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Papers by Prem Basnet

ii Dissertation written by

Bulletin of the American Physical Society, 2009

ABSTRACT

Molecularly thin films have been a subject of great interest for the last several years because o... more Molecularly thin films have been a subject of great interest for the last several years because of their large variety of industrial applications ranging from micro-electronics to bio-medicine. Additionally, molecularly thin films can be used as good models for biomembrane and other systems where surfaces are critical. Many different kinds of molecules can make stable films. My research has considered three such molecules: a polymerizable phospholipid, a bent-core molecules, and a polymer. One common theme of these three molecules is chirality. The phospolipid molecules studied here are strongly chiral, which can be due to intrinsically chiral centers on the molecules and also due to chiral conformations. We find that these molecules give rise to chiral patterns. Bent-core molecules are not intrinsically chiral, but individual molecules and groups of molecules can show chiral structures, which can be changed by surface interactions. One major, unconfirmed hypothesis for the polymer conformation at surface is that it forms helices, which would be chiral. Most experiments were carried out at the air/water interface, in what are called Langmuir films. Our major tools for studying these films are Brewster Angle Microscopy (BAM) coupled with the thermodynamic information that can be deduced from surface pressure isotherms. Phospholipids are one of the important constituents of liposomes -- a spherical vesicle com-posed of a bilayer membrane, typically composed of a phospholipid and cholesterol bilayer. The application of liposomes in drug delivery is well-known. Crumpling of vesicles of polymerizable phospholipids has been observed. With BAM, on Langmuir films of such phospholipids, we see novel spiral/target patterns during compression. We have found that both the patterns and the critical pressure at which they formed depend on temperature (below the transition to a i¬‘uid layer). Bent-core liquid crystals, sometimes knows as banana liquid crystals, have drawn increasing attention because of the richness in phases that they exhibit. Due to the unique coupling between dipole properties and the packing constraints placed by the bent shape, these molecules are emerging as strong candidates in electromechanical devices. However, most applications require that the molecules be aligned, which has proved difficult. Our group has tested such molecules both as Langmuir layers and, when transferred to a solid, as alignment layers with some limited success. However, these molecules do not behave well with the surfaces and the domains at the air/water interface tend to form ill-controlled multilayer structures since attraction with the surfaces is relatively weak. New bent-core molecules obtained from Prof. Dr. C. Tsehiemke from Department of Chemistry Institute of Organic Chemistry, Martin-Luther-University, Germany, have a hydrophilic group at one end. We expect this molecule to behave better on the surface because of the stronger attraction of the hydrophilic group towards the surface than for the bent-core molecules without the hydrophilic group. Polydimethylsiloxane (PDMS) is a polymer which finds many applications in modifying surface properties. It is used in manufacturing lubricants, protective coatings, hair conditioner and glass-coating. However its properties are not well understood. This polymer has been proposed to follow either helical or caterpillar conformations on a surface. The orientational order of CH3 side groups can test for these conformations (they would be predominantly up/down for the caterpillar conformation, but rotating through the entire 360 degree for the helical one). Thus previous work on the Langmuir polymer films at the air/water interface were complemented by deuterium NMR studies to probe their conformations at a surface. These experiments were performed using humid porous solids, in order to provide sufficient surface area for the technique. Previous tests in this group at room temperature were suggestive but inconclusive because of the rapid averaging motion of the molecules. Here, we attempt to freeze the molecules on the surface.

Ferroelectrics, 2012

This article may be used for research, teaching, and private study purposes. Any substantial or s... more This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

At the air-water interface, 4-8-alkyl1,1-biphenyl-4-carbonitrile 8CB domains with different... more At the air-water interface, 4-8-alkyl1,1-biphenyl-4-carbonitrile 8CB domains with different thicknesses
coexist in the same Langmuir film, as multiple bilayers on a monolayer. The edge dislocation at the
domain boundary leads to line tension, which determines the domain shape and dynamics. By observing the
domain relaxation process starting from small distortions, we find that the line tension is linearly dependent
on the thickness difference L between the coexisting phases in the film, =3.3±0.2 mN/m L. Comparisons
with theoretical treatments in the literature suggest that the edge dislocation at the boundary locates near
the center of the film, which means that the 8CB multilayers are almost symmetric with respect to the air-water
interface.

When compressed in the intermediate temperature range below the chain-melting transition yet in t... more When compressed in the intermediate temperature range below the chain-melting transition yet in the
low-pressure liquid phase, Langmuir monolayers made of chiral lipid molecules form hierarchical
structures. Using Brewster angle microscopy to reveal this structure, we found that as the liquid
monolayer is compressed, an optically anisotropic condensed phase nucleates in the form of long, thin
claws. These claws pack closely to form stripes. This appears to be a new mechanism for forming stripes
in Langmuir monolayers. In the lower temperature range, these stripes arrange into spirals within
overall circular domains, while near the chain-melting transition, the stripes arrange into target patterns.
We attributed this transition to a change in boundary conditions at the core of the largest-scale circular
domains.

Langmuir films of 1,2-bis(10,12 Tricosadiynoyl)-sn-Glycero-3-Phosphoethanolamine form spiral and ... more Langmuir films of 1,2-bis(10,12 Tricosadiynoyl)-sn-Glycero-3-Phosphoethanolamine form spiral and target patterns when compressed between two movable barriers in a Langmuir trough above 30^0C, up to the chain-melting transition at ˜37^0C. The critical pressure, at which spirals appear, increases with temperature. The patterns themselves also depend on temperature, with single-armed spirals with many defects forming near 30^0C and defect-free target patterns at higher temperatures. The mechanism of spiral formation could be a competition among elasticity, chirality, and the boundary conditions at the core of the domains. Optical anisotropy and the growth rate of internal structures test this suggested mechanism. .

The difficulty of aligning bent-core liquid crystals at a surface is addressed from three directi... more The difficulty of aligning bent-core liquid crystals at a surface is addressed from three directions: We form Langmuir monolayers of bent core molecules at the air-water interface, and explore their orientation and packing. We transfer these films by Langmuir-Schaefer techniques to a solid surface, and test them for the alignment of bulk liquid crystal. We use atomistic molecular dynamics simulations to directly probe possible molecular orientation at the water surface, for comparison with experiments. We find that relatively small changes in the bent-core molecule affect both the stability of the films and their ability to promote alignment within liquid crystal cells.

The hydrodynamic response of a thin fluid film, whether a Langmuir monolayer at the air/water int... more The hydrodynamic response of a thin fluid film, whether a Langmuir monolayer at the air/water interface or a cell membrane, is difficult to model, since it involves the coupling of both bulk and surfaces phases. However, such hydrodynamic response is not only intrinsically critical for transport within the layer, it also provides the major available means to evaluate an important parameter for phase-separated layers, the line tension. We have developed a line-integral formulation of the hydrodynamic response of phase-separated layers with short-ranged forces, and tested it by comparisons between numerical simulations based on this model and experiment. These experiments both validate the model and demonstrate that the line tension can be determined with unprecedented accuracy and precision. Two systems have been studied to date: a simple smectic liquid crystal multilayer and coexistence between phases in binary lipid/cholesterol mixed layers. For the latter case, long-range dipole-dipole interactions are introduced into the model.

ii Dissertation written by

Bulletin of the American Physical Society, 2009

ABSTRACT

Ferroelectrics, 2012