Motion of a probe driven through a wormlike micellar fluid

Abhishek Ghadai, Pradip Kumar Bera and Sayantan Majumdar

Steady state fluctuations in the case of complex fluids are ubiquitous in various non-linear flow scenario due to interplay of underlying flow and the dynamics of the involved microstructure. Viscoelastic wormlike micellar fluid has been studied as a model system to understand the fluctuations due to flow whether the flow is a classic Taylor-Couette flow, gravity induced sphere sedimentation in a graduated cylinder or the flow past an obstacle in a channel flow. In most of the cases, the nature of the fluctuations is quite different from each other due to various underlying reason such as input type, confinement, nature of the micellar structure, temperature etc. However, understanding the type, nature and origin of such fluctuations are important from both fundamental and practical perspectives. In this study, we try to look at fluctuations due to motion of a pin probe coupled to a rheometer driven in a wormlike micellar solution formed by 2wt. % cetyltrimethylammonium tosylate and 100 mM sodium chloride (2 wt. % CTAT + 100 mM NaCl) at controlled velocities along with in-situ imaging. We find that the fluctuations of the measured force response at a constant applied velocity arise due to breakage of flow induced macro structures of the micellar solution. The length scale of such structures observed from light scattering and particle imaging velocimetry techniques are significantly larger compared to the probe size and independent of them showing a dependence on the nature of the micellar system.