Shear banding in a lyotropic lamellar phase. II. Temporal fluctuations.

We analyze the temporal fluctuations of the flow field associated with a shear-induced transition in a lyotropic lamellar phase: the layering transition of the onion texture. In the first part of this work [Salmon et al., Phys. Rev. E 68, 051503 (2003)], we have evidenced banded flows at the onset of this shear-induced transition which are well accounted for by the classical picture of shear banding. In the present paper, we focus on the temporal fluctuations of the flow field recorded in the coexistence domain. These striking dynamics are very slow (100-1000 s) and cannot be due to external mechanical noise. Using velocimetry coupled to structural measurements, we show that these fluctuations are due to a motion of the interface separating the two differently sheared bands. Such a motion seems to be governed by the fluctuations of sigma(), the local stress at the interface between the two bands. Our results thus provide more evidence for the relevance of the classical mechanical approach of shear banding even if the mechanism leading to the fluctuations of sigma() remains unclear.