Measuring Surface Forces in Salty Solutions

We measure the force acting between micron-scale polystyrene beads and charged surfaces in an ionic fluid. Per the size scale, the fluid may be treated as a continuum, though the beads are small enough to be subject to thermal forces. The electric potential from the surface can be modeled with the Poisson-Boltzmann equation, and the fluid dynamics can be computed by numerically solving the Stokes equations. We use total internal fluorescence microscopy to image the beads, and we form supported lipid bilayers on the surface to modulate the surface charge. Developing this method will enable us to later measure the forces acting on bacteria by charged surfaces.