We use rheological measurements to examine the yielding behavior of a microgel system spanning the range from soft jammed glassy suspensions dominated by inter-particle repulsion to colloidal gels produced by attractive interactions. Under repulsive conditions, the suspensions display a prototypical soft glassy yielding response in which the shear loss modulus exhibits a single peak on increasing strain during the crossover from elastic to viscous behavior. By contrast, under fully attractive conditions the colloidal gel displays a more complex yielding, with two distinct peaks in the loss modulus in the vicinity of the yield strain. It is apparent that the gels yield initially by network rupture, followed by shear induced densification which leads to the formation of compact clusters. We show that the second peak in the loss modulus is consistent with the subsequent breakup of these dense clusters. We quantitatively map the steady progression from simple glassy yielding to the more complex gel response on increasing attraction strength by the evolution of peak locations, magnitudes and frequency dependencies.Notably, the peak locations diverge as the network becomes more fragile and spatially heterogeneous with increasing attraction strength. There is little frequency dependence in the peak positions, but the amplitude of the second yielding peak shows a non-monotonic dependence with a maximum near 5 rad s À1 . Time-resolved measurements and decreasing strain sweeps highlight pronounced differences in the reversibility of the network rupture and cluster breakup processes. Correspondingly, the linear viscoelastic properties of the gel are strongly dependent on mechanical history whereas the glass exhibits no such dependence.

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Foaming in products based on micellar solutions has considerable importance in various consumer applications, such as washing and cleaning. In this work, the effects of surfactant concentration, oil content, and salts containing mono-, di-, and trivalent counterions on foam formation and stability were studied. The foams were generated by employing the Blender Test. The presence of salts caused a significant reduction in foam volume. Effectiveness of the salts followed the sequence Al 3+ > Ca 2+ > Na + . However, the foam collapse rate was slower in the presence of salt. The rate of adsorption of surfactant molecules at the air−water interface was augmented by salt. Oil reduced the foam volume and its stability. The entering, spreading, and bridging coefficients were calculated. These coefficients qualitatively explained the stability of foam in the presence of oil.

Evolution of the energy landscape during physical aging of glassy materials can be understood from the frequency and strain dependence of the shear modulus but the nonstationary nature of these systems frustrates investigation of their instantaneous underlying properties. Using a series of time-dependent measurements we systematically reconstruct the frequency and strain dependence as a function of age for a repulsive colloidal glass undergoing structural arrest. In this manner, we are able to unambiguously observe the structural relaxation time, which increases exponentially with sample age at short times. The yield stress varies logarithmically with time in the arrested state, consistent with recent simulation results, whereas the yield strain is nearly constant in this regime. Strikingly, the frequency dependence at fixed times can be rescaled onto a master curve, implying a simple connection between the aging of the system and the change in the frequency dependent modulus.

We monitor the relaxation of internal stresses in a fractal colloidal gel on cessation of flow and find a weak power-law decay, sigmai approximately t(-alpha) over five decades of time where alpha approximately 0.07. The system exhibits physical aging of the elastic modulus, G' approximately tbeta, with beta approximately alpha . Imposition of zero stress after waiting time tw results in strain recovery as the system relaxes without constraint. Remarkably, recoveries at different tw can be shifted to construct a master curve where data are scaled vertically by 1/sigmai(tw) and plotted horizontally as (t-tw)/twmu, where mu is approximately 1.25, indicative of a superaging response.