Swelling and curling fibers via elastocapillarity

Coalescence of paintbrush bristles removed from a bath of fluid is the result of competing elastic and surface energies. The length scale that emerges out of this energy balance is called the “elastocapillary” length scale. This phenomenon has been well studied both experimentally and theoretically at the desktop scale as well as microscale. However, in many natural and synthetic systems, the fluid between the flexible fibers can swell the material and causes the fibers to curl. A natural example is human hair, which swells in humid conditions, dilating and becoming frizzy. In this presentation, we demonstrate experimental results on this coupled “elastocapillary–elastoswelling” system. Specifically, we identify two distinct regimes dominated by capillarity and swellability, and the transition between these two regimes is governed by the “elastoswelling” length scale. We also show that in the swelling dominated regime a small fluid droplet is being carried upward by the curling fibers that mimic a pipetting mechanism

The relationship between viscoelasticity and elasticity

Soft materials that are subjected to large deformations exhibit an extremely rich phenomenology, with properties lying in between those of simple fluids and those of elastic solids. In the continuum description of these systems, one typically follows either the route of solid mechanics (Lagrangian description) or the route of fluid mechanics (Eulerian description). The purpose of this review is to highlight the relationship between the theories of viscoelasticity and of elasticity, and to leverage this connection in contemporary soft matter problems. We review the principles governing models for viscoelastic liquids, for example solutions of flexible polymers. Such materials are characterized by a relaxation time λ, over which stresses relax. We recall the kinematics and elastic response of large deformations, and show which polymer models do (and which do not) correspond to a nonlinear elastic solid in the limit λ → ∞. With this insight, we split the work done by elastic stresses into reversible and dissipative parts, and establish the general form of the conservation law for the total energy. The elastic correspondence can offer an insightful tool for a broad class of problems; as an illustration, we show how the presence or absence of an elastic limit determines the fate of an elastic thread during capillary instability

Hydrogel menisci: Shape, interaction, and instability

The interface of a soft hydrogel is easily deformed when it is in contact with particles, droplets or cells. Here we compute the intricate shapes of hydrogel menisci due to the indentation of point particles. The analysis is based on a free energy formulation, by which we also assess the interaction laws between neighbouring particles on hydrogel interfaces, similar to the "Cheerios effect". It is shown how the meniscus formed around the particles results from a competition between surface tension, elasticity and hydrostatic pressure inside the gel. We provide a detailed overview of the various scaling laws, which are governed by a characteristic shear modulus $G^*=\sqrt{\gamma\rho g}$ that is based on surface tension $\gamma$ and gravity $\rho g$. Stiffer materials exhibit a solid-like response while softer materials are more liquid-like. The importance of $G^*$ is further illustrated by examining the Rayleigh-Taylor instability of soft hydrogels.Comment: 7 pages, 3 figure

RISK FACTOR ASSESSMENT FOR ACNE VULGARIS IN HUMAN AND IMPLICATIONS FOR PUBLIC HEALTH INTERVENTIONS IN NORTH CENTRAL INDIA: A SURVEY-BASED STUDY

Background: Acne vulgaris is a very common dermatological problem of adolescence since the time immemorial. However, it is neither life threatening nor is a physical disability, but acne affects social and psychological functioning. Acne vulgaris is multifactorial, apart from basic factor of hormonal change and bacterial outbreak; there are several other factors that may influence the prevalence of acne.Methods: In the present study, populations were assessed for influence of various factors on acne prevalence. This cross-sectional study was a population based field study intending to discern the factors that influence the prevalence of acne in adolescents. The study carried out from April 2016 to October 2016 in north central India. For this survey, questionnaires were design to cover all the required information regarding occurrence of acne that include factors like gender, age, skin type, complexion, season of occurrence, dietary habit etc.Results: Acne vulgaris appears to be influenced by gender, age, seasonal variations, breakout area, complexion, skin types and dietary habits. Further, the influence of dietary habit on acne, by the consumption of dairy products or high-carbon diet has also been evaluated. Apart from depicting the vulnerable range of age (p=0.003288), sensitivity on various skin types (p=0.00039) and complexion (p=0.001355) on the basis of gender; This Field study on Acne Vulgaris, also reveals that the season has inordinate role in acne pervasiveness (p=0.115731).Conclusion: This study is helpful in categorizing the risk factors and evidencing the afflictions of acne in population thus, contributing health care planning. Â

Two-dimensional electron-gas-like charge transport at magnetic Heusler alloy-SrTiO3_3 interface

We report remarkably low residual resistivity, giant residual resistivity ratio, free-electron-like Hall resistivity and high mobility ($\approx$ 10$^4$ cm$^2$V$^{-1}$s$^{-1}$) charge transport in epitaxial films of Co$_2$MnSi and Co$_2$FeSi grown on (001) SrTiO$_3$. This unusual behavior is not observed in films deposited on other cubic oxide substrates of comparable lattice parameters. The scaling of the resistivity with thickness of the films allow extraction of interface conductance, which can be attributed to a layer of oxygen vacancies confined within 1.9 nm of the interface as revealed by atomically resolved electron microscopy and spectroscopy. The high mobility transport observed here at the interface of a fully spin polarized metal is potentially important for spintronics applications

Reversal of Solvent Migration in Poroelastic Folds

Polymer networks and biological tissues are often swollen by a solvent, such that their properties emerge from a coupling between swelling and elastic stress. This poroelastic coupling becomes particularly intricate in wetting, adhesion, and creasing, for which sharp folds appear that can even lead to phase separation. Here we resolve the singular nature of poroelastic surface folds and determine the solvent distribution in the vicinity of the fold-tip. Surprisingly, two opposite scenarios emerge depending on the angle of the fold. In obtuse folds such as creases, it is found that the solvent is completely expelled near the crease-tip, according to a nontrivial spatial distribution. For wetting ridges with acute fold angles, the solvent migration is reversed as compared to creasing, and the degree of swelling is maximal at the fold-tip. We discuss how our poroelastic fold-analysis offers an explanation for phase separation, fracture and contact angle hysteresis.Comment: 5 pages, 3 figures, supplementary material attached as anciliary fil