Effect of Self-Assembling Monolayers (SAMs) on Ice Adhesion to Metals
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*Dartmouth College, Thayer School of Engineering, Hanover, NH 03755 ** Hokkaido University, Sapporo, Japan ABSTRACT In this research we used an original method to study the role of hydrogen bonding in ice adhesion and to minimize the effect of this mechanism on ice adhesion. We coated metals (Au and Pt) with a mono-molecular layer of specific organic molecules that had either strong hydrophobic properties (CH3(CH2)IISH) or strong hydrophilic properties (OH(CH 2)1 ISH). To determine the contribution of hydrogen bonding to ice adhesion, self-assembling monolayers (SAMs) of varying degrees of hydrophobicity/hydrophilicity were created by mixing the hydrophobic and hydrophilic components. All of the SAMs were composed of similar molecules that differed only in their outermost groups, OH- and CH3-. Thus, when the SAMs were grown on the same substrate (almost atomically smooth metal coatings), any differences in their adhesion to ice were due to differences in the hydrogen bonding between the ice and SAMs. The SAMs structure and quality were examined with scanning force microscopy (SFM) and the degree of the SAM's hydrophobicity/hydrophilicity was characterized by the contact angle of water on the monolayers. We then froze water on the SAMs and measured the shear strength of the ice/SAM/metal interfaces. Possible damage to the interfaces was examined with SFM after the ice had melted. We found a good correlation between the contact angle of water and the ice adhesion strength and determined the fraction of ice adhesion caused by hydrogen bonding. INTRODUCTION Ice adhesion is the cause of a great many problems for humans, including ice buildup on roads and the icing of airplane wings, power lines, ships, etc. These problems have not been solved yet because the physical mechanisms of ice adhesion are not well understood. The basic mechanisms of ice adhesion can be divided into three main categories: electrostatic interaction, chemical bonding, and dispersion or fluctuating forces (Van der Waals force) 1,2,3. The chemical bonding between ice and other solids is largely hydrogen bonding and has not been studied in detail yet. Van Oss et al. (1992) 4 measured the contact angles of a flat, polycrystalline ice surface with a number of liquids, as well as the adhesion of particles and macromolecules to advancing freezing fronts. They concluded that the contribution of the Lifshitz-van der Waals non-polar component to the ice surface tension was less than that of the polar Lewis acid-base component, which in the case of ice is due to hydrogen bonding. For a long time, some correlation between a material's hydrophilic properties and its adhesion strength to ice has been assumed, but that correlation has not been studied systematically. To fill in this gap in knowledge, we used a novel method to study the role of hydrogen bonding in ice adhesion, a method that can minimize the effect of hydrogen bonds on ice adhesion. Namely, we used self-assembling films or monolayers (SAFs or SAMs) to vary the strength of hydrogen bonding of meta
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