Measuring short-range repulsive forces by imaging directed magnetic-particle assembly title

We present the use of video microscopy of magnetic colloidal particles to measure short range repulsive forces between surfaces. The forces between the particles can be directly measured by monitoring the inter-centroid distances between the particles at various magnetic field strengths. Using a chain consisting of a minimum of 35 micron-sized particles, a distance resolution of similar to 0.21 nm is achieved. Measurements of the interactions between charged colloids agree well with predicted force interactions calculated using DLVO theory. We also report the first measurements of the force versus distance relationship between very short (<75 bases) oligonucleotide-coated surfaces. The interactions between oligonucleotide- coated surfaces agree well with Milner's polymer brush theory for strands greater than 50 bases but deviate for strands less than 35 bases.