collision and adhesion of these like-charged particles. According to this theory, the forces governing the approach of two colloidal particles (or surfaces) are described in terms of Lifshitz-van der Waals (LW) attractive forces and electrostatic (EL) repulsive forces between like-charged particles.1 Particle charge is typically measured in terms of zeta potential based on the velocity of a particle suspended in water between two plates set at a fixed potential. Because of the high salinity of seawater, we expect that the repulsive layer thickness around a particle will be small (less than 1 nm) and, therefore, that electrostatic forces play a small role in particle adhesion (except where salinity undergoes changes from fresh to saline conditions). The reduction in electrostatic repulsion between particles at seawater salinities means that particles could come close to each other without being repelled. Two particles that get close to each other would, therefore, be attracted to each other due to LW forces, but this is clearly not the only force active as not all particles are "sticky" in seawater. Biotic particles, such as bacteria and phytoplankton that form aggregates as large as marine snow in the ocean, can even change their relative stickiness as a function of growth cycle or, for example, the stage of a phytoplankton bloom.2 Thus, it is clear that other factors must be relevant to colloidal and particle coagulation processes in marine systems.

Extended DLVO (XDLVO) theory has been used to describe the interactions of molecules at the surfaces of two particles. XDLVO theory adds additional short-range force due to acid-based (AB) interactions due to electron donor and electron acceptor interactions that affect hydrogen bonding between two surfaces immersed in a polar solvent such as water.3 The AB force can be calculated based on contact angles measured on layers of particles, typically using polar solutions such as glycerol and water, and a nonpolar solution such as diiodomethane.4 The inclusion of this new force has increased our understanding of interactions of particles, but additional factors have yet to be included in these models. These factors include the effect of steric interactions on particle approach and adhesion, and the impact of a small number of molecules at the surface of a particle that may not be detectable using bulk property measurements. For example, Feick and Velegol5 have shown, using a novel rotational electrophoresis technique, that charge density is not constant on some colloidal particles.

Researchers have turned to using techniques that directly measure interaction forces such as the surface force apparatus (SFA) and atomic force microscopy (AFM).6 SFA is more useful for examining properties of different materials as a material or chemical must be placed onto atomically flat surfaces such as mica during measurement. AFM is better suited for examining particle-particle and particle-surface interactions. The AFM tip can be used to probe the topography of a surface, either through direct contact of the tip with the surface (contact mode) or by intermittent tapping of the tip on the surface (tapping mode). Forces can be measured by holding the tip fixed at a specific location in an x-y plane, and then lowering the tip toward the surface (or retracting the tip), measuring the bending of the tip in terms of tip attraction (bending down) or repulsion (bending up). The interaction of the tip with an inorganic surface can be understood as a function of pH and ionic strength using DLVO theory if the tip is assumed to be a sphere of a much larger radius (100s of nm) than the true curvature of the tip at the end (10s of nm).7 Some researchers who have examined bacterial adhesion have bound bacteria onto a tip in order to probe cell interactions with a specific surface.8,9 The disadvantage of this technique is that the geometry of two surfaces (bacteria spread nonuniformly on a pyramid-shaped tip) during the measured interaction is not well defined. Another approach to probe a surface is to bind a colloid to the tip to better define the geometry of the system.10 This latter configuration allows surfaces or colloids to be coated with chemicals, greatly extending the range of interactions that can be examined using the AFM approach.

While the operation of an AFM is quite simple, it can be difficult to interpret the data and extract information on interaction forces. It is not the intent here to provide extensive background on the operation of the AFM or to review the AFM literature, as there are many excellent reviews available.6,11,12 The purpose of this chapter is to briefly review a few findings by my own research group in the area of using AFM to understand bacterial adhesion, and to point out the limitations and future challenges of this new and powerful approach.

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