Page 105 - Handbook of Surface Improvement and Modification

100 Surface Tension and Wetting

Silanes are versatile materials used in a wide range of applications including surface
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modification. A new formula was developed to calculate the surface tension of silicon-
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based molecules. The calculations are based on four input parameters including the radius
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of gyration, critical density, critical temperature, and acentric factor as fluid constants .
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The proposed equation is as follows:
σ B C D E
----- = A1 –( T ) 1 +( Φ ) ω [7.1]
σ 0 r
where:
σ surface tension
σ 0 scaled factor (kT )/G r 2
c
T r reduced temperature
Φ adimensional term = N r G r 3
a c
ω acentric factor
k Boltzmann’s constant
T c critical temperature
G r radius of gyration
N a Avogadro’s number
ρ c critical density
The following coefficients were found after regression analysis of the data for a large
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number of silanes: A=0.00031, B=1.22975, C=0.10397, D=14.93220, and E=0.33689.
Many other influences of the surface tension and wetting are related to the material
structure and morphology and these interferences are outlined now.
One of these interferences was inspired by the adhesive abilities of amphibian toe-
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pads. The amphibians such as tree frogs, torrent frogs, and newts have remarkable abili-
ties of climbing in wet environments which is widely believed to be a result of the specific
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topography on their toe-pads. The toe-pads expel fluid out of the contact area between the
pad epidermis and the substrate to achieve the “close contact” and a boundary friction.
Inspired by this design, polydimethylsiloxane hexagonal pillar arrayed surfaces with vary-
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ing softness were fabricated. The long-range capillary forces and a short-range interac-
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tions caused by the close contact play a significant role in adhesion. The surface softness
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has the short-range contribution for the adhesion of micropillars.
The porosity and morphology are important factor in wettability of PDMS surface. 9
The porous polydimethylsiloxane surfaces were obtained by laser irradiation and addition
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of salt before curing and then washing it out after curing. Both methods cause porosity of
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the surface but the effect of porosity on contact angle were different. Pulsed laser rapidly
changed surface to hydrophobic where salt caused only gradual (and substantially smaller)
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increase in contact angle as the percentage of porosity growth. The hydrophobicity
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caused by the laser treatment depended on chain ordering of PDMS.
A review paper analyzed several methods of determination of the contact angle of
powdery materials with special attention to sessile drop, Wilhelmy plate, and liquid pene-
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tration methods. The Washburn capillary rise method was recommended over other
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methods for estimating water contact angle of powders.
The effect of temperature of polyethylene film fabrication on its wettability has been
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studied. The solutions of polyethylene in hot mesitylene were cast on glass substrate and
o
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solvent evaporated at 25, 5, or -15 C. The idea of this study was to develop a superhy-
drophobic surface, such as needed for self-cleaning coatings, ice-phobic coatings, anti-