Molecular dynamics ͑MD͒ simulations were performed to study nanoscale friction for alkyl monolayers terminated with -CH 3 ͑hydrophobic͒ and -OH ͑hydrophilic͒ on Si͑111͒ in the presence of water molecules. Variation of the -OH/-CH 3 ) ratio results in different surface hydrophobicities. To calculate friction in MD simulations, two Si͑111͒ surfaces coated with the alkyl monolayers were slid against each other. It was shown that the friction coefficient decreased quickly for hydrophilic

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... whereas it remained almost the same for hydrophobic monolayers, as the number of water molecules ͑relative humidity͒ increased. Simulation results are in good agreement with those from scanning force microscopy measurements for both hydrophilic and hydrophobic monolayers. The behavior of water molecules confined between hydrophobic or hydrophilic monolayers was also discussed. A fundamental understanding of nanoscale friction is critical to the design of coatings for microelectromechanical systems.