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A semi-flexible SU-8 polymer microdevice was designed to deflect in response to a change in surface tension at an air-water interface. The suspended microtensiometer encloses a clean water-air interface, while externally the device is surrounded by an interface containing an insoluble component. The difference in surface tension between the inside and outside of the device, called the surface pressure, causes the device to deflect. Finite element simulations were performed to predict device behavior prior to fabrication. Finished devices were tested in a Langmuir trough during multiple compression and expansion cycles, using a platinum Wilhelmy plate for an independent surface pressure measurement. A resolution of 0.16 mN/m was achieved. [ABSTRACT FROM AUTHOR]

Oscillatory measurements are often used to explore the nonlinear response of materials, with recently a strong focus on using large amplitude oscillatory experiments. However, the superposition of an oscillatory motion onto a steady-state shear flow is a method where the kinematic history experienced by the sample is simpler. Such a superposed oscillation can be applied either orthogonal or parallel to the main flow direction. Both superposed deformation modes can now be achieved on rotational rheometers equipped with a force-rebalanced transducer, the orthogonal mode requiring a minor modification to the control loop of the normal force. In the present work, the nonlinear properties of a wormlike micellar (WLM) solution are studied. The results are compared with the predictions of the Giesekus model, which is chosen both for its capability to describe the WLM response and for being one of the simplest continuum models that incorporate an anisotropic microstructure. From the fluid response in the homogeneous flow regime, a rate-dependent relaxation time and a rate-dependent plateau modulus can be derived. The latter provides insight into the structural anisotropy during flow at short length scales, which in this case is isotropic. Further analysis of the superposition moduli can be used to separate and quantify the effects of flow on the reptation and breaking of the chains. In the shear-banding regime, the orthogonal moduli show a weaker dependence on shear rate compared to the predictions of the Giesekus model, yet they remain sensitive to changes in the shear-banded state. [ABSTRACT FROM AUTHOR]

Abstract: Electromechanical resonators are sensitive to the material parameters of a surrounding medium and therefore can be used as sensors for viscoelastic properties and density. In our recent work, we presented a metallic plate resonator excited by Lorentz forces in a permanent magnetic field. We improved the interface circuitry by using signal transformers, so that it is now possible to make accurate measurements using a commonly available low-cost audio- interface. Beside that, the necessary sample volume is reduced to a maximum of 50μl, so that a drop of liquid covers the sensitive area. Using a reflector parallel to the oscillation plane, standing waves in the gap could be generated. A desirable feature of resonator sensors is tunability over a significant frequency range. We investigated mechanisms to change the resonance frequency, e.g. an electric current that induces thermal stress. These advancements pave the way for a versatile low-cost, easy-to-use solution to measure viscoelastic properties in numerous applications. [Copyright &y& Elsevier]

Abstract: Electromechanical resonators are sensitive to the properties of the surrounding medium due to interaction forces onto the surface caused by motions in the medium. In the present contribution, fully metallic Lorentz force resonators exhibiting in-plane oscillation are used to excite shear waves to measure the linear viscoelastic storage and loss-moduli at specific frequencies in the kHz range of complex fluids (e.g. aqueous polymeric solutions). Reflected shear waves in a well defined gap are employed to extend the measurement range as well as the capability to measure at multiple frequencies. Numerical methods and reduced order models are employed to solve for the velocity field and interaction forces to determine the required quantities from the measured frequency response. [ABSTRACT FROM AUTHOR]

Flow-induced structures in suspensions containing spheres in viscoelastic suspending media were investigated by microscopy and rheo-optical methods. Suspensions of monodisperse polystyrene spheres with diameters ranging from 1.2 to 2.8 μm and dispersed in aqueous solutions of hydroxypropylcellulose were studied in simple shear flows. Optical microscopy observations as well as small-angle light-scattering (SALS) experiments were performed using a parallel plate geometry. In agreement with previous work, necklaces of particles aligned in the flow direction were observed when shearing faster then a critical shear rate, which was found to be independent of particle size. In contrast to earlier work, however, the role of particle migration was found to be of prime importance. Particles were shown to migrate toward the plates where the particles assembled and aligned in strings running in the flow direction. For the smallest particles (1 μm diameter), the formation of particle doublets or short strings along the vorticity direction was observed at low shear rates, which flipped to an orientation into the flow direction and grew into longer strings at higher shear rates. SALS experiments were used to quantify the degree of alignment and its dependence on particle size, shear rate, and gap. For the system under investigation, the degree of alignment was found to increase with increasing shear rate and particle size and with decreasing gap. The present results suggest that, depending on the details of the suspending medium and the size and nature of the suspended particles, the formation of aligned structures is affected by the relative magnitude of the colloidal and hydrodynamic forces and the kinetics of string formation versus the kinetics of migration. [ABSTRACT FROM AUTHOR]

In inertialess suspensions of rigid particles, the rotational motion of each particle is governed by the so-called freely rotating condition, whereby the total torque acting on the particle must be zero. In this work, we study the effect of viscoelasticity of the suspending liquid on the rotation period of a sphere by means of three-dimensional finite element simulations, for conditions corresponding to a macroscopic shear flow. The simulation results capture the slowing down of the rotation, relative to the Newtonian case, which was recently observed in experiments. It is shown that such a phenomenon depends on the specific constitutive equation adopted for the viscoelastic liquid. Analysis of transients shows a clear correlation between rotation rate and the development of first normal stress difference. [ABSTRACT FROM AUTHOR]

[Display omitted] The mechanical properties of model air/water interfaces covered by poly(N -isopropylacrylamide) microgels depend on the microgels deformability or in other words on the amount of cross-linker added during synthesis. The study is carried out by measuring the apparent dilational, the compression and the shear moduli using three complementary methods: (1) the pendant drop method with perturbative areas, (2) the Langmuir trough compression, and (3) shear rheology using a double wall ring cell mounted onto a Langmuir through. In the range of surface coverages studied, the interfaces exhibit a solid-like behavior and elasticity goes through a maximum as a function of the surface pressure. This is observable whatever the investigation method. This maximum elasticity depends on the microgel deformability: the softer the microgels the higher the value of the moduli. The mechanical behavior of model interfaces is discussed, taking into account the core–shell structure of the particles and their packing at the interface. [ABSTRACT FROM AUTHOR]

Guanidine-based surfactant ethyl lauroyl arginate (LAE) and cellulose nanocrystals (CNCs) form complexes of enhanced surface activity when compared to pure surfactants. The LAE-CNC mixtures show enhanced foaming properties. The dynamic thin-film balance technique (DTFB) was used to study the morphology, drainage and rupture of LAE-CNC thin liquid films under constant driving pressure. A total of three concentrations of surfactant and the corresponding mixtures of LAE with sulfated (sCNC) and carboxylated (cCNC) cellulose nanocrystals were studied. The sCNC and cCNC suspension with LAE formed thin films, with stability increasing with surfactant concentration and with complex rheological properties. In the presence of LAE, the aggregation of CNC was observed. While the sCNC aggregates were preferentially present in the film volume with a small fraction at the surface, the cCNC aggregates, due to their higher hydrophobicity, were preferentially located at film interfaces, forming compact layers. The presence of both types of aggregates decreased the stability of the thin liquid film compared to the one for the LAE solution with the same concentration. The addition of CNC to LAE was critical for foam formation, and foam stability was in qualitative agreement with the thin films' lifetimes. The foam volume increased with the LAE concentration. However, there was an optimum surfactant concentration to achieve stable foam. In particular, the very resistant foam was obtained with cCNC suspensions that formed the interfaces with a complex structure and rheology. On the other hand, at high LAE concentrations, the aggregates of CNC may exhibit antifoaming properties [ABSTRACT FROM AUTHOR]