A novel experimental setup was developed that provides high-resolution measurement of the kinematics and dynamics of the flow of a gel in a glass capillary tube. The setup was equipped with flow measurement tools as well as an optical coherence tomography device as a high-resolution imaging device. It allows us to directly visualize the flow of tracing particles seeded in the fluid. The measured velocimetric and viscometric data are analyzed to provide the rheological behaviour of the gel.
(a) Schematic diagram of the experimental setup, (b) an image of tracing particles in the capillary tube acquired by the OCT. The white lines in this image indicates the wall positions. (c) The number of velocity vectors, N, that contribute to each bin across the capillary tube. Here, the z-axis represents the distance from the centerline of the cell, normalised by the diameter of the capillary tube. Note that the velocity data were obtained from applying the PTV to 1000 paired images. Then the PTV data were averaged using a spatial binning size of 5µ m.
1. Characterising wall-slip behaviour of carbopol gels in a fully-developed poiseuille flow (PI: Prof. Mark Martinez)
Variation of (a) the slip velocity with the excess shear stress, (b) the thickness of the slip layer with the excess shear stress normalised by the yield stress, (c) the slip velocity normalised by the maximum velocity with the excess shear stress normalised by the yield stress. Note that, in panel (a), the values of slip velocity obtained for Glycerol-solution based Carbopol gel (fluid C7) was scaled by the viscosity ratio of water to solvent.
We examine the wall-slip behaviour of Carbopol gel, as a simple yield-stress fluid, in a Poiseuille flow by analysing its velocity distribution across a glass capillary tube and near the wall. Optical coherence tomography (OCT) is used to image the flow of the gel seeded by buoyant tracing particles, and particle tracking velocimetry (PTV) is applied to the flow images to obtain the velocity profiles near the wall. In addition, the dynamics of the flow is measured through a viscometric unit integrated with the flow cell. Our velocimetric and viscometric measurements reveal apparent slip of Carbopol gels on the capillary wall, in a wide range of wall shear stresses below and above the yield stress of the material. Our results confirm the presence of a fully plugged flow within the precision of our velocimetric measurement, where the wall stress lies below the yielding point. Above the yielding point, the presence of wall slip is confirmed over a wide range of shear rates. Our results indicate that the slip velocity increases with the wall shear stress in both regimes. We relate the slip velocity to the excess shear stress on the wall, i.e. the wall shear stress minus the shear stress at the onset of slip, using a power-law scaling, with an exponent independent of the microstructure of the fluid. The effect of microstructure of the gel on the wall slip is studied by either varying the concentration of the gel or the stirring rate during preparation. We find that the higher the concentration of the gel or the higher the stirring rate is, the lower the slip velocity is. Moreover, our experiments indicate that increasing the viscosity of the solvent leads to a decrease in the slip velocity.
M. Daneshi, A. Pourzahedi, D.M. Martinez, D. Grecov, Characterising wall-slip behaviour of Carbopol gels in a fully-developed Poiseuille flow, Journal of Non-Newtonian Fluid Mechanics. 2019 Jul 1;269:65-72
2. Shear Layers and Plugs in the Capillary Flow of Wormlike Micellar Gels (PI: Prof. Ian Frigaard)
Wormlike micellar solutions formed by long-chained zwitterionic surfactants show gel-like rheology at room temperature and have recently been found to exhibit other complex and interesting rheological features. We study the dynamics of these wormlike micellar gels in a canonical flow scenario using optical coherence tomography-based velocimetry and report the existence of plug flows with strong wall-slip along with non-parabolic velocity profiles with shear-layers for different surfactant concentration and imposed flowrates. We rationalize these results as features of a developing transient flow of a viscoelastic solution in space and time. Our experiments shed light on the transient fluid dynamics of wormlike micelles in simple geometries and inform ongoing attempts to understand the cross-talk between the rheology and flow of soft matter.
Averaged velocity profiles for 1% VES at for Q = 0.1ml/min (blue triangles) and Q = 0.25 ml/min (black circles). Inset : Last few points for both main figure curves fit to a straight line (color+symbols same as main figure)
Time averaged velocity profile before initiating stoppage calcu- lated by PIV for 1% VES flowing at Q = 0.25 ml/min after stepping down from Q = 3 ml/min. Inset : Trajectories of particles sourced from different regions with numbers indicate individual particles and red circles in main figure indicating the y position in the pipe cross-section of the particle
R. R. Gupta, M. Daneshi, I. Frigaarda,b, and G. Elfring, Shear Layers and Plugs in the Capillary Flow of Wormlike Micellar Gels, Soft Matter (submitted 2023).