Abstract
Impact of wall slip on the yield stress measurement is examined for capillary suspensions consisting of cocoa powder as the dispersed phase, vegetable oil as the continuous primary fluid, and water as the secondary fluid using smooth and serrated parallel plates. Using dynamic oscillatory measurements, we investigated the yielding behavior of this ternary solid-fluid-fluid system with varying particle volume fraction, phi, from 0.45 to 0.65 and varying water volume fraction, phi(w), from 0.02 to 0.08. Yield stress is defined as the maximum in the elastic stress (G), which is obtained by plotting the product of elastic modulus (G) and strain amplitude () as a function of applied strain amplitude. With serrated plates, which offer minimal slippage, capillary suspensions with phi 0.45 and a fixed phi(w) =0.06 showed a two-step yielding behavior as indicated by two peaks in the plots of elastic stress as a function of strain amplitude. On the other hand with smooth plates, the capillary suspensions showed strong evidence of wall slip as evident by the presence of three distinct peaks and lowered first yield stresses for all phi and phi(w). These results can be interpreted based on the fact that a particle-depleted layer, which is known to be responsible for slip, is present in the vicinity of the smooth surfaces. The slip layer presents itself as an additional pseudo-microstructure (characteristic length scale) besides the two microstructures, aqueous bridges and solid particle agglomerates, that may occur in the system. With serrated plates, both the yield stresses (sigma(1)sigma(2)) and storage moduli plateau at lower strain (before the first yield point) and at higher strain (before the second yield point) (G were found to increase with phi (at a fixed phi(w) =0.06) following power-law dependences. Similarly with increasing phi(w) (0.02 - 0.08) at a fixed phi =0.62, the system behaved as a solid-like material in a jammed state with particles strongly held together as manifested by rapidly increasing sigma(1) and sigma(2). The usage of smooth surfaces primarily affected sigma(1) which was reflected by an approximately 70-90% decrement in the measured sigma(1) for all values of phi. By contrast, sigma(2) and G were found to be unaffected as shown by close agreement of values obtained using serrated geometry due to vanishing slip layers at higher strain amplitudes.