Dan Schwartz Research Interests

Overview
Our research interests are in the general areas of interfacial phenomena and complex fluids; in particular structure, phase behavior, and dynamics in thin organic film systems.  Our group has used various microscopy techniques, atomic force microscopy (AFM), Brewster angle microscopy (BAM), and polarized fluorescence microscopy (PFM) as probes, along with several supporting techniques such as contact angle goniometry and FTIR spectroscopy.

Self-assembled monolayer growth
We have used AFM as well as spectroscopic and wetting techniques to study the growth process of self-assembled monolayers (SAMs), molecular monolayers adsorbed from solution.  We have demonstrated that many of these systems form via an "islanding" mechanism, in which 2D aggregates of adsorbate molecules nucleate, grow, coalesce, etc. on the substrate.  In the case where adsorbate molecules bind irreversibly to each other, such as octadecyl trichlorosilane (OTS) monolayers, we found that the submonolayer islands had a fractal morphology with a scaling exponent consistent with 2D diffusion-limited aggregation.  This suggested the importance of surface diffusion as a growth mechanism.  More recently, we achieved the first in situ AFM images of SAM growth, observing submonolayer island nucleation and growth of octadecyl phosphonic acid (OPA) on mica.  The OPA islands were compact in shape, in contrast with the fractal OTS islands, consistent with the "softer" intermolecular interactions in OPA which allow rearrangement and annealing of island shapes.  Similar processes are being studied with a variety of adsorbate molecules and substrates.

Quantitative analysis of submonolayer island nucleation and growth kinetics during growth allows us to determine the various physical parameters of the growth process, such as the adsorption rate (sticking coefficient) from solution, the surface diffusion constant, etc.  We have found that the surface morphology during growth, as characterized by the island size distribution, is consistent with that expected from theoretical analysis of vapor phase molecular beam epitaxy growth in ultra high vacuum, for appropriate values of deposition rate and surface diffusion rate.  This establishes the quantitative connection between the growth processes or molecular monolayers from solution and metallic or semiconductor thin films from the vapor phase.

AFM images (300 nm x 700 nm) showing the nucleation and growth of submonolayer islands during the first 25 minutes of monolayer growth of OPA on mica.  The bright areas are islands about 2 nm high.
You can also watch a QuickTime movie showing this growth process.

Phase behavior and rheology of Langmuir monolayers
We have used x-ray scattering and specialized optical microscopy techniques, fluorescence and Brewster angle microscopy (BAM), to study Langmuir monolayers (insoluble surfactant monolayers at the air/water interface).  These experiments have contributed to the understanding of the complicated two-dimensional (2D) phase diagram in model monolayer systems of rod shaped (aliphatic) molecules, such as fatty acids.  We were the first to demonstrate that polarized fluorescence microscopy can be used to visualize phase transitions between crystalline and liquid-crystalline monolayer phases.  More recently, we have used BAM to explore the phase behavior of disk-shaped (discotic) molecular monolayers.  In addition, we have used microscopy and novel interfacial rheometric techniques to explore the effect of flow on Langmuir monolayers.  We explicitly demonstrated the effect of coupling to the viscous monolayer subphase on the interfacial flow.  In addition, we have discovered unusual non-Newtonian viscous behavior in liquid-crystalline (hexatic) monolayer mesophases.
BAM images of an eicosanoic acid monolayer flowing downwards through a channel.  Elongated tilt domains are visible as different gray levels.  The edges of distinctively shaped domains are followed frame by frame to determine the velocity profile across the channel.  Four distinctive features marked A, B, C, and D are marked in this sequence which spans about 2 seconds.  These are representative images; for purposes of data analysis, 30 frames per second are available.
Performing flow experiments in two-dimensions (2D) has several unique advantages:  the theoretical description of the flow is simplified, the entire flow field is directly observable, and we can manipulate the thermodynamic variables to control the orientation and conformation of the surfactant molecules to a greater degree than is possible in 3D fluid phases.  Our recent results demonstrate unusual non-Newtonian behavior in the liquid crystalline (LC) mesophases of fatty acid monolayers.  In contrast to 3D systems, where LC behavior is limited to a small class of specialized molecules, the simplest prototypical monolayer systems display a variety of LC phases.  Understanding the rheology of these phases, therefore, will be directly relevant to processes involving surfactant monolayer flow such as foam stability and emulsion coalescence.

We also study 2D colloidal suspensions.  As a model of multi-phase flow we have observed the channel flow and simple shear flow of a monolayer of a chromophoric fatty acid under conditions where needle-shaped islands of the two-dimensional crystalline phase coexist with a two-dimensional liquid phase.  This serves as an interesting system to study the behavior of rigid rods (a common models for rigid polymers) in shear flow.  Under constant shear conditions, the rods are observed to rotate in the classical Jeffery orbit.  In channel flow, since the overall surface viscosity is very low in the 2D liquid phase, a semi-elliptical velocity profile is observed.  The shear causes the rods to rotate (the classical Jeffery orbit), clockwise in the left half of the channel and counterclockwise in the right .  However, the angular velocity slows dramatically when the rods approach a vertical orientation resulting in an averaged alignment in the shear direction even for dilute rod concentrations.  This is a two-dimensional version of what has been termed a "paranematic" phase in dilute suspensions of rod-like particles.

Thermodynamics of Langmuir-Blodgett films:  two-dimensional phase transitions
We have used AFM to study structure and phase transitions in Langmuir-Blodgett (LB) films, surfactant films transferred layer by layer from the water surface to a solid substrate.  We showed that AFM could be used as a quantitative tool to study molecular packing and systematically determined the structure of fatty acid salt LB films as a function of incorporated cation and film thickness.  We found that the molecular arrangement was typically due to a competition between an average molecular area constraint imposed by the molecular headgroup and the particular packing preferences of the alkyl tails.  We discovered a variety of modulated crystalline phases defined by periodic arrays of line defects.  In many systems, the first monolayer was loosely packed and relatively disordered and the bulk film structure was reached upon the deposition of an additional bilayer.  In other systems, however, unusual structural evolution was observed including a compromise between the previously known types of epitaxial growth - strained layer epitaxy and van der Waals epitaxy ? as well as a system that evolved from liquid to liquid-crystal to crystal as layers were added.

We have also studied phase transitions and pattern formation caused by the LB deposition process itself.  Under the right conditions, monolayers undergo a 2D condensation transition during LB transfer.  We have observed small round molecular islands or dendritic islands depending on thermodynamic conditions.  In addition, we found that phase separation during LB transfer of two-component films can be exploited to form molecular stripes.

Most recently, we have developed the ability to obtain molecular resolution AFM images at high temperatures for the first time.  We found that 2D melting of an LB film was a two step process - the low temperature crystalline phase melted to a 2D smectic phase at 91 °C (with one-dimensional periodicity) before finally melting to a hexatic phase at 95 °C.  This phase sequence was in agreement with the predictions of dislocation-mediated melting theory for an anisotropic 2D crystal, verifying this important theory.

AFM images, 15 nm x 15 nm of cadmium arachidate LB multilayers at 89 °C (left) and 92 °C (right).  The image at 89 °C shows features consistent with individual molecules, i.e. the observed lattice is identical to the known molecular arrangement.  At 92 °C, however, the 2D lattice is no longer visible, only lines of molecules are seen, consistent with a 2D smectic phase.

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