“Fracture” phenomena in shearing flow of viscous liquids

In start-up of steady shearing flow of two viscous unentangled liquids, namely low-molecular-weight polystyrene and α-D-glucose, the shear stress catastrophically collapses if the shear rate is raised above a value corresponding to a critical initial shear stress of around 0.1–0.3 MPa. The time dependence of the shear stress during this process is similar for the two liquids, but visualization of samples in situ and after quenching reveals significant differences. For α-D-glucose, the stress collapse evidently results from debonding of the sample from the rheometer tool, while in polystyrene, bubbles open up within the sample, as occurs in cavitation. Some similarities are pointed out between these phenomena and that of “lubrication failure” reported in the tribology literature.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47212/1/397_2004_Article_BF00368135.pd

The effect of temperature gradients on the sharkskin surface instability in polymer extrusion through a slit die

The sharkskin surface instability is commonly observed in the extrusion of polymer melts. We present a series of experiments in which a specifically designed rectangular slit die with insulated and independently heated sides and is used to induce precise temperature gradients across a flowing polyethylene melt. Our previous experiments demonstrated that the character of the surface distortions produced by the sharkskin instability was a function of the die wall temperature and therefore the extrudate had viscoelastic properties at the surface. In this paper, we explore the role of temperature and viscoelastic property gradients near the capillary wall. The amplitude of the sharkskin instability is quantified and plotted against apparent shear and extension rates. Analysis of the data demonstrates that the amplitude and frequency of the instability is independent of bulk temperature and temperature gradient and is dependent only on wall temperature. The data are normalized using a dimensionless Weissenberg number based on the extension rate to collapse the data collected over all temperatures and gradients onto a single master curve. We conclude with an example of a rectangular extrudate exhibiting varying surface roughness due to differential die heating and discuss the implications of our observations on the sharkskin surface instability mechanism and on commercial application

A phenomenological model for wall effects on the deformation of an ellipsoidal drop in viscous flow

The simple phenomenological model developed by Maffettone and Minale (J Non-Newt Fluid Mech 78:227-241, 1998) for the deformation of a single ellipsoidal drop in a viscous flow is extended here to predict the drop deformation in confined viscous flows. The model is capable of describing the transient evolution of an ellipsoidal drop subjected to a generic flow field. The steady-state predictions are analytical and recover the theoretical limits of Shapira and Haber (Int J Multiph Flow 16:305-321, 1990) for steady small deformation of a drop in a confined simple shear flow. Model predictions are compared with data available in the literature that cover a wide range of parameter values, and the agreement is good

Comprehensive Analysis of Dewetting Profiles to Quantify Hydrodynamic Slip

Hydrodynamic slip of Newtonian liquids is a new phenomenon, the origin of which is not yet clarified. There are various direct and indirect techniques to measure slippage. Here we describe a method to characterize the influence of slippage on the shape of rims surrounding growing holes in thin polymer films. Atomic force microscopy is used to study the shape of the rim; by analyzing its profile and applying an appropriate lubrication model we are able to determine the slip length for polystyrene films. In the experiments we study polymer films below the entanglement length that dewet from hydrophobized (silanized) surfaces. We show that the slip length at the solid/liquid interface increases with increasing viscosity. The correlation between viscosity and slip length is dependent on the type of silanization. This indicates a link between the molecular mechanism of the interaction of polymer chains and silane molecules under flow conditions that we will discuss in detail.Comment: 15 pages, 10 figure