182 research outputs found
Normal stresses, contraction, and stiffening in sheared elastic networks
When elastic solids are sheared, a nonlinear effect named after Poynting
gives rise to normal stresses or changes in volume. We provide a novel relation
between the Poynting effect and the microscopic Gr\"uneisen parameter, which
quantifies how stretching shifts vibrational modes. By applying this relation
to random spring networks, a minimal model for, e.g., biopolymer gels and solid
foams, we find that networks contract or develop tension because they vibrate
faster when stretched. The amplitude of the Poynting effect is sensitive to the
network's linear elastic moduli, which can be tuned via its preparation
protocol and connectivity. Finally, we show that the Poynting effect can be
used to predict the finite strain scale where the material stiffens under
shear.Comment: 5 pages, 5 figure
Beyond linear elasticity: Jammed solids at finite shear strain and rate
The shear response of soft solids can be modeled with linear elasticity,
provided the forcing is slow and weak. Both of these approximations must break
down when the material loses rigidity, such as in foams and emulsions at their
(un)jamming point -- suggesting that the window of linear elastic response near
jamming is exceedingly narrow. Yet precisely when and how this breakdown occurs
remains unclear. To answer these questions, we perform computer simulations of
stress relaxation and shear startup experiments in athermal soft sphere
packings, the canonical model for jamming. By systematically varying the strain
amplitude, strain rate, distance to jamming, and system size, we identify
characteristic strain and time scales that quantify how and when the window of
linear elasticity closes, and relate these scales to changes in the microscopic
contact network. Our findings indicate that the mechanical response of jammed
solids are generically nonlinear and rate-dependent on experimentally
accessible strain and time scales.Comment: 10 pages, 9 figure
Contact Changes of Sheared Systems: Scaling, Correlations, and Mechanisms
We probe the onset and effect of contact changes in 2D soft harmonic particle
packings which are sheared quasistatically under controlled strain. First, we
show that in the majority of cases, the first contact changes correspond to the
creation or breaking of contacts on a single particle, with contact breaking
overwhelmingly likely for low pressures and/or small systems, and contact
making and breaking equally likely for large pressures and in the thermodynamic
limit. The statistics of the corresponding strains are near-Poissonian. The
mean characteristic strains exhibit scaling with the number of particles N and
pressure P, and reveal the existence of finite size effects akin to those seen
for linear response quantities. Second, we show that linear response accurately
predicts the strains of the first contact changes, which allows us to study the
scaling of the characteristic strains of making and breaking contacts
separately. Both of these show finite size scaling, and we formulate scaling
arguments that are consistent with the observed behavior. Third, we probe the
effect of the first contact change on the shear modulus G, and show in detail
how the variation of G remains smooth and bounded in the large system size
limit: even though contact changes occur then at vanishingly small strains,
their cumulative effect, even at a fixed value of the strain, are limited, so
that effectively, linear response remains well-defined. Fourth, we explore
multiple contact changes under shear, and find strong and surprising
correlations between alternating making and breaking events. Fifth, we show
that by making a link with extremal statistics, our data is consistent with a
very slow crossover to self averaging with system size, so that the
thermodynamic limit is reached much more slowly than expected based on finite
size scaling of elastic quantities or contact breaking strains
Evidence of lipid degradation during overnight contact lens wear:gas chromatography mass spectrometry as the diagnostic tool
Purpose. We investigated structural differences in the fatty acid profiles of lipids extracted from ex vivo contact lenses by using gas chromatography mass spectrometry (GCMS). Two lens materials (balafilcon A or lotrafilcon A) were worn on a daily or continuous wear schedule for 30 and 7 days. Methods. Lipids from subject-worn lenses were extracted using 1:1 chloroform: methanol and transmethylated using 5% sulfuric acid in methanol. Fatty acid methyl esters (FAMEs) were collected using hexane and water, and analyzed by GCMS (Varian 3800 GC, Saturn 2000 MS). Results. The gas chromatograms of lens extracts that were worn on a continuous wear schedule showed two predominant peaks, C16:0 and C18:0, both of which are saturated fatty acids. This was the case for balafilcon A and lotrafilcon A lenses. However, the gas chromatograms of lens extracts that were worn on a daily wear schedule showed saturated (C16:0, C18:0) and unsaturated (C16:1 and C18:1) fatty acids. Conclusions. Unsaturated fatty acids are degraded during sleep in contact lenses. Degradation occurred independently of lens material or subject-to-subject variability in lipid deposition. The consequences of lipid degradation are the production of oxidative products, which may be linked to contact lens discomfort
Softening and Yielding of Soft Glassy Materials
Solids deform and fluids flow, but soft glassy materials, such as emulsions,
foams, suspensions, and pastes, exhibit an intricate mix of solid and
liquid-like behavior. While much progress has been made to understand their
elastic (small strain) and flow (infinite strain) properties, such
understanding is lacking for the softening and yielding phenomena that connect
these asymptotic regimes. Here we present a comprehensive framework for
softening and yielding of soft glassy materials, based on extensive numerical
simulations of oscillatory rheological tests, and show that two distinct
scenarios unfold depending on the material's packing density. For dense
systems, there is a single, pressure-independent strain where the elastic
modulus drops and the particle motion becomes diffusive. In contrast, for
weakly jammed systems, a two-step process arises: at an intermediate softening
strain, the elastic and loss moduli both drop down and then reach a new plateau
value, whereas the particle motion becomes diffusive at the distinctly larger
yield strain. We show that softening is associated with an extensive number of
microscopic contact changes leading to a non-analytic rheological signature.
Moreover, the scaling of the softening strain with pressure suggest the
existence of a novel pressure scale above which softening and yielding
coincide, and we verify the existence of this crossover scale numerically. Our
findings thus evidence the existence of two distinct classes of soft glassy
materials -- jamming dominated and dense -- and show how these can be
distinguished by their rheological fingerprint.Comment: 9 pages, 11 figures, to appear in Soft Matte
Mechanical properties of contact lenses:the contribution of measurement techniques and clinical feedback to 50 years of materials development
Purpose: This review summarises the way in which mechanical property measurements combined with clinical perception have influenced the last half century of materials evolution in contact lens development. Methods: Literature concerning the use of . in-vitro testing in assessment of the mechanical behaviour of contact lenses, and the mutual deformation of the lens material and ocular tissue was examined. Tensile measurements of historic and available hydrogel lenses have been collected, in addition to manufacturer-generated figures for the moduli of commercial silicone hydrogel lenses. Results: The three conventional modes of mechanical property testing; compression, tension and shear each represent different perspective in understanding the mutual interaction of the cornea and the contact lens. Tensile testing provides a measure of modulus, together with tensile strength and elongation to break, which all relate to handling and durability. Studies under compression also measure modulus and in particular indicate elastic response to eyelid load. Studies under shear conditions enable dynamic mechanical behaviour of the material to be assessed and the elastic and viscous components of modulus to be determined. These different methods of measurement have contributed to the interpretation of lens behaviour in the ocular environment. An amalgamated frequency distribution of tensile moduli for historic and currently available contact lens materials reveals the modal range to be 0.3-0.6. MPa. Conclusion: Mechanical property measurements of lens materials have enabled calibration of an important aspect of their ocular interaction. This together with clinical feedback has influenced development of new lens materials and assisted clinical rationalisation of in-eye behaviour of different lenses
Sticky Matter: Jamming and rigid cluster statistics with attractive particle interactions
While the large majority of theoretical and numerical studies of the jamming
transition consider athermal packings of purely repulsive spheres, real complex
fluids and soft solids generically display attraction between particles. By
studying the statistics of rigid clusters in simulations of soft particles with
an attractive shell, we present evidence for two distinct jamming scenarios.
Strongly attractive systems undergo a continuous transition in which rigid
clusters grow and ultimately diverge in size at a critical packing fraction.
Purely repulsive and weakly attractive systems jam via a first order
transition, with no growing cluster size. We further show that the weakly
attractive scenario is a finite size effect, so that for any nonzero attraction
strength, a sufficiently large system will fall in the strongly attractive
universality class. We therefore expect attractive jamming to be generic in the
laboratory and in nature.Comment: 4 pages, 5 figure
- …