Shear-induced deformation and interfacial jamming of solid-stabilized droplets.

We use rheo-microscopy to directly investigate the dynamics of solid-stabilized droplets subjected to shear flow of a surrounding bulk fluid. In our system, the stabilizing particles are weakly attractive through the continuous fluid phase and along the droplet interface. Under shear, droplets stabilized by these particles at near-complete surface coverage exhibit a number of previously unforeseen phenomena, including negative-then-positive deviations from the predictions of Taylor and the behavior of bare droplets, evolution toward spherocylindrical shapes, and an earlier onset of rupture than their bare counterparts, which we explain in light of the weak attractive interparticle interactions along the droplet interface. We also demonstrate the formation of long-lived anisotropic particle-coated droplets by flow cessation, and provide evidence that this is due to the formation of a jammed, disordered particle network along the interface at surface coverage lower than the starting conditions. Importantly, these newly observed phenomena are shown to be sensitive to the droplets' initial surface coverage. Our findings provide new technologically-relevant insights into the physics of particle-coated droplets under fluidic or other external stresses, and introduce avenues for future research to better understand the roles of interparticle interactions and surface coverage in mediating the interfacial rheology of particle-laden interfaces and solid-stabilized emulsions.

Effect of Bleaching Gel Viscosity on Tooth Whitening Efficacy and Pulp Chamber Penetration: An In Vitro Study.

OBJECTIVES: Whitening efficacy has been related to hydrogen peroxide (HP) diffusion into tooth structure. However, little information is available relating rheological properties to whitening efficacy. The purpose was to evaluate the whitening efficacy and HP penetration level of a 10% HP gel at three different viscosities and to compare them to a strip delivery system. METHODS AND MATERIALS: Extracted molars (n=120) were randomly assigned into five groups (n=24/ group): NC_MED (negative control; median): medium viscosity gel without HP; LOW: 10% HP gel (low viscosity experimental gel, Ultradent Products Inc); MED: 10% HP gel (medium viscosity experimental gel, Ultradent); HIGH: 10% HP gel (high viscosity gel, Ultradent); and CWS: Crest 3D Whitestrips 1-Hour Express (Procter & Gamble). All teeth were subjected to five 60-minute whitening sessions. Instrumental color measurements were performed at baseline (T0), and 1-day after each application (T1-T5), and 1-month after whitening (T6). HP penetration was estimated with leucocrystal violet and horseradish peroxidase. A Kruskal-Wallis test and post hoc Bonferroni test were performed to assess the difference in tooth color change and HP penetration among the groups (α=0.05). RESULTS: Hydrogen peroxide penetration levels and overall color changes at T6 were 0.24 µg/mL / 2.80; 0.48 µg/mL / 8.48; 0.44 µg/mL / 7.72; 0.35 µg/mL / 8.49; 0.36 µg/mL / 7.30 for groups NC, LOW, MED, HIGH, and CWS, respectively. There was a significant difference for HP penetration, while there was no significant difference among the four experimental groups for tooth color change. CONCLUSION: Rheological properties should be considered when developing new whitening formulations.

Non-monotonic dependence of Pickering emulsion gel rheology on particle volume fraction.

The microstructure of Pickering emulsion gels features a tenuous network of faceted droplets, bridged together by shared monolayers of particles. In this investigation, we use standard oscillatory rheometry in conjunction with confocal microscopy to gain a more comprehensive understanding of the role particle bridged interfaces have on the rheology of Pickering emulsion gels. The zero-shear elastic modulus of Pickering emulsion gels shows a non-monotonic dependence on particle loading, with three separate regimes of power-law and linear gel strengthening, and subsequent gel weakening. The transition from power-law to linear scaling is found to coincide with a peak in the volume fraction of particles that participate in bridging, which we indirectly calculate using measureable quantities, and the transition to gel weakening is shown to result from a loss in network connectivity at high particle loadings. These observations are explained via a simple representation of how Pickering emulsion gels arise from an initial population of partially-covered droplets. Based on these considerations, we propose a combined variable related to the initial droplet coverage, to be used in reporting and rationalizing the rheology of Pickering emulsion gels. We demonstrate the applicability of this variable with Pickering emulsions prepared at variable fluid ratios and with different-sized colloidal particles. The results of our investigation have important implications for many technological applications that utilize solid stabilized multi-phase emulsions and require a priori knowledge or engineering of their flow characteristics.