AFM Pull-off Force Measurements with Polystyrene (Deformable) Colloidal Probes

Pull-off forces were measured for 8-9 pm diameter polystyrene (PSI particles on smooth spin- coated polystyrene films and hexadecanethiol (HDT) self-assemble monolayer surfaces using atomic force microscopy (AFM). Three parameters were controlled during experimentation: (i) the maximum applied load pressing the particle onto the substrate was varied from 50 to 4,000 nN, (ii) loading/unloading rate for the particle to attach and separate from the substrate varied from 0.01 to 9.3 Hz, and (iii) contact time for particle to reside on the substrate surface before retracting the particle to separate it from the surface changed from I µsecond to 200 seconds. the pull-off forces increased with increasing applied loading, increasing loading/unloading rate, and increasing contact time for the PS particle on PS film and on HDT monolayer. The PS particles experience elastic and plastic deformations duringpull-08 force measurements. Plastic deformation is primarily associated with flattening of surface irregularities. The results obtained in this research suggest that the particle-substrate contact areas of the magnitude comparable to those predicted by the JKR contact mechanical model can be reached in the AFM experiments by using high applied loads and long particle-substrate contact times. This result re-opens the prospect of' using AFM for studying surface energies of deformable micro-particles, even if the micro-particles have small irregularities on the surface.