Yield stress "in a flash": investigation of nonlinearity and yielding in soft materials with an optofluidic microrheometer

Fabrication and mechanical characterization of hydrogel-based 3D cell-like structures

In this article, we demonstrate the fabrication of 3D cell-like structures using a femtosecond laser-based two-photon polymerization technique. By employing poly(ethylene glycol) diacrylate monomers as a precursor solution, we fabricate 3D hemispheres that resemble morphological and biomechanical characteristics of natural cells. We employ an optical tweezers-based microrheology technique to measure the viscoelastic properties of the precursor solutions inside and outside the structures. In addition, we demonstrate the interchangeability of the precursor solution within fabricated structures without impairing the microstructures. The combination of two-photon polymerization and microrheological measurements by optical tweezers demonstrated here represents a powerful toolbox for future investigations into cell mimic and artificial cell studies.

Sliding and rolling of yield stress fluid droplets on highly slippery lubricated surfaces

HYPOTHESIS

Droplets of yield stress fluids (YSFs), i.e. fluids that can flow only if they are subjected to a stress above a critical value and otherwise deform like solids, hardly move on solid surfaces due to their high viscosity. The use of highly slippery lubricated surfaces can shed light on the mobility of YSF droplets, which include everyday soft materials, such as toothpaste or mayonnaise, and biological fluids, such as mucus.

EXPERIMENTS

The spreading and mobility of droplets of aqueous solutions of swollen Carbopol microgels were studied on lubricant infused surfaces. These solutions represent a model system of YSFs. Dynamical phase diagrams were established by varying the concentration of the solutions and the inclination angle of the surfaces.

FINDINGS

Carbopol droplets deposited on lubricated surfaces could move even at low inclination angles. The droplets were found to slide because of the slip of the flowing oil that covered the solid substrate. However, as the descending speed increased, the droplets rolled down. Rolling was favored at high inclinations and low concentrations. A simple criterion based on the ratio between the yield stress of the Carbopol suspensions and the gravitational stress acting on the Carbopol droplets was found to nicely identify the transition between the two regimes.

Deformation profiles and microscopic dynamics of complex fluids during oscillatory shear experiments

Oscillatory shear tests are widely used in rheology to characterize the linear and non-linear mechanical response of complex fluids, including the yielding transition. There is an increasing urge to acquire detailed knowledge of the deformation field that is effectively present across the sample during these tests; at the same time, there is mounting evidence that the macroscopic rheological response depends on the elusive microscopic behavior of the material constituents. Here we employ a strain-controlled shear-cell with transparent walls to visualize and quantify the dynamics of tracers embedded in various cyclically sheared complex fluids, ranging from almost-ideal elastic to yield stress fluids. For each sample, we use image correlation processing to measure the macroscopic deformation field, and echo-differential dynamic microscopy to probe the microscopic irreversible sample dynamics in reciprocal space; finally, we devise a simple scheme to spatially map the rearrangements in the sheared sample, once again without tracking the tracers. For the yield stress sample, we obtain a wave-vector dependent characterization of shear-induced diffusion across the yielding transition, which is accompanied by a three-order-of-magnitude speed-up of the dynamics and by a transition from localized, intermittent rearrangements to a more spatially homogeneous and temporally uniform activity. Our tracking free approach is intrinsically multi-scale, can successfully discriminate between different types of dynamics, and can be automated to minimize user intervention. Applications are many, as it can be translated to other imaging modes, including fluorescence, and can be used with sub-resolution tracers and even without tracers, for samples that provide intrinsic optical contrast.