Tribological Study on Microelectromechanical Systems (MEMS) in Humidity Air by Molecular Dynamics Simulations

Micromachined devices, such as microelectromechanical systems (MEMS), high-density sensor arrays, and microfluidics, are poised to bring the next technology revolution. Friction is a crucial factor that controls the efficiency and durability of moving mechanical assemblies in these devices. It is affected by environmental conditions, such as humidity and solvents. However, the mechanism of the friction and the effect of humidity on friction are not well understood yet. To minimize stiction and reducing adhesion and friction in MEMS, one of strategies is the use of self-assembled monolayers (SAMs). The alternative approach is the direct reaction between alkene and H-terminated silicon surface to form thin organic films via a linkage of covalent bond Si-C. The major advantage of the monolayers over conventional SAMs of thiols on gold is their high stability. In this work, molecular simulations were performed to study friction for alkyl monolayers with different terminal groups ranging from hydrophobic to hydrophilic on Si(111) surface. 

A simulation setup was constructed based on molecular mechanics study of alkyl monolayers on Si(111). It consists of two Si(111) surfaces coated by alkyl monolayers with mixed terminal groups (-OH and -CH3) and water molecules confined between the surfaces. Variation of -OH/-CH3 ratio results in different surface hydrophobicities. The two surfaces were slid against each other in molecular dynamics (MD) simulations. The number of confined water molecules was determined by grand canonical ensemble Monte Carlo (GCMC) under a certain relative humidity. Simulation results were compared with those from experimental measurements by scanning force microscopy. MD simulations in this study will not only provide a fundamental understanding of friction, but also guide the design of coatings for MEMS devices. Here are the MD simulations of MEMS in either humidity air or liquid solvents.


(1) MEMS in Humidity Air

Simulation setup
Simulation results

(configurations and friction coefficients)

Hydrophobic surface

Hydrophilic surface

Friction coefficient from MD

Friction coefficient from SFM

Water density profiles

Friction coefficients for other pore sizes

Friction coefficient vs. pore size

Movie from MD 

[download the movie file, 4.9 MB]

(Water molecules will not be showed in the movie.)

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