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Porous materials possess numerous useful functions because of their high surface area and ability to modulate the transport of heat, mass, fluids, and electromagnetic waves. Unlike highly ordered structures, disordered porous structures offer the advantages of ease of fabrication and high fault tolerance. Bicontinuous interfacially jammed emulsion gels (bijels) are kinetically trapped disordered biphasic materials that can be converted to porous materials with tunable features. Current methods of bijel fabrication result in domains that are micrometers or larger, and non-uniform in size, limiting their surface area, mechanical strength, and interaction with electromagnetic waves. In this work, scalable synthesis of bijels with uniform and sub-micrometer domains is achieved via a two-step solvent removal process. The resulting bijels are characterized quantitatively to verify the uniformity and sub-micrometer scale of the domains. Moreover, these bijels have structures that resemble the microstructure of the scale of the white beetle Cyphochilus. We find that such bijel films with relatively small thicknesses (< 150 um) exhibit strong solar reflectance as well as high brightness and whiteness in the visible range. Considering their scalability in manufacturing, we believe that VIPS-STRIPS bijels have great potential in large-scale applications including passive cooling, solar cells, and light emitting diodes (LEDs)., Comment: 20 pages, 4 figures in main text, 4 figures in Supporting information

Physically confining polymers into nanoscale pores induces significant changes in their dynamics. Although different results on the effect of confinement on the dynamics of polymers have been reported, changes in the segmental mobility of polymers typically are correlated with changes in their chain mobility due to increased monomeric relaxation times. In this study, we show that translational and segmental dynamics of polymers confined in disordered packings of nanoparticles can exhibit completely opposite behavior. We monitor the capillary rise dynamics of entangled polystyrene (PS) in disordered packings of silica nanoparticles (NPs) of 7 and 27 nm diameter. The effective viscosity of PS in 27 nm $SiO_2$ NP packings, inferred based on the Lucas-Washburn equation, is significantly smaller than the bulk viscosity, and the extent of reduction in the translational motion due to confinement increases with the molecular weight of PS, reaching 4 orders of magnitude reduction for PS with a molecular weight of 4M g/mol. The glass transition temperature of entangled PS in the packings of 27 nm $SiO_2$ NPs, however, increases by 45 K, indicating significant slowdown of segmental motion. Interestingly, confinement of the polymers into packings made of 7 nm $SiO_2$ NPs results in molecular weight-independent effective viscosity. The segmental dynamics of PS in 7 nm $SiO_2$ NP packings are slowed down even further as evidenced by 65 K increase in glass transition temperature. These seemingly disparate effects are explained by the microscopic reptation-like transport controlling the translational motion and the physical confinement affecting the segmental dynamics under extreme nanoconfinement., Comment: 34 pages, 6 figures Supplementary Information: 10 pages, 6 figures

Nanocomposite films containing a high volume fraction (> 50vol%) of nanoparticles (NPs) in a polymer matrix are promising for their functionality and use as structural coatings, and also provide a unique platform to understand polymer behavior under strong confinement. Previously, we developed a novel technique to fabricate such nanocomposites at room temperature using solvent-driven infiltration of polymer (SIP) into NP packings. In the SIP process, a bilayer made of an underlying polymer film and a dense packing of NPs is exposed to solvent vapor which induces condensation of the solvent into the voids of the packing. The condensed solvent plasticizes the underlying polymer film, inducing polymer infiltration into the solvent-filled voids in the NP packing. In this work, we study the effect of confinement on the kinetics of SIP and the final partitioning of polymer into the interstices of the NP packing. We find that, while the dynamics of infiltration during SIP are strongly dependent on confinement, the final extent of infiltration is independent of confinement. The time for infiltration obeys a power law with confinement, as defined by the ratio of the chain size and the pore size. Qualitatively, the observed time scale is attributed to changes in concentration regimes as infiltration proceeds, which lead to shifting characteristic length scales in the system over time. When the concentration in the pore exceeds the critical overlap concentration, the characteristic length scale of the polymer is no longer that of the entire chain, but rather the correlation length, which is smaller than the pore size. Therefore, at long times, the extent of infiltration is independent of the confinement ratio. Furthermore, favorable surface interactions between the polymer and the nanoparticles enhance partitioning into the NP packing., Comment: 8 pages, 5 figures. Supporting Information: 4 pages, 3 figures

Recent advances in our understanding of thermal transport in nanocrystalline systems are responsible for the integration of new technologies into advanced energy systems, including thermoelectric refrigeration systems and renewable energy platforms. However, there is little understanding of heat energy transport mechanisms that govern the thermal properties of disordered nanocomposites. In this work, we explore thermal transport mechanisms in disordered packings of amorphous nanoparticles with and without a polymer filling the interstices in order to quantify the impact of thermal boundary scattering introduced at nanoparticle edges in an already amorphous system and within the context of a minimum thermal conductivity approximation. By fitting a modified minimum thermal conductivity model to temperature-dependent measurements of thermal conductivity from 80 K to 300 K, we find that the interstitial polymer {\it eliminates} boundary scattering in the disordered nanoparticle packing, which surprisingly leads to an {\it increase} in the overall thermal conductivity of the disordered nanoparticle thin-film composite. This is contrary to our expectations relative to effective medium theory and our understanding of a minimum thermal conductivity limit. Instead, we find that a stiff interstitial material improves the transmission of heat through a nanoparticle boundary, improving the thermal properties of disordered nanoparticle packing. We expect these results to provide insight into the tunability of thermal properties in disordered solids that exhibit already low thermal conductivities through the use of nanostructuring and vibrational thermal bridging.

We study the effect of physical confinement on the capillary infiltration of polymers into cylindrical nanopores using molecular dynamics simulations. In particular, we probe whether the critical contact angle above which capillary rise infiltration ceases to occur changes for long chain polymers, possibly due to loss of conformation entropy induced by chain confinement. Surprisingly, the critical contact angle does not strongly depend on the length of polymer chains and stays constant for large N. A free energy model is developed to show that the critical angle for infiltration depends strongly on the size of statistical segments rather than the total chain length, which we confirm by performing MD simulations of infiltration with semi-flexible polymers. These results could provide guidelines in manufacturing polymer nanostructures and nanocomposites using capillary rise infiltration.

Bacteria at fluid interfaces endure physical and chemical stresses unique to these highly asymmetric environments. The responses of Pseudomonas aeruginosa PAO1 and PA14 to a hexadecane-water interface are compared. PAO1 cells form elastic films of bacteria, excreted polysaccharides and proteins, whereas PA14 cells move actively without forming an elastic film. Studies of PAO1 mutants show that, unlike solid-supported biofilms, elastic interfacial film formation occurs in the absence of flagella, pili, or certain polysaccharides. Highly induced genes identified in transcriptional profiling include those for putative enzymes and a carbohydrate metabolism enzyme, alkB2; this latter gene is not upregulated in PA14 cells. Notably, PAO1 mutants lacking the alkB2 gene fail to form an elastic layer. Rather, they form an active film like that formed by PA14. These findings demonstrate that genetic expression is altered by interfacial confinement, and suggest that the ability to metabolize alkanes may play a role in elastic film formation at oil-water interfaces.

Chirality, ubiquitous in complex biological systems, can be controlled and quantified in synthetic materials such as cholesteric liquid crystal (CLC) systems. In this work, we study spherical shells of CLC under weak anchoring conditions. We induce anchoring transitions at the inner and outer boundaries using two independent methods: by changing the surfactant concentration or by raising the temperature close to the clearing point. The shell confinement leads to new states and associated surface structures: a state where large stripes on the shell can be filled with smaller, perpendicular sub-stripes, and a focal conic domain (FCD) state, where thin stripes wrap into at least two, topologically required, double spirals. Focusing on the latter state, we use a Landau-de Gennes model of the CLC to simulate its detailed configurations as a function of anchoring strength. By abruptly changing the topological constraints on the shell, we are able to study the interconversion between director defects and pitch defects, a phenomenon usually restricted by the complexity of the cholesteric phase. This work extends the knowledge of cholesteric patterns, structures that not only have potential for use as intricate, self-assembly blueprints but are pervasive in biological systems., Comment: 27 pages, 10 figures

Colloids and proteins alike can bind to lipid bilayers and move laterally in these two-dimensional fluids. Inspired by proteins that generate membrane curvature, sense the underlying membrane geometry, and migrate to high curvature sites, we explore the question: Can colloids, adhered to lipid bilayers, also sense and respond to membrane geometry? We report the curvature migration of Janus microparticles adhered to giant unilamellar vesicles elongated to present well defined curvature fields. However, unlike proteins, which migrate to minimize membrane bending energy, colloids migrate by an entirely different mechanism. By determining the energy dissipated along a trajectory, the energy field mediating these interactions is inferred to be linear in the local deviatoric curvature, as reported previously for colloids trapped at curved interfaces between immiscible fluids. In this latter system, however, the energy gradients are far larger, so particles move deterministically, whereas the colloids on vesicles move with significant fluctuations in regions of weak curvature gradient. By addressing the role of Brownian motion, we show that the observed curvature migration of colloids on bilayers is indeed a case of curvature capillary migration, with membrane tension playing the role of interfacial tension. Furthermore, since this motion is mediated by membrane tension and shape, it can be modulated, or even turned "on" and "off", by simply varying these parameters. While particle-particle interactions on lipid membranes have been considered in many contributions, we report here an exciting and previously unexplored modality to actively direct the migration of colloids to desired locations on lipid bilayers., Comment: 24 pages, 12 figures

We study the assembly of colloids in a two phase water-water system that provides an environment that can sustain bacteria, providing a new structure with rich potential to confine and structure living colloids. The water-water system, formed via phase separation of a casein and xanthan mixture, forms a 3-D structure of coexisting casein-rich and xanthan-rich phases. Fluorescent labelling and confocal microscopy reveal the attachment of these living colloids, including Escherichia coli and Pseudomonas aeruginosa, at the interface between the two phases. Inert colloids also become trapped at the interfaces, suggesting that the observed attachment can be attributed to capillarity. Over time, these structures coarsen and eventually degrade, illustrating the dynamic nature of these systems. This system lays the foundation for future studies of the interplay of physicochemical properties of the fluid interfaces and bulk phases and microbial responses they provoke to induce complex spatial organization, to study species which occupy distinct niches, and to optimize efficient microbial cross-feeding or protection from competitors., Comment: 13 pages, 8 figures (includes supplementary)

We study the uniaxial compressive behavior of disordered colloidal free-standing micropillars composed of a bidisperse mixture of 3 and 6 um polystyrene particles. Mechanical annealing of confined pillars enables variation of the packing fraction across the phase space of colloidal glasses. The measured normalized strengths and elastic moduli of the annealed freestanding micropillars span almost three orders-of-magnitude despite similar plastic morphology governed by shear banding. We measure a robust correlation between ultimate strengths and elastic constants that is invariant to relative humidity, implying a critical strain of $\sim$0.01 that is strikingly similar to that observed in metallic glasses (MGs) [W.L. Johnson, K. Samwer, Phys. Rev. Lett. 95, 195501, 2005] and suggestive of a universal mode of cooperative plastic deformation. We estimate the characteristic strain of the underlying cooperative plastic event by considering the energy necessary to create an Eshelby-like ellipsoidal inclusion in an elastic matrix. We find that the characteristic strain is similar to that found in experiments and simulations of other disordered solids with distinct bonding and particle sizes, suggesting a universal criterion for the elastic to plastic transition in glassy materials with the capacity for finite plastic flow., Comment: 7 pages, 3 figures