Self-assembling materials spontaneously form structures at length scales of interest in nanotechnology. In the particular case of block copolymers, the thermodynamic driving forces for self-assembly are small, and low-energy defects can get easily trapped. We directed the assembly of defect-free arrays of isolated block copolymer domains at densities up to 1 terabit per square inch on chemically patterned surfaces. In comparing the assembled structures to the chemical pattern, the density is increased by a factor of four, the size is reduced by a factor of two, and the dimensional uniformity is vastly improved.

A coarse grain model and a Monte Carlo sampling formalism are proposed for simulations of self-assembly in block copolymer melts and nanoparticle−copolymer composites. Our approach relies on a particle-based representation of the system, it does not invoke a saddle point approximation, and it permits treatment of large three-dimensional systems. We provide a detailed description of the model and methods and discuss their relationship to results from self-consistent-field theory and single chain in mean field simulations. The validity of the proposed approach is addressed by applying it to study systems whose description within existing approaches would be demanding. In particular, we use it to examine the directed assembly of copolymer blends and nanoparticles on nanopatterned substrates. We show that results from simulations are in good agreement with experiment, and we use our theoretical findings to help explain the experimental observations.

The ubiquitous aquaporin channels are able to conduct water across cell membranes, combining the seemingly antagonist functions of a very high selectivity with a remarkable permeability. Whereas molecular details are obvious keys to perform these tasks, the overall efficiency of transport in such nanopores is also strongly limited by viscous dissipation arising at the connection between the nanoconstriction and the nearby bulk reservoirs. In this contribution, we focus on these so-called entrance effects and specifically examine whether the characteristic hourglass shape of aquaporins may arise from a geometrical optimum for such hydrodynamic dissipation. Using a combination of finite-element calculations and analytical modeling, we show that conical entrances with suitable opening angle can indeed provide a large increase of the overall channel permeability. Moreover, the optimal opening angles that maximize the permeability are found to compare well with the angles measured in a large variety of aquaporins. This suggests that the hourglass shape of aquaporins could be the result of a natural selection process toward optimal hydrodynamic transport. Finally, in a biomimetic perspective, these results provide guidelines to design artificial nanopores with optimal performances. nanofluidics | hydrodynamic permeability | biochannels

A remarkable feature of active matter is the propensity to self-organize. One striking instance of this ability to generate spatial structures is the cluster phase, where clusters broadly distributed in size constantly move and evolve through particle exchange, breaking or merging. Here we propose an exhaustive description of the cluster dynamics in apolar active matter. Exploiting large statistics gathered on thousands of Janus colloids, we measure the aggregation and fragmentation rates and rationalize the resulting cluster size distribution and fluctuations. We also show that the motion of individual clusters is entirely consistent with a model positing random orientation of colloids. Our findings establish a simple, generic model of cluster phase, and pave the way for a thorough understanding of clustering in active matter.

A Monte Carlo formalism for the study of polymeric melts is described. The model is particle-based, but the interaction is derived from a local density functional that appears in the field-based model. The method enables Monte Carlo simulations in the nVT, nPT, semigrandcanonical and Gibbs ensembles, and direct calculation of free energies. The approach is illustrated in the context of two examples. In the first, we consider the phase separation of a binary homopolymer blend and present results for the phase diagram and the critical point. In the second, we address the microphase separation of a symmetric diblock copolymer, examine the distribution of local stresses in lamellae, and determine the order-disorder transition temperature.

A combined theoretical and experimental approach is used to study the directed assembly of a lamellae-forming block copolymer on chemically patterned substrates. The period of the pattern is lower than that of the copolymer, whose characteristic morphology is then used to interpolate the features of the substrate. The pattern considered in this study consists of stripes of width W repeated over the background substrate with period L P = 2L 0, where L 0 is the copolymer natural period. The stripe and background areas are characterized by their affinities Λs and Λb for the blocks of the polymer. Using theoretically informed Monte Carlo simulations of a coarse-grained model, we investigate in a systematic manner the influence of the pattern parameters W, Λs, and Λb on the morphology of copolymer thin films. Thermodynamic integration is used to compute the free energy difference and the relative stability of competing morphologies. It is found that the parameter space considered here is dominated by nonbulk and often metastable morphologies. The conditions that yield successful interpolation of lamellae are identified. Consistent with theoretical predictions, experiments on patterned substrates with carefully controlled interfacial characteristics reveal new, three-dimensional morphologies that do not arise in the bulk. The sought-after vertical lamellae, which are desirable for pattern interpolation and lithography, are found to occur only when the interaction between one of the blocks and the background area is relatively weak.

We apply local mean-field (i.e., density functional) theory to a lattice model of a fluid in contact with a dilute, disordered gel network. The gel structure is described by a diffusion-limited cluster aggregation model. We focus on the influence of porosity on both the hysteretic and the equilibrium behavior of the fluid as one varies the chemical potential at low temperature. We show that the shape of the hysteresis loop changes from smooth to rectangular as the porosity increases and that this change is associated with disorder-induced out-of-equilibrium phase transitions that differ in adsorption and in desorption. Our results provide insight in the behavior of 4He in silica aerogels.

We introduce a particle-based Monte Carlo formalism for the study of polymeric melts, where the interaction energy is given by a local density functional, as is done in traditional field-theoretic models. The method enables Monte Carlo simulations in arbitrary ensembles and direct calculation of free energies. We present results for the phase diagram and the critical point of a binary homopolymer blend. For a symmetric diblock copolymer, we compute the distribution of local stress in lamellae and locate the order-disorder transition.