, granular materials, and many biological materials, have in common that they are organized on mesoscopic length scales, with structural features that are much larger than an atom, but much smaller than the overall size of the material. The large size of the basic structural units and the relatively weak interactions that hold them together are responsible for the characteristic softness of these materials1, but they also lead to many other distinct features of soft materials [1], such as sensitivity toward thermal fluctuations and external stimuli and a slow response with long relaxation times, often resulting in non-trivial flow behavior and arrest in non-equilibrium states. These features make soft matter problems challenging. In hard condensed matter physics, it is often possible to accurately predict material properties based on the interactions between the individual atoms, which are typically organized on a regular crystalline lattice. For soft matter systems, with their intrinsically heterogeneous structure, complex interactions across different length scales, and slow dynamics, this is much more difficult. The subtle interplay between interactions and thermal fluctuations can lead to complex emergent behavior, such as spontaneous pattern formation, self-assembly, and a large response to small external stimuli. Because of the wide range of materials and systems that can be classified as soft matter, soft matter science is an inherently interdisciplinary field, in which physics, chemistry, materials science, biology, nanotechnology, and engineering come together. For a field that is so broad in scope, it is impossible to do justice to the entire range of outstanding problems or even to identify two or three key challenges. For this reason, I will only highlight a small (and highly personal) selection of current challenges in the field. The interdisciplinary nature of the field will be evident from these examples
Strand plasticity governs fatigue in colloidal gels
Repeated loading of a solid leads to microstructural damage that ultimately
results in catastrophic material failure. While posing a major threat to the
stability of virtually all materials, the microscopic origins of fatigue,
especially for soft solids, remain elusive. Here we explore fatigue in
colloidal gels as prototypical inhomogeneous soft solids by combining
experiments and computer simulations. Our results reveal how mechanical loading
leads to irreversible strand stretching, which builds slack into the network
that softens the solid at small strains and causes strain hardening at larger
deformations. We thus find that microscopic plasticity governs fatigue at much
larger scales. This gives rise to a new picture of fatigue in soft thermal
solids and calls for new theoretical descriptions of soft gel mechanics in
which local plasticity is taken into account.Comment: 5 pages, 4 figure
Cracking up: symmetry breaking in cellular systems
The shape of animal cells is, to a large extent, determined by the cortical actin network that underlies the cell membrane. Because of the presence of myosin motors, the actin cortex is under tension, and local relaxation of this tension can result in cortical flows that lead to deformation and polarization of the cell. Cortex relaxation is often regulated by polarizing signals, but the cortex can also rupture and relax spontaneously. A similar tension-induced polarization is observed in actin gels growing around beads, and we propose that a common mechanism governs actin gel rupture in both systems
Microscopic insights into the failure of elastic double networks
The toughness of a polymer material can increase significantly if two
networks are combined into one material. This toughening effect is a
consequence of a transition from a brittle to a ductile failure response.
Although this transition and the accompanying toughening effect have been
demonstrated in hydrogels first, the concept has been proven effective in
elastomers and in macroscopic composites as well. This suggests that the
transition is not caused by a specific molecular architecture, but rather by a
general physical principle related to the mechanical interplay between two
interpenetrating networks. Here we employ theory and computer simulations,
inspired by this general principle, to investigate how disorder controls the
brittle-to-ductile transition both at the macroscopic and the microscopic
level. A random spring network model featuring two different spring types,
enables us to study the joined effect of initial disorder and network-induced
stress heterogeneity on this transition. We reveal that a mechanical force
balance gives a good description of the brittle-to-ductile transition. In
addition, the inclusion of disorder in the spring model predicts four different
failure regimes along the brittle-to-ductile response in agreement with
experimental findings. Finally, we show that the network structure can result
in stress concentration, diffuse damage and loss of percolation depending on
the failure regime. This work thus provides a framework for the design and
optimization of double network materials and underlines the importance of
network structure in the toughness of polymer materials.Comment: main text: 18 pages, 9 figures. Supplemental material: 5 pages, 6
figure
Stress management in composite biopolymer networks
Living tissues show an extraordinary adaptiveness to strain, which is crucial
for their proper biological functioning. The physical origin of this mechanical
behaviour has been widely investigated using reconstituted networks of collagen
fibres, the principal load-bearing component of tissues. However, collagen
fibres in tissues are embedded in a soft hydrated polysaccharide matrix which
generates substantial internal stresses whose effect on tissue mechanics is
unknown. Here, by combining mechanical measurements and computer simulations,
we show that networks composed of collagen fibres and a hyaluronan matrix
exhibit synergistic mechanics characterized by an enhanced stiffness and
delayed strain-stiffening. We demonstrate that the polysaccharide matrix has a
dual effect on the composite response involving both internal stress and
elastic reinforcement. Our findings elucidate how tissues can tune their
strain-sensitivity over a wide range and provide a novel design principle for
synthetic materials with programmable mechanical properties
Crosslinker mobility governs fracture behavior of catch-bonded networks
While most chemical bonds weaken under the action of mechanical force (called
slip bond behavior), nature has developed bonds that do the opposite: their
lifetime increases as force is applied. While such catch bonds have been
studied quite extensively at the single molecule level and in adhesive
contacts, recent work has shown that they are also abundantly present as
crosslinkers in the actin cytoskeleton. However, their role and the mechanism
by which they operate in these networks have remained unclear. Here, we present
computer simulations that show how polymer networks crosslinked with either
slip or catch bonds respond to mechanical stress. Our results reveal that catch
bonding may be required to protect dynamic networks against fracture, in
particular for mobile linkers that can diffuse freely after unbinding. While
mobile slip bonds lead to networks that are very weak at high stresses, mobile
catch bonds accumulate in high stress regions and thereby stabilize cracks,
leading to a more ductile fracture behavior. This allows cells to combine
structural adaptivity at low stresses with mechanical stability at high
stresses
Elongated particles discharged with a conveyor belt in a two-dimensional silo
The flow of elliptical particles out of a 2-dimensional silo when extracted
with a conveyor belt is analyzed experimentally. The conveyor belt - placed
directly below the silo outlet - reduces the flow rate, increases the size of
the stagnant zone, and it has a very strong influence on the relative velocity
fluctuations as they strongly increase everywhere in the silo with decreasing
belt speed. In other words, instead of slower but smooth flow, flow reduction
by belt leads to intermittent flow. Interestingly, we show that this
intermittency correlates with a strong reduction of the orientational order of
the particles at the orifice region. Moreover, we observe that the average
orientation of the grains passing through the outlet is modified when they are
extracted with the belt, a feature that becomes more evident for large
orifices.Comment: 11 pages, 11 figures, final version published in Phys. Rev.
Slippery paints:Eco-friendly coatings that cause ants to slip
Many insects are considered to be pests and can be serious threats to buildings. Insecticides represent an effective way to control pest insects but are harmful to the environment. As an eco-friendlier alternative, we have formulated waterborne, organic paints which provided a slippery physical barrier for leafcutter ants (Atta cephalotes) on vertical surfaces. Different paints were produced by varying the Pigment Volume Concentration (PVC) and amount of TiO2 and CaCO3 particles, and characterised in terms of contact angles, surface roughness and scrub resistance. The paints' slipperiness for A. cephalotes ants was evaluated in climbing tests on vertical paint panels (by recording the percentage of fallen ants). Two main factors reduced the insects' attachment to vertical paint surfaces: (1) the PVC: in paints above a critical PVC, more loose particles detach from the coating and thereby reduce insect attachment; and (2) the type, dimensions and shape of solid particles: CaCO3 particles detach more easily from the paint than TiO2, probably due to their larger size and platelet shape. Paints formulated at PVC 70 and containing 20 wt% CaCO3 showed the best performance in terms of slipperiness, as well as providing good scrub resistance
Temperature-Responsive Polyelectrolyte Complexes for Bio-Inspired Underwater Adhesives
Adhesive proteins of marine organisms contain significant amounts of hydrophobic amino acids. Therefore, inter- and intramolecular hydrophobic interactions are expected to play an important role in both adhesion and cohesion. Here, we mimic the hydrophobicity of adhesive proteins by using temperature-responsive polyelectrolyte complexes (TERPOCs) with a high poly(N-isopropylacrylamide) (PNIPAM) content. Upon mixing aqueous solutions of PNIPAM-b-poly(acrylic acid)-b-PNIPAM and poly(2-(dimethylamino) ethyl methacrylate) (PDMAEMA), complexation between the oppositely charged polyelectrolytes occurs. At low temperatures, complex coacervate core micelles (C3Ms) with a PNIPAM corona are formed, and upon a temperature increase, the solution turns into a hydrogel by the formation of a network of hydrophobic PNIPAM domains. Consequently, an abrupt increase in viscosity is observed upon heating which facilitates injectability of the adhesive. The gelation temperature, Tgel, and (adhesive) strength of the TERPOC can be adjusted by altering the salt and polymer concentration, which changes the balance between the electrostatic and hydrophobic interactions. Despite the importance of hydrophobic groups in strong underwater adhesives, we conclude that TERPOCs with a high PNIPAM content (70 wt%) are unstable due to water release. Consequently, there is a limited amount of hydrophobic groups that can be inserted in this type of systems. Nevertheless, TERPOCs show promising and tunable properties for application as injectable underwater adhesives, for example in biomedical applications
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