1.3 Rheology of surfactant and polymeric solutions.

In the rheology area, our work has touched a number of subjects. Earlier on, we have conducted experimental work on extensional deformation of polymer solutions, which focused on flow visualization and velocity measurements. Using high-speed videography and a combination of photochromic dies and particle tracking, we were able to measure velocity profiles in several flow fields, showing for example, that significant shear may take place in tubeless siphons or aspirated jets or that regions of compression may exist in fiber spinning. We also developed some techniques for customized microencapsulation of foodstuffs in biopolymer shells, for specialized aquaculture applications.

More recently, given my interest in surfactant drag-reducers, we have investigated the rheology of the surfactant solutions we use for our drag reduction studies, using rheometers, rheo-optical devices, and light-scattering setups. The main issue we are focusing on is flow / fluid interactions. One area in particular that we have examined is the transient behavior of the fluids when subjected to shear, and especially the characteristic times of the solutions under various flow conditions, which are of interest given the macroscopic recovery studies we conducted. First, we showed that the cationic drag-reducing additives proposed for use in hydronic systems do exhibit shear-induced structure formation behavior and we quantified this behavior extensively. In particular, we measured the rebuilding times of the structures after disruption by high shear, in order to generate information useful for prediction of drag reduction after a disturbance.

Given that the surfactant solutions do show large variations in viscosity and elasticity as a function of the level and duration of shear, we have investigated the effect of the viscosity on the drag reduction data and found that it is imperative in most cases to know accurately the viscosity of the fluid over the range of shear rate encountered in the turbulent flow. For this purpose we built an ultra-high-shear rate capillary viscometer that enables us to measure the viscosity of the solutions from about 1 s-1 to 106 s-1, and therefore enables us to conduct much more physically meaningful drag reduction analyses. We also conducted a study showing that the laminar viscosity measurements are a function of the diameter of the capillary up to a point, which suggests wall slip or a macroscopic characteristic length for the micellar structure. Similar results were obtained with rotating viscometers in a gap study undertaken recently.

Next, we studied the effect of counterion to surfactant ratio, and showed that the transient characteristics of the fluid are very different depending on this ratio. We also showed that the characteristic times show a maximum at a ratio different than that leading to the maximum in viscoelasticity, which would allow us to customize our drag-reducing fluids for specific purposes. A parametric study was then conducted to study the equilibrium structures of the fluids using dynamic light-scattering and low-shear rheometry. This study enabled us to quantify the concentration, ratio, and temperature at which the micelles become entangled, another very useful piece of information for optimization of the additives.

Given that we showed that the drag reduction level exhibited by the fluid can be greatly reduced by chemical contamination, we have studied the effect of various metal ions and compounds on the viscoelasticity of the cationic surfactants, both to characterize this effect and also to see if this type of measurements could be used as a predicting tool for drag-reducing characteristics. In a striking parallel with the drag reduction results, we did indeed see that some contaminants eliminate readily the shear-induced structures in the fluid, through a combination of adsorption and chemical reactions with the counterion. We also showed that the viscoelasticity can be readily recovered with chelating agents, again in similarity to the drag reduction results. During this study we also developed some spectrophotometric techniques that allowed us to measure directly the concentration of these additives in solution, which enables us to follow surfactant adsorption during the field tests as well.

These results confirming that the cationic surfactants are adversely affected by metal compounds, we have then investigated some more promising non-ionic additives. Interestingly, we were able to document for the first time in these solutions the formation of shear-induced structures that are very similar to those documented previously for cationic surfactants. We also showed that these non-ionic surfactants are much less sensitive to chemical contamination than cationic surfactants. These are the additives we used successfully in our second field test.

We have recently undertaken a study aimed at simultaneous viscometric characterization and direct light-scattering visualization of the supramolecular structure of some of our solutions. This work extends our earlier viscometric characterization work on shear-thickening thanks to the implementation of a powerful light-scattering visualization technique developed by D. Pine (UCSB). In this work, it was shown that the shear-induced structure formation leads to the appearance of a gel-like phase which can be seen in the gap of a concentric cylinder and which fills it over a time scale consistent with the induction and plateau times we measured previously. Further experiments generated visualization results explaining the sudden jump in viscosity observed at a critical shear rate and showed that the shear-thickening (i.e. gel phase appearance) is controlled by shear stress rather than shear rate as previously believed. Indeed, the gel phase can fill only part of the gap under steady-state conditions during a constant shear stress. The presence of two "different" fluids in the gap revealed that great care must be taken to achieve a correct interpretation of viscometric measurements for these fluids. For example, some additional velocity measurements using particle tracking within the viscometer gap revealed the existence of large "slip" between the wall and the gel phase, which explained the apparent lack of dependence of the viscosity on concentration and shear rate, and the effect of viscometer gap on the measurements. We also documented and quantified the fractures of the gel phase which resulted in constant shear stress in the high shear rate range and in a phase instability between the dilute and gel phases that leads to the large oscillations in apparent viscosity data observed in the transition stress region. This combination of simultaneous advanced rheology measurements and direct visualization provided us therefore with a striking rheologically-quantified visual observation of shear-induced structures in surfactant solutions, and led to explanations for several phenomena previously not well understood.

[Mechanical Engineering Department] [College of Engineering] [UCSB] [UCSB Web Sites]
For problems with- or comments about- this Web page, please contact matthys@engineering.ucsb.edu . Thank you !