The behavior of supermolecular structures and assemblies under spatially confined conditions is a central
problem in the physics and chemistry of "soft matter." As the degree of confinement approaches a relevant
supramolecular length scale, significant deviations from bulk behavior are generally observed. These
deviations often play a central role in both technological and biological processes, in applications as
diverse as lubrication, liquid spreading, filtration, mesoscopic materials synthesis, and biological
Interfacial Surfactant Aggregation
An interesting area of supermolecular confinement is the aggregation of surfactant molecules at the
solid/liquid interface. The adsorption of surfactants is essential in many commercially important
processes such as detergency, lubrication, water purification and ore flotation. Despite the importance
of surfactant aggregation at the solid/liquid interface, very little was known about the structure of
these aggregates until recently. Interfacial surfactant structure had been indirectly studied by a
variety of techniques such as calorimetry, solution depletion methods, neutron and x-ray scattering,
ellipsometry and fluorescence probes. AFM, however, has the unique ability to directly visualize the
structure of surfactant aggregates at the solid/liquid interface. AFM is able to directly visualize the
structure of surfactant aggregates at the solid/liquid interface by mapping the electric double layer or
stearic repulsion forces between the adsorbed micelles and the AFM probe.
The Atomic Force Microscope (AFM) can be modified and extended to study a wide variety of phenomenon at
nanometer length scales and pico-newton force scales. The following sections are some modification we've
made to our microscopes.
Self assembly: experimental models
A central preoccupation of condensed matter physics is the emergence of material properties,
such as crystal structure, lattice dynamics, and phase transitions, from the local interactions
between constituent atoms. In real materials, however, this connection is incomplete because exact interatomic
potentials generally cannot be derived ab initio, or readily be measured in isolation, or freely be
varied to investigate their effects on material properties. The advent of colloidal crystals,
prepared from particles with simple and tunable interactions, has helped bridge this gap.
However, colloidal crystals have some limitations as model systems for real materials and as
reliable test-beds for statistical theories. Experiments on charge-stabilized colloids have revealed
unexpected long-range attractions between like-charged particles and the effective pair
potential for these systems is now a matter of some controversy. Using dipole-dipole
interactions between paramagnetic colloidal particles circumvents this problem, although
hydrodynamic coupling and electrostatics can still play a role. Colloidal systems require extensive
sample preparation, and long equilibration times (several days) to form the delicately
ordered lattices. Kinetic traps and prohibitively slow approach to equilibrium may pose special
problems for binary colloid mixtures (the mesoscopic analogue of alloys or compounds), for
which few results have been reported to date.
A new and conceptually simple experimental model for condensed phases has been developed, which
combines accurately known and tunable pair potentials, equilibration times of a few seconds, an
independently adjustable “temperature,” and collective behavior visible to the naked eye. We
use this system to quantify lattice and interfacial structures, phonon velocities, binary phases and
phase transitions; these results are quantitatively related to the known pair potential, confirming
the applicability of statistical theories to this ensemble.
Self assembly: theoretical models
Micelles can be modeled...
Non-Covalent microcontact printing
Microcontact printing is a technique developed by Prof. George
Whitesides' group at Harvard. It is conceptually very simple: An uncured silicone rubber is cast
onto a "master" surface that has the desired surface features. The silicone is cured and removed from
the master surface. The now embossed silicone "stamp" is "inked" with molecules and stamped down on a
surface just as you would use an office rubber stamp. This technique is able to reproduce patterns on
length scales below 100 nm. This technique is typically used to stamp molecules which covalently bond to
a surface, such as thiols on gold. We are interested in "softer" materials which form non-covalent bonds
with a surface, such as lipids and proteins. More information about microcontact printing
Past research has included crystal growth and dissolution, electrochemistry, molecular crystals.