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Molecular dynamics prediction of nanofluidic contact angle offset by an AFM

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Abstract

This paper investigates the nano-fluidic contact angle measurement by performing molecular dynamics simulations. The contact angle between a nano-water droplet and a platinum surface is important for the design of the porous catalyst layer in low-temperature fuel cells. The measurement can generally be conducted by an atomic force microscope (AFM). However, the interaction force between the water droplet and the probe tip of the microscope may influence the measurement results. This paper employs the molecular dynamics technique to investigate the offset of the contact angle measurement. Calculations are in two sets, one simulated the water molecules clustering on the platinum surface, and the other involved the AFM measurement of the contact angle. The former case presents the original contact angle between the nano-scale water droplet and the platinum surface; the offset of the contact angle measurement due to intrusion of the AFM probe is predictable from the latter case. For engineering purposes, we present a correlation between the offset angle and the AFM measurement locations.

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Abbreviations

e b :

unit vector in body-fixed frame

e s :

unit vector in space-fixed frame

h :

the height of droplet spreading on solid surface (nm)

q :

charge of atoms

r ij :

distance between atoms in each pair (nm)

r :

curvature radius of the droplet surface spreading on solid surface (nm)

r′:

radius of contact area between the droplet and the solid surface (nm)

ɛ0 :

dielectric constant (8.852 × 10−6)

ɛoo :

energy at the minimum of the pair potential for water (0.6502 kJ/mol)

ϕ, ξ, ψ:

Euler angles (degree)

θ:

contact angle between the droplet and the solid surface (degree)

σoo :

distance to the zero in pair potential for water (0.3166 nm)

ω:

length projection of the distance vector on the x−y plane (nm)

i, j :

separate atoms

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Acknowledgments

National Science Council provided support under contract no. NSC 95-2218-E-007-001; National Center for High-performance Computing provided the high-speed computer facility.

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Authors and Affiliations

  1. Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu, 30013, Taiwan

    K. Fei, C. P. Chiu & C. W. Hong

Authors
  1. K. Fei
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  2. C. P. Chiu
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  3. C. W. Hong
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Correspondence to C. W. Hong.

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Fei, K., Chiu, C.P. & Hong, C.W. Molecular dynamics prediction of nanofluidic contact angle offset by an AFM. Microfluid Nanofluid 4, 321–330 (2008). https://doi.org/10.1007/s10404-007-0187-y

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  • DOI: https://doi.org/10.1007/s10404-007-0187-y

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