Gravitational collapse of colloidal gels

We investigate the phenomenon of gravitational collapse in colloidal gels via dynamic simulation in moderately concentrated gels formed via arrested phase separation. In such gels, rupture and re-formation of bonds of strength O(kT) permit ongoing structural rearrangements that lead to temporal evolution—aging—of structure and rheology [1]. The reversible nature of the bonds permits a transition from solid-like to liquid-like behavior under external forcing, and back to solid-like behavior when forcing is removed. But such gels have also been reported to undergo sudden and catastrophic collapse of the entire structural network, eliminating any intended functionality of the network scaffold. Although the phenomenon is well studied in the experimental literature, the microscopic mechanisms underlying the collapse remain murky [2-18]. Here we conduct large-scale dynamic simulation to model structural and rheological evolution of a gel subjected to gravitational stress. The model comprises 750,000 Brownian particles interacting via a hard-core repulsion and short-range attractive interactions that lead to formation of a gel, periodically replicated to an infinite system [1]. A body force is applied to the gel, and particle positions, velocities, and pressure are measured throughout simulation, as well as the bulk strain of the gel. Three temporal regimes emerge: slow, pre-collapse evolution; collapse and rapid sedimentation; and long-time compaction producing, to our knowledge, the first large-scale dynamic simulation of gravitational gel collapse. We connect the temporal regimes to distinct phases of structural and rheological evolution. A range of attraction strengths, and their effect on the critical force that triggers collapse, are studied. We find that the initial deformation is slow and linear, and the transition to and scaling of the fast strain rate depends on the strength of gravitational forcing, as is the transition to and rate of the final sedimentation regime, in excellent agreement with experimentally reported behavior [3,9,14]: The detailed microstructural evolution is reported here, along with the dependence of the delay time and speed with attraction strength and magnitude of the applied stress relative to Brownian forces. Please click Additional Files below to see the full abstract

Engineering Ordered Bicontinuous Networks Formed By Block Copolymers With Additives Using Molecular Modeling

Unlike other simpler morphologies that AB diblock copolymers (DBCs) can form, ordered bicontinuous phases are made of two interweaving network structures of the minority phase A in a matrix of the majority phase B. These network structures are attractive for applications involving ordered nanoscale porous materials including solar cell active membranes, filters, catalysts, and nanolithographic templates. Key challenges in obtaining these structures successfully are: (i) the need for very precise chemistries due to their very limited region of stability in pure block copolymer melts, (ii) the limited tunability of the morphology feature size for specific applications, and (iii) their proclivity for defect formation. The use of additives could provide more handles to tailor feature size and to enhance the stability of bicontinuous phases (e.g., by alleviating the packing frustration of the short A-blocks which creates an entropic penalty that hampers the stability of such phases). We used molecular modeling to delineate phase diagrams, provide design guidelines for lithographic applications, and to explore the nucleation behavior of one of these phases from a disordered melt. We have used different strategies to modify the stability of bicontinuous phases by exploring the effect of distinct additives: (i) A cosurfactant (short DBC) that straddles the interface and alleviates packing frustration in both A and B-domains, (ii) two solvents selective to each phase that swells both domains unevenly (for potential nanolithographic applica- tions), and (iii) an A-selective homopolymer that swells only one of the domains but provides additional configurational entropy to access bicontinuous phases beyond those found in pure DBC melts. A combination of theory and molecular simulations is used to study these systems. Self-consistent field theory is fast and used to calculate free energies of prespecified morphologies but fails to include molecular fluctuations. Coarse-grained molecular simulations are slower and require more sophisticated techniques for calculating free energies but can capture molecular fluctuations and more realistically describe defects and kinetically trapped phases. Bicontinuous phases in the DBC + homopolymer system (namely the Gyroid, Double diamond and Plumbers Nightmare) are particularly challenging because they possess large unit cells with hundreds of molecules (and thousands of monomers) per unit cell, and the observed morphology depends strongly on simulation box size, which is unknown a priori. Accurate free energy estimates are required to ascertain the stable phase, particularly when multiple competing phases spontaneously form at the conditions of interest. A variant of thermodynamic integration was implemented to obtain free energies and hence identify the stable phases and their optimal box sizes. Clear evidence was found of phase coexistence between bicontinuos phases, consistent with previous predictions for the same blend using Self-consistent field theory. Our simulations also allowed us to examine the microscopic details of these coexisting bicontinuous phases and detect key differences between the microstructure of their nodes and struts

Exploring one-to-one computing on the ground: the "100-dollar" laptop as a learning tool for socially-disadvantaged school children in India

In recent years, there has been an influx of single-user laptop projects in educational settings. The rationales are to address purported access gaps, foster student-led learning, and encourage the development of 21st century digital literacies. More recently, low-cost laptops are being distributed in developing country contexts, with the goal of providing children with new educational opportunities. However, technologies are being distributed even before the socioeconomic and cultural contexts of their use are fully understood. This work aimed to study one implementation of the One Laptop Per Child (OLPC) initiative in a South Indian city. A naturalistic case study methodology was adopted to explore how the program manifested in a school for below-poverty-line children. The findings revealed that social, cultural, economic, and curricular considerations both enabled and constrained the program in many ways. This suggests that an understanding of the social-embeddedness of technology is vital to the success of such programs

Consumers’ Attitudes towards Surcharges on Distributed Renewable Energy Generation and Energy Efficiency Programs

Increasing penetration of energy efficiency programs and distributed renewable energy generation has imposed significant challenges for utilities to recoup their large upfront costs. There is a heated debate on what surcharges should be implemented to help the utilities recover their fixed costs; however, very few studies focus on consumers’ attitudes regarding this topic. This study surveys about 190 residential consumers throughout the United States in November 2015, investigating their preferences and attitudes towards extra demand charges and volumetric energy price increases. We apply probit models and regress consumers’ attitudes on selected socio-demographic and behavioral variables. The results indicate the homeowners are more likely to prefer demand charges when compared to renters. The demographic and behavioral factors impact consumers’ perception of bill savings from energy efficiency programs or solar panel installation and also influence how consumers perceive the fairness of utilities recovering revenue losses by increasing volumetric energy price. In this paper, we demonstrate there is preference heterogeneity among consumers and that policy makers should be aware of such preference heterogeneity and apply policy targeting based on the identified demographics and behavioral factors impacting consumer preferences

A Theoretical and Simulation Study of the Self-assembly of a Binary Blend of Diblock Copolymers

Pure diblock copolymer melts exhibit a narrow range of conditions at which bicontinuous and cocontinuous phases are stable; such conditions and the morphology of such phases can be tuned by the use of additives. In this work, we have studied a bidisperse system of diblock copolymers using theory and simulation. In particular, we elucidated how a short, lamellar-forming diblock copolymer modifies the phase behavior of a longer, cylinder-forming diblock copolymer. In a narrow range of intermediate compositions, self-consistent field theory predicts the formation of a gyroid phase although particle-based simulations show that three phases compete: the gyroid phase, a disordered cocontinuous phase, and the cylinder phase, all having free energies within error bars of each other. Former experimental studies of a similar system have yielded an unidentified, partially irregular bicontinuous phase, and our simulations suggest that at such conditions the formation of a partially transformed network phase is indeed plausible. Close examination of the spatial distribution of chains reveals that packing frustration (manifested by chain stretching and low density spots) occurs in the majority-block domains of the three competing phases simulated. In all cases, a double interface around the minority-block domains is also detected with the outer one formed by the short chains, and the inner one formed by the longer chains.Publisher Version is available at: http://jcp.aip.org/resource/1/jcpsa6/v136/i23/p234905_s1?view=fulltextThis work was supported by Grant CBET 0756248 from the National Science Foundation. This publication is also based on work supported in part by Award No. KUS-C1-018-02, made by King Abdullah University of Science and Technology (KAUST). F.J.M.V. was supported by the ERC (Advanced Grant Agreement No. 227758). We are thankful to Professor David Morse from University of Minnesota for the code for implementing SCFT