We have designed and validated a platform capable of assessing minuscule changes of the viscous drag force that different fluid environments exert on individual, micrometer-sized particles. Based on a combination of optical tweezers and a microfluidic device, this platform allows us to expose a laser-trapped particle to a series of microenvironments, created through microfluidic laminar flow, and to monitor biochemical interactions on the surface of the particle. In addition, we can determine the viscosity of the different environments with excellent resolution. We have used this setup to examine the interaction between human IgG and both protein-A as well as protein-G. At saturation, bound IgG added an apparent thickness of 12.6±.77(SD)nm and 16.0±1.4(SD) nm to protein-A- and protein-G-coated beads, respectively. This is in agreement with the expected size of human IgG of ∼10-16 nm dependent on orientation. Moreover, we measured equilibrium constants of KA=4.21x107 M−1 for the protein-A:IgG interaction and KA=7.94x107 M−1 for the protein-G:IgG interaction, in agreement with literature values. Finally, we measured the viscosity of different solutions of bovine serum albumin, human serum, and glycerol with µPa·s resolution. Measurements were performed in a microfluidic flow chamber and in microwells on a glass slide. The microwells allowed us to carry out comparative viscosity measurements with very small volumes of solution, less than 50 µL. The results obtained in each case agree with published models. This platform opens up a new playing field for the study of biochemical surface interactions. Its potential applications include the investigation of biomolecular interactions at the surface of live biological particles, such as individual bacteria, fungal cells, or large viruses, and the determination of the viscosity of costly solutions or small samples.