Viscoelastic behavior of an ordering latex suspension in a steady shear flow

Abstract

The linear viscoelastic behavior of an ordering polystyrene latex during steady shear flow has been investigated. In this study a home-made instrument has been used that superimposes a small-amplitude oscillatory shear orthogonal onto the steady shear flow. The measurements can be interpreted as due to the coexistence of two phases under flow. For increasing shear rate the ordered ͑or solidlike͒ phase melts away until the suspension becomes completely disordered ͑or fluidlike͒. A model is presented in which the fluid is conceived as a viscoelastic matrix containing spherical viscoelastic domains. The model predicts a similar linear viscoelastic behavior of sheared lattices as found experimentally. The resulting parameters give information about the microstructure of the dispersion under shear flow. ͓S1063-651X͑98͒15002-X͔ PACS number͑s͒: 83.50.Fc

A colloidal crystal modeled by bead–spring cubes

“…On the other hand, the change of system ͑ii͒ into system ͑iii͒ represents the coalescence of crystallites and one observes that coalescence results in an increase of GЈ for low frequencies ͑while it remains constant for intermediate and high frequencies͒ and it results in an increase of GЉ for low frequencies and a decrease of GЉ for intermediate and high frequencies. We have already noted that the measurements carried out by Van der Vorst et al 55 showed an increase of GЈ and a decrease of GЉ as a function of the preshear time. Thus, the growing and alignment of the crystallites due to preshearing is reflected by the increase of GЈ(), while the coalescence of crystallites is reflected by the decrease of GЉ()Ϫ ϱ .…”

“…In Van der Vorst et al 55 measurements are given of the dynamic moduli GЈ() and GЉ() as a function of the preshear time, i.e., the duration of the applied preshear was 0, 4, 6, 10, and 36 min. These experiments show that preshearing of a colloidal crystal results in an enhanced ͑long range͒ ordering in the sample, i.e., it was observed that due to the preshearing the storage modulus GЈ() was increasing, while, simultaneously, the loss modulus GЉ() was decreasing.…”

“…[33][34][35][36][39][40][41][42][43][44][45][53][54][55] The applied preshear in these papers varies from 1 to 200 s Ϫ1 , which is below the shear rate needed to melt a colloidal crystal into an amorphous suspension.…”

“…The storage modulus GЈ() denotes the elastic part of the dynamical stress response, while the viscous part is denoted by the loss modulus GЉ(). To our knowledge there exists only one paper in which measurements are given of these dynamic moduli as a function of the preshear time; these experiments of Van der Vorst et al 55 showed that the preshearing of a colloidal crystal results in an enhanced ͑long range͒ ordering in the sample, i.e., it was observed that due to the preshearing the storage modulus GЈ() was increasing, while, simultaneously, the loss modulus GЉ() was decreasing. It was also observed that the moduli GЈ() and GЉ() belonging to a preshear of 10 min were almost the same as the ones belonging to longer preshear times, which indicates that the rest configuration of a colloidal crystal remains the same for preshear times longer than 10 min.…”

“…The experimental results obtained so far can be sum-marized as follows: ͑i͒ GЈ() is constant and GЉ() ϭ ϱ for տ300 rad s Ϫ1 ͑the dynamical viscosity ϱ is much smaller than the steady shear viscosity of the colloidal crystal͒, 55-66 ͑ii͒ both GЈ() and GЉ() are constant for 0.3 rad s Ϫ1 ՇՇ300 rad s Ϫ1 ͓since GЈ()ӷGЉ() the experimental inaccuracy to determine GЉ() is rather high; consequently, although GЉ() appears to be constant in these measurements, its exact frequency dependency is not known yet͔, 20,[31][32][33][42][43][44][45][46]55,[64][65][66][67][68][69][70] and ͑iii͒ GЈ() is still constant and GЉ() is proportional to Ϫ1/2 for Շ0.3 rad s Ϫ1 ͑more experiments are needed to demonstrate that the frequency dependency Ϫ1/2 is generally valid͒. 46,55,64 Up to now, many researchers have tried to find an expression for the frequency independent GЈ(), [61][62][63][64][71][72][73][74][75][76][77][78] but, to our knowledge, no paper exists in which a theoretical model is given that explains the clear transition in the frequency behavior of GЉ().…”