When a non-Brownian suspension is subjected to an oscillatory shear, even in the Stokes regime, a change in its microstructure occurs. Since the pioneering paper of Gadala-Maria and Acrivos in 1980 it was shown that particle migration is induced by simple shear and more recently, in 2005, Pine et al. proved experimentally and numerically the flow irreversibility of a non Brownian suspension in Stokes regime. The linearity loss is basically due to the, often negligible, multibody interactions that, though small, are large enough to break the symmetry typical of the Stokes regime. The same phenomenon was invoked to justify the existence of a single steady state under simple shear of a non-Brownian suspension in the semidilute regime (Rexha and Minale, 2011).

Focusing on the oscillatory shear, in 2006 and 2007, Bricker and Butler showed a non-monotonic dependence of the complex viscosity Vs. the imposed strain at large accumulated strains. In particular they showed that the complex viscosity monotonically decreases with the imposed strain until an imposed value of 100% from where it starts to increase again going towards the steady state value of the shear viscosity. They explained this behaviour showing numerically that a macroscopic microstructure reorganisation occurs during the oscillatory shear giving rise to different structures depending on the imposed strain. Later on in 2011 Park et al. showed the existence of two different restructuring mechanisms: One at small accumulated strains, the other at large accumulated strains.

We investigate the very early stage of the structure reorganisation driven by the oscillatory shear, focusing only on what happens in the first oscillation. We span several imposed strains ranging from 0.005 to 5 that are applied on Newtonian non Brownian suspensions made of hollow glass beads suspended in a Newtonian poly-isobutene with particle volume fraction: 0.35, 0.4 and 0.45. We show experimentally that even at the smallest imposed strain it is enough a single oscillation to obtain a non linear response such that the apparent complex viscosity results smaller than the steady shear viscosity. We confirm this numerically and we show that the microstructure is dilated during the single oscillation by evaluating the suspension pair distribution function. The non-monotonic behaviour of the apparent complex viscosity is also observed with a single oscillation and the re-increase of the complex viscosity occurs when the imposed strain is large enough to give to the suspension the opportunity to reorganise its structure as in a sort of steady simple shear.

Also the effect of the imposed frequency on the micro structure reorganisation under oscillatory shear is investigated.

J.M. Bricker, J.E. Butler (2006) *J. Rheol.* **50**:711.

J.M. Bricker, J.E. Butler (2007) *J. Rheol.* **51**:735.

F. Gadala-Maria, A. Acrivos (1980) *J. Rheol.* **24**:799.

H. Park, J.M. Bricker, M.J. Roy, J.E. Butler (2011) *Phys Fluids* **23**:013302

D.J. Pine, J.P. Gollub, J.F. Brady, A.M. Leshansky (2005) *Nature* **438**:997

G. Rexha, M. Minale (2011) *J. Rheol.* **55**:1319.

Date: 17/2/2015

Time:12:00 (coffee & cookies will be served at 11:45)

Place:FORTH Seminar Room 2