Shear microscopy of the ‘‘butterfly pattern’’ in polymer mixtures

Abstract

The concentration Auctuations induced in a semidilute polymer solution by shear flow are directly visualized for the first time. The spatial structure of the "butterAy pattern" observed by light scattering is elucidated, being the result of hydrodynamic rippling waves which have a distribution of tilt angles.The minimal tilt angle depends on shear rate in a power law fashion, causing the butterAy to almost close upon itself at high shear rates. The concentration waves have a time scale related to a shear rate j -l sec and periodicity A, -l 0 pm, indicating a link to reptation processes. PACS numbers: 61.2S. Hq, 47.50.+d, 83.50.Ax Shear induced phase separation is one of the striking results of combining hydrodynamics with the special elastic and viscous properties of polymers, and has been the center of a concentrated effort in recent years fl-10]. It originates in the profound effects of entanglement and coil stretching upon the flow of semidilute, high molecular weight polymer solution. While our notion of mixing by shear flow is intuitively consistent with shear induced homogenization in lower molecular weight systems [11], the appearance of systems where shear induces phase separation was quite remarkable. When shear induced separation was first observed experimentally [1,2], it was attributed to an elastic term in the free energy [2). Such a contribution could move the critical curve suSciently to allow phase separation at elevated temperatures. More recently, Helfand and Frederickson (HF) suggested [3) that this separation is not a thermodynamic phase transition, but rather a nonequilibrium result of the coupling between the stress field and the polymer concentration field.A first clue to the hydrodynamic origin of the separation can be found in light scattering experiments, which showed a strong anisotropy of the scattering pattern with respect to the flow direction [6,8]. In these experiments the salient feature is the "butterfly pattern" obtained from small-angle light scattering (SALS). It is characterized by a dark streak in the direction perpendicular to the flow, where there is no enhancement of the scattering. In the direction of the flow a strong enhancement of the scattering occurs that is limited in width to an opening angle 2a which increases with shear rate. Such anisotropy would not occur if the transition were a thermodynamic phase transition. A qualitative confirmation of the HF theory was given in a Couette flow light scattering experiment [7), where the experimental axes of the scattering rays were made to correspond to those in the calculation of HF. In this paper we present the first direct observation and visualization of the actual participating hydrodynamic modes.The butterfly pattern has also been observed in other polymer systems, and is attracting much attention [12][13][14][15][16][17]. It has shown up in stretched rubber, polymer gels, and polymer melts. There it is found at much smaller wavelengths, by use of neutron scattering. It is worth noting that the butterfly pattern w...

Butterfly Light Scattering Pattern and Rheology of a Sheared Thixotropic Clay Gel

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“…When a fluid dispersed in a second fluid is subjected to shear, the droplets deform and burst, and the use of shear flow to emulsify immiscible liquids is ubiquitous in the processing of soft materials. Although the pioneering work of Taylor [1][2][3][4] on isolated Newtonian emulsions remains at the foundation of our understanding, with the underlying intuition that the droplets elongate along the direction of flow at sufficiently high shear rates, the shear response of fluids that exhibit rheological complexity, such as semidilute entangled polymer solutions [5][6][7][8], wormlike micelles [9], and thixotropic clay gels [10], suggests that the shear can induce domains with extended correlation along the direction of vorticity, perpendicular to the flow field. In this Letter, stroboscopic video microscopy is used to measure the shear response of dilute emulsions composed of viscoelastic polymer melts.…”

Shear‐Induced Phase Separation of Entangled Polymer Solutions: Formation of Optically Anisotropic Strings as Precursor Structures of Shish‐Kebab

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