Shear-Induced Detachment of Polystyrene Beads from SAM-Coated Surfaces.

In this work we experimentally and theoretically analyze the detachment of microscopic polystyrene beads from different self-assembled monolayer (SAM) surfaces in a shear flow in order to develop a mechanistic model for the removal of cells from surfaces. The detachment of the beads from the surface is treated as a thermally activated process applying an Arrhenius Ansatz to determine the activation barrier and attempt frequency of the rate determing step in bead removal. The statistical analysis of the experimental shear detachment data obtained in phosphate-buffered saline buffer results in an activation energy around 20 kJ/mol, which is orders of magnitude lower than the adhesion energy measured by atomic force microscopy (AFM). The same order of magnitude for the adhesion energy measured by AFM is derived from ab initio calculations of the van der Waals interaction energy between the polystyrene beads and the SAM-covered gold surface. We conclude that the rate determing step for detachment of the beads is the initiation of rolling on the surface (overcoming static friction) and not physical detachment, i.e., lifting the particle off the surface.

Digital in-line holographic microscopy.

We first briefly review the state of the art of digital in-line holographic microscopy (DIHM) with numerical reconstruction and then discuss some technical issues, such as lateral and depth resolution, depth of field, twin image, four-dimensional tracking, and reconstruction algorithm. We then present a host of examples from microfluidics and biology of tracking the motion of spheres, algae, and bacteria. Finally, we introduce an underwater version of DIHM that is suitable for in situ studies in an ocean environment that show the motion of various plankton species.

Hydroxide ion adsorption on self-assembled monolayers.

It is argued, on the basis of density functional calculations, that a self-assembled monolayer of oligo(ethylene glycol) or n-alkanes in contact with water will preferentially adsorb hydroxyl ions (either from autoionization of water or added to the solution) on both methoxy- and hydroxide-terminated endgroups, thus charging the surface region of the SAM negatively with an estimated charge density of about 1 microC/cm(2) in agreement with recent experiments. The negative charging can explain long-ranged forces between opposing SAM surfaces. On dense SAMs, hydroxyl ions are highly mobile. Hydronium ions can absorb by penetration into the SAM provided there is enough lateral space for their encapsulation. The important role of hydration is demonstrated by calculating the excess binding energy of adsorption using a Born-Haber cycle.