Growth kinetics of colloidal chains and labyrinths

Particles interacting by a combination of isotropic short-range attraction and long-range repulsion have been shown to form complex phases despite the apparent simplicity of the interparticle potential. Using computer simulations we study the behavior of two-dimensional systems of colloids with such an interaction, focusing on how area fraction and repulsion range at fixed repulsion gradient may be used to tune the resulting kinetics and nonequilibrium structure. While the short-range attraction leads to aggregation, the long-range repulsion encourages growth of chains of particles due to repulsive intercluster interactions. Depending on area fraction/ repulsion range we observe chain labyrinths, chain-compact aggregate coexistence, and connected networks of chains. The kinetics of cluster growth displays a sequence of connected networks and disconnected cluster or chain systems with increasing repulsion range, indicating the competing roles of connectivity of growing chains and repulsion-driven breakup of chains into compact aggregates. Chain-dominated systems show approximately logarithmic coarsening at late time that we interpret as the result of chains performing random walks in the randomly fluctuating potential landscape created by their neighbors, a situation reminiscent of glassy systems

Finding bridges in packings of colloidal spheres

We identify putative load-bearing structures (bridges) in experimental colloidal systems studied by confocal microscopy. Bridges are co-operative structures that have been used to explain stability and inhomogeneous force transmission in simulated granular packings with a range of densities. We show that bridges similar to those found in granular simulations are present in real experimental colloidal packings. We describe critically the bridge-finding procedure for real experimental data and propose a new criterion-Lowest Mean Squared Separation (LSQS)-for selecting optimum stabilisations

Jamming and unjamming of concentrated colloidal dispersions in channel flow

We investigated the pressure driven flow of concentrated colloidal dispersions in a converging channel geometry. Optical microscopy and image analysis were used to track tracer particles mixed into dispersions of sterically stabilized poly(methyl methacrylate) (PMMA) spheres. The dispersions were drawn into a round \unit[0.5]{mm} capillary at one of two pump speeds ($\equiv$ applied pressure): v_1=\unit[0.245]{ml\,\, min^{-1}} and v_2=\unit[0.612]{ml\,\, min^{-1}}. We observed that the dispersions at particle volume fractions $\phi\leqslant0.50$ followed Hagen-Poiseuille flow for a simple fluid; i.e. the mean flow rate $\langle V\rangle$ is approximately proportional to pressure drop (pump speed) and inversely proportional viscosity $\eta$. Above this concentration ($\phi\geqslant0.505$), the dispersions exhibit granular-like jamming behavior with $\langle V\rangle$ becoming independent of the pressure drop. However, at the highest applied pressure ($v_2$), the dispersions are able to unjam and switch from granular-like behaviour back to a simple hard-sphere liquid like system, due to the formation of rotating vortices in the spatial flow pattern. This mechanism is consistent with computer simulations of granular systems and supports for example proposed explanations of anomalously low friction in earthquake faults

Structure and kinetics in colloidal films with competing interactions

Using computer simulation we explore how two-dimensional systems of colloids with a combination of short-range attractive and long-range repulsive interactions generate complex structures and kinetics. Cooperative effects mean the attractive potential, despite being very short-ranged compared to the repulsion, can have significant effects on large scale structure. By considering the number of particles occupying a notional `repulsion zone' defined by the repulsion length scale, we classify different characteristic structural regimes in which the combination of attraction and repulsion leads to different structural and kinetic outcomes, such as compact clustering, chain labyrinths and coexisting clusters and chains. In some regimes small changes in repulsion range and/or area fraction can change timescales of structural evolution by many orders of magnitude

Using the perceptions of chemical engineering students and graduates to develop employability skills

Recent years have seen increased global industry sector demand for chemical engineers, subsequent growth of Chemical Engineering (CE) degrees, producing additional qualified graduates. The Confederation of Business Industry have regularly indicated that employers are dissatisfied with skills sets offered by graduates; a 2004 World Chemical Engineering Council (WCEC) survey of experienced and newly employed chemical engineers’ perceptions of their own work skills indicated highest importance for general transferrable skills, with technical knowledge ranked considerably lower. A decade later, we investigate whether chemical engineers, both employed and in education, have similar skills perceptions, by surveying CE undergraduates in penultimate and final years of study, and CE alumni employed in CE roles; all from the University of Strathclyde. Again, transferrable skills were perceived as most important to respondents; as undergraduates gained industrial experience, a shift in perceived relative importance of technical knowledge occurred, again similar to the WCEC survey, otherwise, alumni and students had similar opinions regarding perceived degree of learning of various skills. Alumni were more critical of the quality of education with regards to management and transferrable skills, while female participants perceived business skills as undertaught, feeling considerably overexposed to the potential of research compared to male colleagues. Focus groups showed that male undergraduates valued ‘technical knowledge’ and ‘communicating professionally’; by contrast, female graduates highlighted ‘initiative’ and ‘business skills’. Consequently, training sessions were developed, focussing on transferable skills identified as important by all groups, to be delivered during academic year inductions, aligning skills to year curricula

Effects of secondary metal carbonate addition on the porous character of resorcinol-formaldehyde xerogels

A deeper understanding of the chemistry and physics of growth, aggregation and gelation processes involved in the formation of xerogels is key to providing greater control of the porous characteristics of such materials, increasing the range of applications for which they may be utilised. Time-resolved dynamic light scattering has been used to study the formation of resorcinol-formaldehyde gels in the presence of combinations of Group I (Na and Cs) and Group II (Ca and Ba) metal carbonates. It was found that the combined catalyst composition, including species and times of addition, is crucial in determining the end properties of the xerogels, via its effect on growth of clusters involved in formation of the gel network. Combination materials have textural characteristics within the full gamut offered by each catalyst alone; however, in addition, combination materials which retain the small pores associated with sodium carbonate catalysed xerogels exhibit a narrowing of the pore size distribution, providing an increased pore volume within an application-specific range of pore sizes. We also show evidence of pore size tunability while maintaining ionic strength, which significantly increases the potential of such systems for biological applications

Group work experiences of women students in a Scottish chemical engineering programme

Chemical Engineering, similar to other Engineering courses, has seen an undergraduate gender shift 10 in recent years towards greater women student representation. This raises the issue of the inclusion, 11 in terms of equality of participation and opportunities, of these women students in learning activities 12 and also the role that they can play in encouraging inclusion and development of others, which can 13 have implications, not only for their current studies, but their future careers. This paper provides both 14 statistical evaluation of students’ attainment from group working activities, and a narrative account of 15 the students’ experiences along with the resulting impact on their inclusion, engagement and group 16 interactions. We highlight the changing role filled by women students and their awareness of these 17 changes and impacts. Notably, the work identifies a change in attitude with regards to roles for 18 women in facilitating group work with many women students purposefully avoiding the additional 19 work-load that past studies have identified

Influence of controlled fluid shear on nucleation rates in glycine aqueous solutions

The nucleation of glycine was investigated in supersaturated aqueous solutions exposed to well-controlled fluid shear under isothermal conditions. Shear rates between 25 s-1 and 1000 s-1 were studied using Couette and capillary flow devices. Induction times were obtained from imaging, transmission and scattering measurements, or visual monitoring. Great care was taken to eliminate any seeding in order to avoid secondary nucleation preceding formation of first crystals through primary nucleation. The observed induction times of sheared solutions were considerably lower than those of unsheared solutions. Increasing the average shear rate was found to reduce the mean induction time through a power law relationship. A detailed statistical analysis showed that the number of experimental repetitions used was sufficient to obtain statistically significant trends for the system studied. Induction times appeared to closely follow a probability distribution based on a previously published model of Jiang and ter Horst. Using their model, where the induction time is related to the rate of formation of primary nuclei and the time it takes them to grow to the size where the secondary nucleation becomes significant, it was found that both the primary nucleation rate and the growth time were strongly dependent on the shear rate imposed