In investigating the growth kinetics of epitaxial films in situ by force microscopy, we have observed several instances where the lateral force contrast on the growing monolayer exhibits a strong dependence on the driving force for growth (i.e., solute concentration). We present results for three epitaxial growth systems in aqueous solutions: CaSO3 on CaCO3, PbSO4 on BaSO4, and BaSO3 on BaSO4. In each system, material grown at higher solute concentrations exhibits a friction higher than that of material grown at lower concentrations. These observations suggest a link between defect density and friction contrast in growing epitaxial films. An additional time-dependent behavior is observed in the CaSO3/CaCO3 system, indicating an annealing process.