Modulated Optical Nanoprobes (MOONs) are micron sized particles, with one fluorescent hemisphere and one dark hemisphere, formed by coating one side of a monolayer of fluorescent colloids with metal. As a result, when imaged using fluorescence microscopy, variation is observed of both the intensity and the shape of the fluorescent images. Exploiting this property, we have developed tracking routines that allow us to monitor simultaneously two translational and two rotational degrees of freedom. As can be seen below, the angular orientation of MOON particles can readily be determined with high accuracy.

These newly developed capabilities make possible several interesting avenues of research. One direction I am currently pursuing in conjunction with Liang Hong, is using MOONs to explore the interactions inside of concentrated colloidal suspensions. By virtue of the ability to observe the rotational and translational motion simultaneously, the possibility of translational rotational coupling in such systems can be examined.

The no-slip boundary condition, or the assumption that the velocity of a fluid in contact with a wall is zero, is almost universally assumed. While generally a reasonable assumption for typical devices, as devices become ever smaller, the possibility of slip becomes ever more relevant. Experiments have suggested a wide variety of slip lengths for hydrophilic surfaces. However, to date, these measurements have been handicapped by either limited resolution or their indirect nature. In conjunction with Dr. Sung Chul Bae, by using a combination of resonance energy transfer and fluorescence microscopy, I hope to measure fluid flow or the absence thereof within nanometers of the surface, providing clear information regarding the nature of the appropriate boundary condition.