Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P30: Focus Session: Rheology and Hydrodynamics of Wormlike Micellar Fluids |
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Sponsoring Units: DFD Chair: Andrew Belmonte, Pennsylvania State University Room: Colorado Convention Center 304 |
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P30.00001: Dynamics of the Shear Banding Instability Invited Speaker: A variety of complex fluids, such as liquid crystals, polymers, and surfactant solutions (lamellae or cylindrical micelles), are easily perturbed by shear flow and exhibit apparent ``phase transitions'' and complex nonlinear dynamics. ``Shear banding,'' or separation of material into bands of different apparent viscosities, has been reliably observed in wormlike micelles and many other systems. Despite a general one dimensional (1D) theory that predicts stable bands, recent experiments suggest that the generic situation is dynamic, rather than steady bands. After an overview I will discuss recent calculations that address the possibility of rheo-chaos, and two dimensional calculations to verify or refute the previously found 1D solutions. I will also discuss the possibility of shear banding in the closely related system of entangled polymer solutions. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P30.00002: Light-Induced Gelling in a Micellar Fluid Based on a Zwitterionic Surfactant. Rakesh Kumar, Srinivasa Raghavan Fluids with photoresponsive rheological properties (i.e. photorheological or PR fluids) can be useful in a range of applications, such as in dampers, sensors, and valves for microfluidic or MEMS devices. Previously, we have demonstrated a cationic surfactant-based PR fluid whose viscosity can be rapidly decreased by UV irradiation. This viscosity decrease was not reversible. Here, we describe a different formulation based on a zwitterionic surfactant that shows a rapid increase in viscosity (gelling) upon exposure to UV radiation. The formulation consists of the zwitterionic surfactant and a photosensitive cinnamic acid derivative. Initially, the viscosity of the fluid is low indicating the presence of small micelles. Upon UV irradiation, the cinnamic acid derivative is photoisomerized from trans to cis. In turn, the small micelles transform into long wormlike micelles, thus increasing the solution viscosity by more than five orders of magnitude. Small angle neutron scattering (SANS) data confirms the dramatic increase in micelle length. Possible reasons for such changes in micelle dimensions will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P30.00003: PB-PEO wormlike micelles under oscillatory shear flow as probed by Time-resolved SANS Minne Paul Lettinga, Barbara Lonetti, Joerg Stellbrink, Joachim Kohlbrecher Polybutadiene-poly(ethylene oxide) (2.5 kd:2.5 kd) diblock copolymers form wormlike micelles, known to undergo an isotropic to nematic phase transition at 5{\%} w/w. The rheological properties of this system display similarities to surfactant wormlike micelles. The theory describing this type of `living' polymers uses the concept of reptation in combination with the kinetics of breaking and re-formation of the micelles to predict the dynamical response of such systems. In practice the dynamical characterization is limited to the determination of the crossover point between the storage and lost moduli and thus to the linear properties of the system. Here we present an \textit{in situ} study of the response of pb-peo in the vicinity of the I-N transition to an oscillatory shear field. We determine the (non-) linear response of the Kuhn-segments applying a novel approach to obtain high time-resolution Small Angle Neutron Scattering ($\Delta $t$\ge $5 ms). We interpret our data using the reptation time as determined by high-speed confocal microscopy on labeled pb-peo. Thus we obtain a microscopic understanding of the dynamics of `living' polymers. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P30.00004: A rheological study of wormlike micelles flows in microchannel Jean-Baptiste Salmon, Chlo\'e Masselon, Annie Colin Complex fluids show non linear properties under simple shear flow leading to flow induced phase transitions and instabilities. The flow curve of wormlike micelles exhibit a stress plateau separating high and low viscosity branches, corresponding to shear-banding flows. Our aim is to understand the structure/concentration/flow coupling of wormlike micelles. A microfluidic chip is easy to couple with many analytical methods; it is hence well adapted to our study. We both perform particle image velocimetry and microscopy on a microfluidic chip consisting in channels with dimensions 250 $\mu $m large and 1 mm deep. Such a canyon geometry enables us to relate the measured velocity profiles to the local rheology. We evidence shear banding and slip at the walls. Strikingly there is no single rheological law that describes the velocity profiles at different pressure drops. Using microscopy, we point out turbid bands at the walls corresponding to the highly sheared bands. At low pressure drops, these bands are stable in time and their widths increase with increasing pressure until a limit where they fluctuate in space and time. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P30.00005: Nonlinear microrheology of wormlike micelle solutions using ferromagnetic nanowire probes N. Cappallo, C. Lapointe, D. H. Reich, R. L. Leheny We describe the application of high-aspect-ratio ferromagnetic nanowires to the microrheology of wormlike micelle solutions composed of equimolar cetylpyridinium chloride/sodium salicylate (CPCl/NaSal). Employing video microscopy to track the rotation of suspended nanowires in response to external magnetic fields, we access both the linear and nonlinear rheology of the fluid. The linear viscosity at low rotation rates is strongly temperature dependent as expected from macroscopic rheometry. At high rotation rates the viscosity exhibits pronounced shear thinning. The onset of the nonlinear response is characterized by a temperature-dependent shear thickening that has no apparent counterpart in the macroscopic rheometry. Time-resolved measurements involving step changes in rotation rate reveal that, once the fluid has been prepared into a shear-induced state, it exhibits nonlinear viscosity within the expected linear regime. Further, the shear-induced state of the fluid generates an out-of-plane torque on the wire that we have characterized by time-resolved studies. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P30.00006: Disentanglement behavior of DNA and wormlike micellar solutions as probed with particle-tracking velocimetry. Pouyan Boukany, Shi-Qing Wang We study an ideal entanglement network to test a number of emerging ideas about how topological entanglement reorganizes in presence of shear flow. Aqueous DNA solutions and wormlike micellar solutions can be highly entangled at very low concentrations and thus very soft yet sluggish. A particle tracking velocimetric method, which was developed recently in our lab [1], was applied to determine the velocity profile of these solutions in simple shear under several flow conditions including large step strain, large amplitude oscillatory shear, startup continuous shear and creep. It is shown [2] that all of the nonlinear viscoelastic flow behavior is associated with development of inhomogeneous shear when nucleation of chain disentanglement takes place in reaction to imposed shear deformation. [1] \textit{Phys. Rev. Lett. }\textbf{96}, 016001 (2006); \textit{ibid}. \textbf{96}, 196001; \textit{ibid.} \textbf{97}, 187801. [2] Manuscripts to be submitted to \textit{Macromolecules} and \textit{Langmuir}. [Preview Abstract] |
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