The interaction between the microgel particles can be tuned from attractive to repulsive by increasing the pH of the system. The figure above shows log(G”) vs log(strain) for samples having different amounts of 1M NaOH (indicated in the legend of the figure). y0 is the log(G”) in the linear regime, centers and heights are the positions and peak heights of the two peaks. It was observed that: (a)The height of both the peaks decrease with increasing pH. (b) The position of the 2 nd peak remains constant whereas the position of the 1 st peak shifts to higher strain value. (a) The modulus is higher for higher pH value. It is understandable as the microgel particles are more swollen at higher pH. (b) G” show two peaks at lower pH but only one peak at higher pH. ORIGIN OF TWO PEAKS (Pham et al, EPL 75, 624 (2006) (a) (b) (c) 1.The first peak arises because of the bond breaking between the particles (fig b). 2.The second peak arises because of the escape of the particle from the cage formed by its neighbors (fig c). PREDICTIONS (a)1 st peak (which is due to bond-breaking) should decrease as the interaction between the particles reduce. (b) 2 nd peak (which is due to cage-escape) should be frequency dependent while the 1 st peak should be frequency independent. The yielding behavior of a dense soft matter system shows qualitative differences with the variation in the interaction between the constituents of the system. A system with repulsive interaction shows a single step yielding. During a strain sweep the system shows a linear regime (constant elastic and viscous moduli) at low strain values followed by decrease in G’ and G” (a single peak precedes the decrease in G”) beyond the yield strain. In contrast, a two-step yielding mechanism has been proposed for dense attractive systems where during the strain sweep, two peaks have been observed in the viscous moduli. These two peaks have been attributed to bond breaking and escape from the cage formed by neighboring particles. We have studied the yielding mechanism in microgel suspension in water with interactions modified by tuning the pH of the system. At low pH, when the interaction is attractive, the system shows two peaks in G”. At high pH, when the interaction is repulsive, the system shows a single step yielding. We also have looked at the frequency dependence of yielding by performing strain sweeps at different frequencies. The figure above shows log(G”) vs log(strain) for different applied frequencies. (a) With increasing frequency the modulus increases while the heights of both peaks decrease. (b)The center of 1 st peak remains constant whereas the center of 2 nd peak shifts slightly towards the higher strain with increasing frequency. We observed the transition from two-step to single step yielding as the attractive interaction is reduced, however, our pH and frequency measurements do not confirm the present model. ACKNOWLEDGEMENTS: Frequency and Interaction Dependence of Yielding in Microgel Particles Ajay Singh Negi and Chinedum Osuji Department of Chemical Engineering, Yale University AbstractTwo Step YieldingYielding: pH DependenceYielding: Freq Dependence Conclusions System: 0.2 wt% Carbopol Ultrez10 in water. The pH of the system was controlled by addition of 1M NaOH Experimental Technique: Bulk rheology (MCR 301 rheometer). 2 degree cone with 5 cm diameter. (a) Preshear at /s for 600s. (b) Waiting for 1200s. (c) Strain sweep at a fixed frequency. (d) Above three steps were repeated for different frequency. Methods