Leonardo Golubovic

Structural properties of the sliding columnar phase in layered liquid crystalline systems.

Under appropriate conditions, mixtures of cationic and neutral lipids and DNA in water condense into complexes in which DNA strands form local two-dimensional (2D) smectic lattices intercalated between lipid bilayer membranes in a lamellar stack. These lamellar DNA-cationic-lipid complexes can in principle exhibit a variety of equilibrium phases, including a columnar phase in which parallel DNA strands form a 2D lattice, a nematic lamellar phase in which DNA strands align along a common direction but exhibit no long-range positional order, and a possible new intermediate phase, the sliding columnar (SC) phase, characterized by a vanishing shear modulus for relative displacement of DNA lattices but a nonvanishing modulus for compressing these lattices. We develop a model capable of describing all phases and transitions among them and use it to calculate structural properties of the sliding columnar phase. We calculate displacement and density correlation functions and x-ray scattering intensities in this phase and show, in particular, that density correlations within a layer have an unusual exp(-const x ln(2)r) dependence on separation r. We investigate the stability of the SC phase with respect to shear couplings leading to the columnar phase and dislocation unbinding leading to the lamellar nematic phase. For models with interactions only between nearest neighbor planes, we conclude that the SC phase is not thermodynamically stable. Correlation functions in the nematic lamellar phase, however, exhibit SC behavior over a range of length scales.

Effects of Antimicrobial Peptide Revealed by Simulations: Translocation, Pore Formation, Membrane Corrugation and Euler Buckling

We explore the effects of the peripheral and transmembrane antimicrobial peptides on the lipid bilayer membrane by using the coarse grained Dissipative Particle Dynamics simulations. We study peptide/lipid membrane complexes by considering peptides with various structure, hydrophobicity and peptide/lipid interaction strength. The role of lipid/water interaction is also discussed. We discuss a rich variety of membrane morphological changes induced by peptides, such as pore formation, membrane corrugation and Euler buckling.

Steric Entropy and Phase Equilibria in Microemulsions

A coarse-grained spin model is constructed and used to study global phase behavior of ensembles of fluid membranes. This model encompasses both bending rigidity renormalization and steric entropy arising from coarse-graining of short-scale membrane fluctuations. It thus enables us to obtain, in a unified way, phase diagrams containing both uniform and periodic phases in microemulsions and binary systems of nonionic surfactant bilayers in a single solvent.

Classical and statistical mechanics of celestial-scale spinning strings: Rotating space elevators

We introduce novel and unique class of dynamical systems, Rotating Space Elevators (RSE). The RSEs are multiply rotating systems of strings reaching into outer space. Objects sliding along RSE strings do not require internal engines or propulsion to be transported from the Earths surface into outer space. The RSEs exhibit interesting nonlinear dynamics and statistical physics phenomena.

Physics of Celestial Scale Dumbbells

The physics of manmade celestial scale objects, such as Space Elevators connecting the Earth with outer space, has recently attracted increased attention of diverse researchers. In this article we review basic physics of celestial scale dumbbells such as the Analemma Tower suspended from an asteroid orbiting the Earth (Clouds, 2017). Celestial dumbbells involve two large masses (top and bottom) connected by strings. The two masses move geosynchronously with the Earth, with the bottom mass remaining close to the Earth and the top mass moving above the Earth’s geosynchronous satellite orbit. Appealing examples of celestial scale dumbbells are untied Rotating Space Elevators (RSE) (Knudsen & Golubovic, 2015). Physics of untied rotating space elevators. European Physical Journal Plus 130 , 243.]. Celestial scale dumbbells exhibit rich and interesting nonlinear dynamics caused by instabilities of dumbbell geosynchronous motion discussed in this review article. We also point out that celestial scale dumbbells are physically feasible (in terms of nowadays available materials strengths) on dwarf planets in the main asteroid belt of the Solar system such as Ceres.

Nonequlibrium Statistical Mechanics of An Ensemble of Vesicles

We investigate far-from-equilibrium dynamics of a polydisperse ensemble of vesicles ( such as liposomes). For that purpose, we construct of a Smoluchowsky-type transport equation incorporating vesicle diffusion and the processes of vesicle fusions and fissions. This approach is used to study the time evolution of an initially monodisperse vesicle ensemble and its important quantities such as the internal aqueous, encapsulated volume. We find three stages of the evolution: (i) an early stage during which EV remains nearly constant, followed by (ii) a stage with a rapid decay of the EV, and, finally (iii) a late stage in which the ensemble approaches the thermodynamic equilibrium. In the far-fromequilibrium stages (i) and (ii), the vesicle ensemble is a “two-fluid” system composed of a polydisperse fluid of small, nearly equilibrated vesicles, coexisting with a fluid of nearly monodisperse vesicles which size evolves due to evaporation-recondensation of the small vesicles. Our picture agrees with experimental data on the EV of lecithin liposomes.

Mechanism of thermally assisted creep crack growth

The authors use atomistic Monte-Carlo simulations to investigate the dynamics of cracks which sizes are smaller than the Griffith length. They demonstrate that such cracks can irreversibly grow proviso their size is larger than a certain critical length which is smaller than the Griffith length, as recently suggested [L. Golubovic and A. Peredea, Phys. Rev. E51, 2799 (1995)]. They show here that this thermally assisted creep crack growth is dominated by irreversible changes in the region of the crack tip, primarily in the form of dislocation emissions and nucleation of microcavities and voids. These processes act together during the crack growth: the crack tip region acts as a source for emissions of dislocations which subsequently serve as seeds for creation of vacancy clusters in a region away but still close to the crack tip. Eventually, passages between these vacancy clusters and the mother crack are formed and the crack thus increases in size. As this process repeats, the crack grows.

LIQUID CRYSTAL EQUILIBRIUM FLUCTUATIONS UNDERSTOOD BY A RELATIONSHIP TO THE DYNAMICS OF GROWING INTERFACES

We review a recent theoretical progress in understanding equilibrium thermal fluctuations of smectic A liquid crystals. It is based on a relationship of smectics A to a rather different physical problem, namely, the Kardar-Parisi-Zhang (KPZ) dynamical model for interfaces growing in the presence of a flux of incoming particles. This relationship provides an exact approach to study Landau-Peierls phenomena in two-dimensional smectics and reveals the existence of novel elastic critical point in three-dimensional smectics A with broken inversion symmetry.

Spinodal Decomposition of the Interface in a Nonlinear Growth Model with Noise

We consider a non-linear version of Edwards-Wilkinson interface model for the growth in the presence of a collimated partjcie flux. We include both surface tension and surface diffusion relaxation. In a range of positive small values of the surface tension, the interface develops a state which fulfills standard criteria for spinodal decomposition. This suggests the existence of a phase transition from the ordinary state with the interface perpendicular to the incoming flux to a novel, noue.induced state with the interface apontaneously tilted with respect to the direction of the flux.

Entropic Elasticity of Lamellar Tethered Membrane Phases

Entropic elastic constants of lamellar tethered membrane phases are considered both in the vicinity of and at temperatures well below the membrane crumpling transition critical point. We calculate entropic forces acting on boundaries of these systems. Various predictions of the statistical physics of tethered membranes, such as the breakdown of the classical membrane elasticity theory at low temperatures or the existence of the crumpling transition, can be tested experimentally by using our results.