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Utilizing dynamic contact angle tensiometry to quantify the kinetics of switchable surface rearrangement
Wednesday, Jun 13, 2012, 1:50 PM - 2:10 PM
J. A. Kleingartner
, H. Lee, G. H. McKinley, R. E. Cohen;
Massachusetts Institute of Technology, Cambridge, MA.
Hydrogen bonded thin films consisting of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) were prepared via the layer-by-layer assembly method at pH 2.0. The abundance of free hydroxyl and carboxylic acid groups in the as-assembled films allows for thermal and chemical modifications that enhance the stability of the materials at higher pH conditions. Hydroxyl terminated poly(ethylene glycol) (PEG) molecules were grafted to the stabilized films at pH 7.4 using glutaraldehyde chemistry. This resulted in a coating that rearranges on a molecular scale with time when contacted with water. In the dry state these films are hydrophobic exhibiting advancing water contact angles near 120°. However, the contact angle decreases as the surface rearranges to allow for more favorable interactions between the water and film surface. Observations of sessile drops revealed a transient decay from 120° to about 55° over a 10 minute time interval. Utilizing dynamic contact angle tensiometry the kinetics of this surface rearrangement were probed in more detail by advancing and receding water over these surfaces at various rates ranging from 0.01 mm/sec to 1 mm/sec and over a range of temperatures from near 0°C to 30°C. The dimensionless group τ
(where τ is the time constant for surface rearrangement,
is the immersion rate, and
is the capillary length) describes a ratio of a time-scale for molecular rearrangement (τ) to a convective time scale for meniscus advancement (
). A transition from a high contact angle regime to a rearrangement-dominated, low contact angle regime is observed as this dimensionless group passes through unity. These films exhibit complete reversibility in their wetting response, and return to their hydrophobic state upon drying in air. The kinetics of this reverse rearrangement are also being investigated.
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