A DISSIPATIVE PARTICLE DYNAMICS MODEL OF LIQUID-BRIDGE FORMED BETWEEN ATOMIC FORCE MICROSCOPY TIP AND FLAT SUBSTRATE

Atomic force microscopy (AFM) is a scanning probe microscopy technique extensively used in many applications. The typical radius of AFM tips varies from 5-100 nm while the gap between the tip and the substrate is similar to 0-50 nm. At these length scales, experimental investigation of the liquid bridge shape and accurate quantitative measurement of the capillary force and contact angles of the liquid bridge at the tip and the substrate interface is difficult even with state-of-the-art technology. Several theoretical investigations of capillary forces were attempted on AFM by applying Kelvin-Laplace and Young-Laplace equations but the critical issue of understanding the interaction between a nanoscale AFM tip and the substrate is not well understood. For this reason, a mesoscale model of AFM tip and fluid interaction is conducted in this work using multi-body dissipative particle dynamics (MDPD). First, we qualitatively and quantitatively validate our model by comparing our results with those of well-established behaviors of the liquid-bridge reported in literature. Then, the ability of MDPD to capture the meniscus shape and behavior versus the contact surface wettability distribution is examined. Our model is then used to design control strategies for wettability-based liquid-bridge manipulation to reduce the effect of capillary condensation on AFM imaging and also for liquid-bridge mediated nanoscale lithography techniques. Our model can be used as a powerful design tool for meniscus manipulation technology, such as dip-pen nanolithography, as well as for studying dynamic, e.g., tapping mode AFM tip interactions with liquid bridge.