Yield stress and scaling of polyelectrolyte multilayer modified suspensions: effect of polyelectrolyte conformation during multilayer assembly.

The yield stress of polyelectrolyte multilayer modified suspensions exhibits a surprising dependence on the polyelectrolyte conformation of multilayer films. The rheological data scale onto a universal master curve for each polyelectrolyte conformation as the particle volume fraction, φ, and the ionic strength of the background fluid, I, are varied. It is shown that rough films with highly coiled, brushy polyelectrolytes significantly enhance the yield stress. Moreover, via the ionic strength I of the background fluid, the dynamic yield stress of brushy polyelectrolyte multilayers can be finely adjusted over 2 decades.

Yielding and structural relaxation in soft materials: evaluation of strain-rate frequency superposition data by the stress decomposition method.

Rheological properties of soft materials are often investigated in oscillatory shear and characterized by the storage and loss modulus, G' and G'', respectively. Unfortunately, the relaxation dynamics of most soft materials is too slow to be directly probed by commercial rheometers. Recently, it was shown by Wyss et al. [Phys. Rev. Lett. 98, 238303 (2007)] that the application of an oscillating strain-rate drives such soft materials and shifts the structural relaxation to higher times. They called this experimental technique strain-rate frequency superposition (SRFS). The great benefit of SRFS is the extremely extended frequency range. As viscoelastic measures, Wyss et al. proposed the familiar storage and loss modulus. Using these moduli results in a serious drawback: When the material yields, nonlinearities appear and the physical interpretation of the storage and loss modulus breaks down. Thus, SRFS as proposed by Wyss et al. is limited to the linear regime and the benefit of the extended frequency regime vanishes. In the present work, we validate an alternative data analysis technique, recently established as the stress decomposition method [K. S. Cho et al., J. Rheol. 49, 747 (2005); R. H. Ewoldt et al., J. Rheol. 52, 1427 (2008)], for combination with SRFS. Use of the stress decomposition method provides a physical interpretation of linear and nonlinear SRFS data in terms of strain stiffening and softening as well as shear thickening and thinning.

Systematic modification of the rheological properties of colloidal suspensions with polyelectrolyte multilayers.

Tailoring rheological properties of colloidal suspensions with the adsorption of polyelectrolyte multilayers (PEMs) is based on the idea of controlling macroscopic mechanical properties by modifying the particle surface in a reproducible and well-understood manner. With layer-by-layer self-assembly, monodisperse polystyrene particles are coated with up to ten layers of the oppositely charged strong polyelectrolytes: poly(diallyl dimethyl ammonium chloride) and poly(styrene sulfonate). The conformation of the adsorbed polyelectrolyte is controlled by the ionic strength of the used aqueous polyelectrolyte solution. For 1M NaCl solution, a brushlike adsorption of the polyelectrolyte is expected. The ability of PEMs to serve on a nanoscale level as surface modifiers and influence macroscopic rheological properties like viscoelasticity, yield stress, and shear banding is discussed. The mechanical behavior of these suspensions is qualitatively described by the theory of Derjaguin-Landau-Verwey-Overbeek with short-range repulsion and long-range attraction. A scaling rule is proposed which distinguishes between the precusor and the multilayer regime.

Barchan dunes in two dimensions: experimental tests for minimal models.

A well-defined two-dimensional single barchan dune under the force of a shearing water flow is investigated experimentally. From an initially prepared triangular heap a rapid relaxation to a steady-state solution is observed with constant mass, shape, and velocity. This attractor exhibits all characteristic features of barchan dunes found in nature, namely a gently inclined windward side, crest, brink, and steep lee face. The relaxation time towards the steady state increases with mass. For small dunes we find significant deviations from a fixed height-length aspect ratio. As predicted by recent theoretical models, the migration velocity scales reciprocal to the length of the dune.

Transition pathways between solid and liquid state in suspensions.

Suspensions containing rigid monodisperse spherical particles in a Newtonian carrier liquid are investigated experimentally, providing evidence for solid and liquid states in a transient shear rate from rest. Between these two states a transition takes place; the transition pathways from solid to liquid and from liquid to solid being different. The dynamics of the transition are shown, with the material in this regime reacting as a highly nonlinear system. This involves inverting the input to output and vice versa and comparing them. A key feature of the transition regime is a material instability caused by the collapse of the particle network structure.

Viscoelasticity of mono- and polydisperse inverse ferrofluids.

We report on measurements of a magnetorheological model fluid created by dispersing nonmagnetic microparticles of polystyrene in a commercial ferrofluid. The linear viscoelastic properties as a function of magnetic field strength, particle size, and particle size distribution are studied by oscillatory measurements. We compare the results with a magnetostatic theory proposed by De Gans et al. [Phys. Rev. E 60, 4518 (1999)] for the case of gap spanning chains of particles. We observe these chain structures via a long distance microscope. For monodisperse particles we find good agreement of the measured storage modulus with theory, even for an extended range, where the linear magnetization law is no longer strictly valid. Moreover we compare for the first time results for mono- and polydisperse particles. For the latter, we observe an enhanced storage modulus in the linear regime of the magnetization.