Multiscale Investigation of the Nonlinear Rheology of Wormlike Micelles

Wormlike micelles (WLMs) are formed by reversible self-assembly of amphiphilic (e.g. surfactant) molecules usually with the aid of salt which acts to screen the electrostatic repulsion between the micelles to achieve giant structures. Their unique dynamics such as reversible scission give them interesting rheological properties which are sought in several industrial applications. It is therefore imperative to understand and predict these phenomena in WLMs using constitutive equations that incorporate reversible scission into the deformation dynamics of WLMs. We compare the predictions of the Vasquez-Cook-McKinley (VCM) (2007) model which treats WLMs as Hookean dumbbells that break at half-length to form two shorter dumbbells, to an analogous Brownian dynamics (BD) simulation of the same physical model. We find a discrepancy between their predictions and trace it to the absence in the VCM model of the internal position of the nascent breakage point in the long micelle, which is needed to satisfy microscopic reversibility of breakage and fusion. We correct this deficiency in the VCM model by extending an ensemble-averaged bead-spring phase space model of Wiest et al. (1989) to include reversible scission of two-spring chains. The revision tracks the conformations of the two halves of the long micelle and transmits this information to the short micelles upon breakage and thereby recovers complete agreement with the BD results. We extend the reversible scission kinetics to the slip-link tube model of Likhtman (2005) originally formulated for single polymer chains with entanglements. This facilitates the simulation of entangled WLMs and enables us to study the effects of entanglements on WLM rheology. We observe increased stresses for start-up shear flows in the entangled WLMs when breakage time was equal to reptation time. We propose that reversible scission acts as a means of constraint release which re-orientates chain segments in the velocity gradient direction and prevent retraction in the tube thereby causing increased stress. However, stress overshoot caused by the relaxation of the peak stress to a lower steady stress was observed in the fast-breaking regime. This suggests that reversible scission functions as both stress relaxation and constraint release mechanisms. We then investigate strain hardening, a nonlinear rheological property. We explore different kinds of WLMs for strain hardening and systematically study their strain hardening dependence on salt concentration and temperature. By measuring stress relaxation following a step strain, we observe that strain hardening is prevalent over a temperature range of 15 - 25 C for a solution of cetyl trimethyl ammonium bromide (CTAB) with the added hydrotrope, sodium salicylate at hydrotrope-to-surfactant concentration ratios between 0.5 - 3.0. The extent of strain hardening upon nonlinear step-strain deformation varies non-monotonically as a function of salt-to-surfactant ratio for different temperatures. A transition from strain hardening to softening or linear response is observed at strains that are dependent on temperature and concentration. Strain hardening was also observed in solutions of CTAB and hydrotrope, sodium 3-hydroxy-2-naphthoate. However, solutions of anionic sodium lauryl sulfate surfactants without hydrotrope but simple salt, sodium chloride strain softened, indicating that the hydrotrope is crucial to obtaining strain hardening in step strains. The results indicate a stress relaxation mechanism that is more complex than that of simple disentanglement and reversible scission, possibly involving strain-induced associations between micelles facilitated by hydrotropes that may act as physical crosslinkers.PHDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/145970/1/adamsaa_1.pd

E-Government, Information and Communications Technology Support and Paperless Environment in Nigerian Public Universities: Issues and Challenges

The move toward a paperless environment has become the driving force behind sustainable development and e-government usage in many public sector institutions. It is equally at the heart of the government campaign to make service delivery in public institutions cost-effective, seamless, and efficient. The universities are supposed to be at the front-line of this campaign due to their operations which involve the heavy usage of papers at huge costs. Thus, making it important for university management to provide ICT support in order to promote paperless exchange of information and presentations. It is, however, understood recently, that there are issues which surround the low usage of ICT among university management and ultimately paperless environment. The purpose of this paper, therefore, is to conceptually discuss the mode of university operations and how paperless environment can be attained. The paper highlights the cyclical order of document generation, document management and document sharing as the process in which a paperless environment can take place within both the academic and the administrative settings in the university. The paper further discusses the challenges hindering the attainment of paperless environment among which are infrastructural gap, inadequate ICT support and attitudinal challenge. It is recommended that for the smooth operation of a paperless environment, the government must bridge the infrastructure gap especially power as well as train and retrain staff on the path ICT usage

Experimental Control of Nodality VIA Equal Presentations of Conditional Discriminations in Different Equivalence Protocols Under Speed and No-speed Conditions

A within-participant comparison of simple-to-complex, complex-to-simple, and simultaneous protocols was conducted establishing different sets of three 7-member equivalence classes for 4 undergraduate students. The protocols were implemented under either accuracy-only or accuracy-plus-speed conditions while keeping number of presentations of training and testing trials equal. The results partially support previous reports of differential effects on acquisition, with participants completing more blocks in training under the simultaneous than the complex-to-simple and the simple-to-complex protocols. Across the protocols, however, the number of trials completed to criterion did not vary systematically. More important, response speed and accuracy did not decrease as a function of nodal number, with or without the speed contingency, or under any protocol. The latter results challenge the generality of previous reports of the nodality effect and the notion of “relatedness” of equivalence-class members, and support a reinforcement-contingency, instead of a structural, perspective on equivalence-class formation

Test order effects in simultaneous protocols

Simultaneous protocols typically yield poorer stimulus equivalence outcomes than do other protocols commonly used in equivalence research. Two independent groups of three 3-member equivalence sets of stimuli were used in conditional discrimination procedures in two conditions, one using the standard simultaneous protocol and the other using a hybrid simultaneous training and simple-to-complex testing. Participants completed the two conditions in one long session in Experiment 1, but in separate sessions in Experiment 2. The same stimulus sets used in Experiment 1 were randomized for the two conditions in Experiment 2. Overall, accuracy was better with the hybrid than with the standard protocol in both experiments. The equivalence yield was also better under the hybrid than under the standard protocol in each experiment. The results suggest that the order of testing for emergent relations may account for the difficulty often encountered with the standard simultaneous protocol