title: Dynamics of a disturbed sessile drop measured by atomic force microscopy (AFM).
abstract: A new method for studying the dynamics of a sessile drop by atomic force microscopy (AFM) is demonstrated. A hydrophobic microsphere (radius, r ∼ 20-30 μm) is brought into contact with a small sessile water drop resting on a polytetrafluoroethylene (PTFE) surface. When the microsphere touches the liquid surface, the meniscus rises onto it because of capillary forces. Although the microsphere volume is 6 orders of magnitude smaller than the drop, it excites the normal resonance modes of the liquid interface. The sphere is pinned at the interface, whose small (<100 nm) oscillations are readily measured with AFM. Resonance oscillation frequencies were measured for drop volumes between 5 and 200 μL. The results for the two lowest normal modes are quantitatively consistent with continuum calculations for the natural frequency of hemispherical drops with no adjustable parameters. The method may enable sensitive measurements of volume, surface tension, and viscosity of small drops.
authors.affiliation: Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218, USA. patricia.mcguiggan@jhu.edu
authors.forename: Patricia M
authors.initials: PM
authors.lastname: McGuiggan
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Grave
authors.forename: Jay S
authors.initials: JS
authors.lastname: Wallace
authors.forename: Shengfeng
authors.initials: S
authors.lastname: Cheng
authors.forename: Andrea
authors.initials: A
authors.lastname: Prosperetti
authors.forename: Mark O
authors.initials: MO
authors.lastname: Robbins
mesh_terms: D057927:Hydrophobic and Hydrophilic Interactions; D018625:Microscopy, Atomic Force / Q000379:methods*; D010316:Particle Size; D011138:Polytetrafluoroethylene / Q000737:chemistry*; D013499:Surface Properties; D013816:Thermodynamics
chemical_list: D011138:Polytetrafluoroethylene