www.acsprf.org

The proposed research seeks to understand at the nano-scale the mode of translation of one object directly abutting another object.  In particular, as the linear dimensions decrease, does a spherical object continue to translate by rotation, or does it transition to a preferred sliding mode.  Such a transition has been proposed though how the physical size, the work of adhesion, and the viscoelastic properties of the two objects affect the point at which it occurs has never been systematically explored.  To address these questions, we have undertaken a study investigating the relative motion of polystyrene microspheres across planar surfaces.

            In the proposed work, we sought to use the tip of an atomic force microscope (AFM) cantilever to apply a shearing force to polymeric spheres, recording this force while the spheres translate laterally across planar surfaces.  The spheres were commercially available polystyrene microspheres, and the underlying substrate was polished silicon wafer.  Both of these materials were chosen because of the relative ease with which their surface chemistries may be modified, thereby changing the work of adhesion in a controllable fashion.  Moreover, the microspheres' elastic moduli can be modified by treatment with various solvents, allowing us to investigate the role of this property as well.  For added control, the experiments required that they be performed while submerged in fluid environments.

            In an exploratory series of experiments, we found difficulty determining whether the spheres were rolling or sliding.  The original plan called for applying a large force to the spheres with the AFM tip, creating a plastic deformation – a permanent indentation.  Then, if the sphere were rolling while the lateral force induced translation across the surface, the asymmetry created by the indentation would be detectable in Fourier transforms of the recorded force trace.  No indications of rolling were observed in the traces; however, doubt remained as to whether the sphere were sliding or whether the indentation rotated to the side where it would not interact with the surface even if rolling were occurring.  Thus, a new approach had to be developed.

We have since been working to develop a method in which microspheres sparsely labeled with fluorescent markers are translated by the shear flow of fluids in microfluidic channels.  When imaged from below while exciting in total internal reflection, the intensity of the fluorescent emission will fluctuate if the particles roll – the surface bound dyes will be excited at varying distances from the substrate surface, producing fluctuating intensities in emission intensity.

To produce the sparsely labeled spheres, a new technique had to be developed.  We used relatively large amino-functionalized microspheres which had been incubated with smaller carboxy- functionalized beads, where the smaller beads were a mixture of one part fluorescently labeled to 10 parts unlabeled spheres.  This resulted in microspheres that had other adherent microspheres coating the surface, with fluorescently labeled microspheres separated by the unlabeled microspheres.  The increased surface roughness and size might yet present some difficulties.

We then needed to develop a microfluidic platform in which to perform the translation experiments.  We used a photoresist (Shipley SU-8) and photolithography to produce a master structure from which microfluidic channels were cast in polydimethylsiloxane (PDMS, Dow Corning).  The PDMS replica that had been cast from the master was then exposed to oxygen plasma and sealed with a glass coverslip.  Connections to the fluid reservoirs at the inlet and outlet ends of the microfluidic system were made with 20-g stainless steel needle stock tubing.  Polyethylene tubing was then affixed to these inlets and outlets, and the fluid was driven using a peristaltic pump..

With the technique for producing the labeled microspheres and the microfluidic channels in place, we are now working to build the TIRF system.  After the completion of this final technical component, we will be able to proceed with the proposed work.