Atomic-Scale Measurement of Liquid Metal Wetting and Flow

The flow behavior of liquid metals at solid interfaces is critically important to successful welding, brazing, soldering and the synthesis of metal/ceramic composites. Continuum flow models frequently fail to reliably predict wetting behavior because they are based upon bulk fluid properties, rather than microscopic flow processes at the actual solid/liquid interface. Improved understanding of interfacial liquid flow is hindered by the paucity of experimental measurements at this microscopic level. This report describes a new approach, Acoustic Wave Damping (AWD), to measuring viscoelastic properties of liquid metal layers in the nanometer thickness regime. The AWD experiment measures the frequency response of a quartz crystal microbalance in contact with an viscoelastic layer. An equivalent circuit model and continuum acoustic theory relate this electrical response to mechanical energy storage and dissipative loss. For viscoelastic layers of known thickness and density, a quantitative complex shear modulus can be determined from the AWD data. Studies of self-assembled monolayers (SAMs) demonstrate sensitivity to monolayer structure and bonding. Molecular dynamics simulations relate these atomistic properties to the ensemble response. AWD measurements of ultra-thin liquid indium layers reveal metastable undercooling for 10-50nm thick indium layers. Continued refinement of the AWD technique and the addition of complementary interface characterization techniques will enable definitive studies of ultra-thin molten metals. † Present Address: U.S. Naval Research Laboratory, Code 6670, Washington, DC, 20375. # Present Address: Department of Chemical Engineering, The University of Texas at Austin, Austin, TX, 78712.