High Shear-Induced Gelation of Charge-Stabilized Colloids in a Microchannel without Adding Electrolytes

We study shear-induced gelation in a microchannel without adding any salt, for a polymer colloid that is fully stable under stagnant conditions. The initial stability is achieved by negative charges from the polymer chain end groups. Then, sulfonate surfactants are added to the system. The surfactant adsorption is characterized by coexistence of domains of gaseous-like noninteracting molecules (G) and condensed patches of interacting molecules (K). It is found that, for a fixed, substantially high shear rate of 1 x 10(6) 1/s (Peclet number 4.5 x 10(4)), in the absence of surfactants, the shear-induced gelation of the system does occur, and when the surfactants are added, as the surfactant surface coverage on the particles increases to a certain value, the shear-induced gelation or even small extent of aggregation becomes impossible. We have estimated the Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy barrier from the measured zeta-potentials and found that in all the cases the corresponding shear-induced collision energy is orders of magnitude larger than the required energy to overcome the barrier. Thus, one would expect occurrence of gelation for all the systems. This clearly indicates presence of additional non-DLVO interactions, which under very low ionic strength are related to the adsorbed surfactant patches that generate strongly repulsive, short-range hydration force. Moreover, when no gelation but only aggregation occurs after passing through the microchannel, the cluster size distribution exhibits distinct bimodality. The structure of the obtained gels also depends on the surfactant surface coverage.