Experimental Evidence for an Intrinsic Route to Polymer Melt Fracture Phenomena: A Nonlinear Instability of Viscoelastic Poiseuille Flow

Experimental evidence of a helical, supercritical instability in pipe flow of shear thinning fluids

“…of [25]). Interestingly, this value, which could be considered a priori quite large for an "infinite die" regime to be reached, is compatible with our own finding that the "infinite pipe" limit is reached for L/R 40 (12). From this comparison we conclude that the main body of experiments on PS (resp.…”

“…In the first case, dealing with the peculiar class of non shear thinning fluids, it has been shown that the Poiseuille flow, although linearly stable, exhibits a weakly non-linear subcritical instability: beyond a critical value of the Weissenberg number Wi, the flow is unstable against a finite but small noise amplitude, therefore resulting in a hysteretic behavior [8,9]. The existence of such an instability has later been confirmed by the experimental studies of Bertola et al [12] Bonn et al [13] and Pan et al [14] on axisymmetric pipe and 2D channel flows.…”

“…In large pipes, it yields an new type of symmetry-breaking supercritical transition [31]. In our case, as well as probably in microchannel planar flow [16], "strong" shear-thinning is responsible for the linear instability of the base flow, whereas Boger fluids designed to exhibit negligible shearthinning are only non-linearly unstable [12][13][14].…”

“…As will be discussed in the next section, this is in strong contradistinction with the usually reported phenomenology for polymer melt extrusion where shorter dies, mostly used in practice, are also the most prone to the so-called "melt fracture" helical instability. [12] and Pan et al [14] who evidenced a pipe instability controlled by the Weissenberg number but exhibiting a large hysteresis -an obvious subcritical feature, while the instability is, in our case, supercritical. Since the two cases differ essentially by the steady flow characteristics of the fluids under study, namely their being shear-thinning or not, we are led to conclude that it is this rheological feature which primarily controls the nature of the pipe instability.…”

“…The instability is supercritical -A possible subcritical nature of the instability would be revealed by the occurence of hysteresis in the oscillation amplitude when ramping V up and down [12]. Fig.7 shows that, within experimental uncertainties, no hysteresis is measurable, either on the amplitude or on the spatial frequency of the undulations.…”