Wetting of microstructured surfaces

Aqil, S., 2006. Wetting of microstructured surfaces. PhD, Nottingham Trent University.

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This thesis reports a study of the effect of topography on wetting. In the first part of this study the development of a reliable method for producing microstructured patterned SU-8 50 surfaces, using a photolithography technique, is described. These patterns involve tall circular pillars of vertical walls with diameters and spacing that range between 4 and 40 ?m, and with aspect ratio (defined as pillars height/pillars diameter) sometimes greater than 4. The contact angle, θ, of water droplets on these patterned surfaces is shown to exceed 150° compared to 80° on the flat surface, dependent on the pattern dimensions; thus surface with wetting tendencies is converted into a nonwetting (hydrophobic) surface as a consequence only of surface topography variation. Subsequently these patterned surfaces were used to study the dynamics of spreading of drops of a non-volatile viscous polydimethylsiloxane (PDMS) oil, which spreads completely on a flat SU-8 surface. The exponent, n, of the power law of the dynamic contact angle-time relation θ ∞ t-n is shown to increase from 0.3 on a flat towards 0.75 on surfaces of very tall pillars. The patern is also shown to convert the drop edge speed, ve from a cubic function in the dynamic contact angle, ve ∞ θ3 , towards a linear function, ve ∞ θ.

In the final part of this thesis, qualitative and quantitative features of the evaporation of water droplets with initial contact angles of ~ 130° -150°are described. Both pinned contact area and retreating contact line evaporation were observed corresponding to droplets in initial states of either Wenzel or Cassie states (i. e. penetrating surface features or suspended upon them). For droplets beginning in the Cassie state, a transition to the Wenzel state was observed. A model for the constant contact area mode of evaporation was applied to the data and a diffusion coefficient of water vapour into the surrounding air ((2.6 ± 0.2)x10-5 m2s-1) was obtained consistent with reference literature to within 8%.

Item Type: Thesis
Creators: Aqil, S.
Date: 2006
ISBN: 9781369316247
Divisions: Schools > School of Science and Technology
Record created by: Linda Sullivan
Date Added: 25 Sep 2020 14:11
Last Modified: 23 Aug 2023 13:27
URI: https://irep.ntu.ac.uk/id/eprint/40953

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