Universal scaling of the osmotic pressure for dense, quasi-two-dimensionally confined polymer melts reveals transitions between fractal dimensions.

A scaling law for the osmotic pressure of quasi-two-dimensional polymer melts as a function of concentration is obtained, which shows fractal characteristics. Structural properties such as the chains' contour length and their inner-monomer pair distribution function display fractal scaling properties as well. These predictions are confirmed with mesoscale numerical simulations. The chains are swollen and highly entangled, yet Flory's exponent is always ν = 1/2. The melt can be considered a fluid of "blobs" whose size becomes renormalized in terms of the contour's length while the fractal dimension df increases monotonically between 5/4 and 2, as the monomer concentration is increased. The semidilute scaling of the pressure is recovered when df = 1. Our results agree with recent experiments and with numerical reports on quasi-2d melts. This work provides a new paradigm to study and interpret thermodynamic and structural data in low-dimensional polymer melts, namely as fractal macromolecular objects.

Synergy between surfactants' stiffness and concentration on their self-assembly into reverse micelles as water droplet carriers in nonpolar solvents.

A study of the self-assembly process into reverse micelles (RMs) of linear surfactants and monomeric aqueous solutes dissolved in nonpolar solvents, varying the concentration (cs) and the persistence length (Lp) of the surfactants is presented here. The influence of cs and Lp on the structural and dynamic properties of the aggregates is investigated through mesoscopic simulations carried out with the dissipative particle dynamics method. All simulations are performed at a fixed water/surfactant molecular ratio of 2:1, varying the surfactant concentration from c = 6 wt% up to c = 12 wt%, for increasing surfactants' rigidity from Lp = 0.73 nm up to Lp = 44.99 nm. It is found that there exists a collaborative interplay between cs and Lp that enhances the number of RMs assembled and their diffusion as carriers of water droplets. These results should be useful as guidelines to understand and improve processes where the RMs are implemented to carry aqueous solutes in nonpolar solvents.

Hyperscaling of the correlation length, interfacial tension, and specific heat in two-dimensional and quasi-two-dimensional liquids.

The line tension of two immiscible liquids under two-dimensional and quasi-two dimensional conditions is calculated as a function of temperature, using mesoscale numerical simulations, finding that it decays linearly. The liquid-liquid correlation length, defined as the thickness of their interface, is also predicted as the temperature is varied, and it diverges as the temperature becomes close to the critical temperature. These results are compared with recent experiments on lipid membranes and good agreement is obtained. The scaling exponents of the line tension (µ) and the spatial correlation length (ν) with temperature are extracted, finding that they fulfill the hyperscaling relationship, µ=d-1ν, where d is the dimension. The scaling of specific heat with temperature of the binary mixture is obtained as well. This is the first report of the successful test of the hyperscaling relation between µ and ν for d = 2 and for the non-trivial case of quasi-two dimensions. This work can help to understand experiments that test properties of nanomaterials using simple scaling laws, without needing to know specific chemical details of those materials.

The effective interfacial tensions between pure liquids and rough solids: a coarse-grained simulation study.

The effective solid-liquid interfacial tension (SL-IFT) between pure liquids and rough solid surfaces is studied through coarse-grained simulations. Using the dissipative particle dynamics method, we design solid-liquid interfaces, confining a pure liquid between two explicit solid surfaces with different roughness degrees. The roughness of the solid phase is characterized by Wenzel's roughness factor and the effective SL-IFT is reported as a function of it also. Two solid-liquid systems, different from each other by their solid-liquid repulsion strength, are studied to measure the effects caused by the surface roughness on the calculation of . It is found that the roughness changes the structure of the liquid, which is observed in the first layer of liquid near the solid. These changes are responsible for the effective SL-IFT increase, as surface roughness increases. Although there is a predominance of surface roughness in the calculation of it is found that the effective SL-IFT is directly proportional to the magnitude of the solid-liquid repulsion strength. The insights provided by these simulations suggest that the increase of Wenzel's roughness factor increases the number of effective solid-liquid interactions between particles, yielding significant changes in the local values of the normal and tangential components of the pressure tensor.

The viscosity of polyelectrolyte solutions and its dependence on their persistence length, concentration and solvent quality.

In this work, a comprehensive study of the influence on shear viscosity of polyelectrolyte concentration, persistence length, salt concentration and solvent quality is reported, using numerical simulations of confined solutions under stationary Poiseuille flow. Various scaling regimes for the viscosity are reproduced, both under good solvent and theta solvent conditions. The key role played by the electrostatic interactions in the viscosity is borne out when the ionic strength is varied. It is argued that these results are helpful for the understanding of viscosity scaling in entangled polyelectrolyte solutions for both rigid and flexible polyelectrolytes in different solvents, which is needed to perform intelligent design of new polyelectrolytes capable of fine tuning the viscosity in complex fluids.

Ab Initio Modeling Of Friction Reducing Agents Shows Quantum Mechanical Interactions Can Have Macroscopic Manifestation.

Two of the most commonly encountered friction-reducing agents used in plastic sheet production are the amides known as erucamide and behenamide, which despite being almost identical chemically, lead to markedly different values of the friction coefficient. To understand the origin of this contrasting behavior, in this work we model brushes made of these two types of linear-chain molecules using quantum mechanical numerical simulations under the density functional theory at the B97D/6-31G(d,p) level of theory. Four chains of erucamide and behenamide were linked to a 2 × 10 zigzag graphene sheet and optimized both in vacuum and in continuous solvent using the SMD implicit solvation model. We find that erucamide chains tend to remain closer together through π-π stacking interactions arising from the double bonds located at C13-C14, a feature behenamide lacks, and thus a more spread configuration is obtained with the latter. It is argued that this arrangement of the erucamide chains is responsible for the lower friction coefficient of erucamide brushes, compared with behenamide brushes, which is a macroscopic consequence of cooperative quantum mechanical interactions. While only quantum level interactions are modeled here, we show that behenamide chains are more spread out in the brush than erucamide chains as a consequence of those interactions. The spread-out configuration allows more solvent particles to penetrate the brush, leading in turn to more friction, in agreement with macroscopic measurements and mesoscale simulations of the friction coefficient reported in the literature.