(245e) Interfacial Nanomechanics In Atomic Force Microscopy of Complex Polymer-Surfactant Thin Films

A complex polymer-amphiphile lubricant film deposited from an aqueous phase is common to many personal care products. Edisonian engineering has produced multi-ingredient formulations to deliver optimum tribological performance. Much of this product development has proceeded blind to the nano- to micro-scale characteristics of film morphology. Even murkier have been tribological behaviors at nanometer-regime length scales. Long-sought understandings have included basic phenomenology such as (a) wear/healing processes and (b) the “fly height” of a countersurface sliding over film domains that may be liquid-like, considering the known bulk characteristics of the polymer, surfactant, fatty alcohol and water ingredients.

Atomic force microscopy (AFM), while long used for simple topographic imaging of solid surfaces in many industrial R&D labs, has the simultaneous ability to interrogate tribological response functions of organic thin films. Alternatively the microcantilever to which the AFM imaging tip is attached can be vertically vibrated in such a way that weak van der Waals attractive forces enable non-contact imaging of liquidy film domains or droplets. Here liquid versus solid behaviors are assessed domain by domain across the film in non-contact and intermittent-contact imaging modes, the latter in a quasistatic variant called digital pulsed force mode, D-PFM (also known as peak-force tapping). Moreover frictional response is probed while sliding the same nanoasperity continuously across/through the film domains. Vertical displacement of the asperity is measured during this dynamic shear process to assess fly height. Subsequent imaging of the same domains in non-contact assesses changes produced by the preceding tribological experiment. Force-distance relationships during the pulling of tip away from film provide further glimpses into solid-like versus liquid-like behaviors (“wicking”). In D-PFM these behaviors are mapped at high point density to provide a rich data cube.

In this work we present results of all of the above measurement/imaging modes on complex thin films deposited from an aqueous gel phase of polydimethylsiloxane, n-alkyl ammonium chloride surfactant and fatty alcohol onto mica substrates. Velocity-dependent sliding further characterizes the nanorheological performance of different phase segregated film domains.