Normal stresses in surface shear experiments

When viscoelastic bulk phases are sheared, the deformation of the sample induces not only shear stresses, but also normal stresses. This is a well known and well understood effect, that leads to phenomena such as rod climbing, when such phases are stirred with an overhead stirrer, or to die swell in extrusion. Viscoelastic interfaces share many commonalities with viscoelastic bulk phases, with respect to their response to deformations. There is however little experimental evidence that shear deformations of interfaces can induce in-plane normal stresses (not to be confused with stresses normal to the interface). Theoretical models for the stress-deformation behavior of complex fluid-fluid interfaces subjected to shear, predict the existence of in-plane normal stresses. In this paper we suggest methods to confirm the existence of such stresses experimentally.     

Fluctuations of permeable interfaces in water-in-water emulsions

The fluctuations of highly permeable interfaces, encountered in phase-separated biopolymer solutions, liposomes, polymersomes, or colloidosomes, are investigated. An expression for the power spectrum of the height correlation function is derived for a multicomponent system, incorporating the effects of mass transfer across the interface, using nonequilibrium thermodynamics. We also derive an expression for the relaxation time of the height correlation function, and calculate the relaxation time for a phase-separated gelatin-dextran-water system. Comparing our expression with the expression for an impermeable interface shows that mass transfer has a significant impact on the relaxation time of the interface.     

Generalized surface momentum balances for the analysis of surface dilatational data

Dilatational rheological properties of interfaces are often determined using drop tensiometers, in which the interface of the droplet is subjected to oscillatory area changes. A dynamic surface tension is determined either by image analysis of the droplet profile or by measuring the capillary pressure. Both analysis modes tend to use the Young-Laplace equation for determining the dynamic surface tension. For complex fluid-fluid interfaces there is experimental evidence that this equation does not describe the response of the interface to deformations adequately. Generalizations of this equation are available, and in this comment we will discuss these generalizations, and the conditions for which they reduce to the Young-Laplace equation.     

Rapid crack growth in a thermoelastic solid under mixed-mode thermomechanical loading

A two-dimensional steady-sate analysis of semi-infinite brittlecrack growth at a constant subcritical rate in an unboundedfully-coupled thermoelastic solid under mixed-mode thermomechanicalloading is made. The loading consists of normal and shear tractionsand heat fluxes applied as point sources (line loads in theout-of-plane direction). A related problem is solved exactly in an integral transformspace, and robust asymptotic forms used to reduce the originalproblem to a set of integral equations. The equations are partiallycoupled and exhibit operators of both Cauchy and Abel types,yet can be solved analytically. The temperature change field at a distance from the moving crackedge is then constructed, and its dominant term is found tobe controlled by the imposed heat fluxes. The role of this termis, indeed, enhanced if the heat fluxes serve to render thecrack as a net heat source/sink for the solid, as opposed tobeing a transmitter of heat across its plane. More generally,the influence of the thermoelastic coupling on this field, aswell as other functions, is found to increase with crack speed.     

Shear-induced aggregation and break up of fibril clusters close to the percolation concentration

To probe the behaviour of fibrillar assemblies of ovalbumin under oscillatory shear, close to the percolation concentration, c_p (7.5%), rheo-optical measurements and Fourier transform rheology were performed. Different results were found close to c_p (7.3%), compared to slightly further away from c_p (6.9 and 7.1%). For 6.9 and 7.1%, a decrease in complex viscosity, and a linear increase in birefringence, n, with increasing strain was observed, indicating deformation and orientation of the fibril clusters. For 7.3%, a decrease in complex viscosity was followed by an increase in complex viscosity with increasing strain, which coincided with a strong increase in n, dichroism, n, and the intensity of the normalized third harmonic (I₃/I₁). This regime was followed by a second decrease in complex viscosity, where n,n and I₃/I₁ decreased. In the first regime where the viscosity was decreasing with increasing strain, deformation and orientation of existing clusters takes place. At higher oscillatory shear, a larger deformation occurs and larger structures are formed, which is most likely aggregation of the clusters. Finally, at even higher strains, the clusters break up again. An increase in complex viscosity, n, n and I₃/I₁ was observed when a second strain sweep was performed 30 min after the first. This indicates that the shear-induced cluster formation and break up are not completely reversible, and the initial cluster size distribution is not recovered after cessation of flow.     

The surface shear viscosity of a planar liquid-vapor interface I. Theory

In this paper we develop a theory for the calculation of the surface shear viscosity of a planar liquid-vapor interface. The theory is an extension of the generalized hydrodynamics formalism, originally developed for the calculation of linear transport coefficients in isotropic bulk fluids. We develop an expression for the surface shear viscosity in terms of the actual shear viscosity profile in the interfacial region. We derive an expression for this profile in terms of the first four moments of the autocorrelation function of the transverse parallel velocity (the component of velocity parallel to k, which is the projection of k on to the interface). Finally, we calculate these moments for a planar liquid-vapor interface.     

Mesoscopic properties of semiflexible amyloid fibrils

We have determined the contour length, persistence length, bending rigidity, and critical percolation concentration for semiflexible amyloid fibrils formed from the globular proteins beta-lactoglobulin, bovine serum albumin, and ovalbumin. The persistence length was estimated using an adjusted random contact model for highly charged semiflexible chains. We have found contour lengths in the range of 50 nm to 10 microm and persistence lengths in the range of 16 nm to 1.6 microm. This wide range of contour and persistence lengths and the ease of preparation of these amyloid fibrils make them ideal model systems for the study of semiflexible polymers.     

Synthetic magnetic opals

We present studies of novel nanocomposites of BiNi impregnated into the structure of opals as well as inverse opals. Atomic force microscopy and high resolution elemental analyses show a highly ordered structure and uniform distribution of the BiNi filler in the matrix. These BiNi-based nanocomposites are found to exhibit distinct ferromagnetic-like ordering with transition temperature of about 675 K. As far as we know there exists no report in literature on any BiNi compound which is magnetic.     

Dynamics of encapsulation and controlled release systems based on water-in-water emulsions: negligible surface rheology

A nonequilibrium thermodynamic model based on the interfacial transport phenomena (ITP) formalism was used to study deformation-relaxation behavior of water-in-water emulsions. The ITP formalism allows us to describe all water-in-water emulsions with one single theory. Phase-separated biopolymer solutions, hydrogel beads, liposomes, polymersomes, colloidosomes, and aqueous polymer microcapsules are all limiting cases of this general theory with respect to rheological behavior of the bulk phases and interfaces. Here we have studied two limiting cases of the general theory, with negligible surface rheology: phase-separated biopolymer solutions and hydrogel beads. We have determined the longest relaxation time for a small perturbation of the interfaces in these systems. Parameter maps were calculated which can be used to determine when surface tension, bending rigidity, permeability, and bulk viscoelasticity dominate the response of a droplet or gel bead. In phase-separated biopolymer solutions and dispersions of hydrogel beads six different scaling regimes can be identified for the relaxation time of a deformation. Hydrogel beads may also have a damped oscillatory response to a deformation. The results presented here provide new insight into the complex dynamics of water-in-water emulsions and also suggest new experiments that can be used to characterize the interfacial properties of these systems.