Surface is of my particular interest because it is the playground of so many unique forces. One of the traditional surface techniques used in our group is SFA (Surface Forces Apparatus), which basically pushes two ideal mica surfaces together and squeezes the material into a confined geometry within nanometers. Then an oscillatory force will be applied and the properties of the surface can be estimated by measuring the responding forces.

However, there are several drawbacks of this method. Measuring forces is like you can only use your hand to “feel” the surface, but you cannot see. The state of art of latest laser technology enables us to really “see” those molecules close to surfaces.

Here we are trying to combine the SFA with Raman spectroscopy. The setup is originally designed by Dr. Sung Chul Bae. With this setup we can get valuable information about the orientation and conformation of molecules under confinement. The preliminary results have been published in Langmuir. Now we are studying the details of chain molecules, such as PDMS, under confinement. And in the same time we are trying to improve this method to small molecules system by taking the advantage of SERS (Surface Enhanced Raman Spectroscopy) effect.

Another method I am studying to probe the surface is SFG (sum frequency spectroscopy). This method by its nature is a surface technique. By taking the advantage of both non-linear optics and the femtosecond laser, we can study the fast dynamics of the material near surfaces.

A simple, generalizable, inexpensive method is developed by our group to synthesize “Janus” colloidal particles in large quantity.

A traditional goal in colloid and nanoparticle science was to obtain particles that are homogenous in their chemical composition, and applications of this kind continue to have value in applications such as painting, ceramics, and photonic materials. Of emerging interest is the alternative idea of obtaining particles whose surface chemical composition is differs on two sides of the particle ("Janus" as suggested by de Gennes). Anisotropic surface chemical composition, superposed on a homogeneous shape, could introduce asymmetric interactions that induce particles to self-assemble. Janus-type spherical particles can be used as microrheological probes. Using magnetic fields to exert force on one side of the particle, Janus-type spherical particles can be used to manipulate particles with controlled magnetic fields. Janus particles can also be used as potential building blocks for new three-dimensional self-assembled structures, but this purpose requires a method to produce large quantities of material.

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