The Orientation Dynamics of Rigid Rod Suspensions Under Extensional Flow
Résumé
The orientation dynamics of a lyotropic colloidal suspension of sepiolite clay under extensional flow have been explored by combined dichroism and SALS measurements. Extensional flow was applied using a four-roll mill to a thin film of sepiolite suspension (rigid rods 1 micrometer long and 0.010 micrometers in diameter). Analysis of transient extensional flow reversals revealed important characteristics of the orientation dynamics of these suspensions: i) the existence of a critical volume fraction separating isotropic behavior, where no orientation persists after stretching, from nematic behavior, where permanent orientations persist during the relaxation phase. ii) in the nematic domain, a critical strain rate separates two flow regimes corresponding to a stable, so-called strong flow regime above the critical strain rate and an unstable, so-called weak flow regime below it. These experimental observations agree with the theoretical predictions of the model proposed by Marrucci and Maffettone (1989, 1990) who have examined the two dimensional form of the simple molecular model of Hess (1976) and Doi (1981). What is new in the present case is that the colloidal suspension is a 3-dimensional system, whereas previous experimental validations of the model concerned only 2-dimensional rodlike polymers systems (Maffettone et al., 1996) (Maruyama et al., 1998a and b).