Award Abstract #0103514
NER: Nano-Textured Surfaces: Super-Hydrophobicity and Liquid Adhesion

NSF Org:	CMMI Division of Civil, Mechanical, and Manufacturing Innovation
Initial Amendment Date:	July 23, 2001
Latest Amendment Date:	July 23, 2001
Award Number:	0103514
Award Instrument:	Standard Grant
Program Manager:	Jorn Larsen-Basse CMMI Division of Civil, Mechanical, and Manufacturing Innovation ENG Directorate for Engineering
Start Date:	August 1, 2001
Expires:	July 31, 2002 (Estimated)
Awarded Amount to Date:	$99999
Investigator(s):	Frederick Lange flange@engineering.ucsb.edu (Principal Investigator)
Sponsor:	University of California-Santa Barbara Office of Research SANTA BARBARA, CA 93106 805/893-4188
NSF Program(s):	DMR SHORT TERM SUPPORT, MATERIALS AND SURFACE ENG
Field Application(s):	0308000 Industrial Technology
Program Reference Code(s):	AMPP, 9161, 1762, 1676
Program Element Code(s):	1712, 1633

ABSTRACT

0103514

Lange

This proposal was submitted in response to the solicitation "Nanoscale Science and Engineering" (NSF 00-119)

Surfaces can be made either super-hydrophobic or super-hydrophilic by producing a textured surface that resembles hills and valleys, provided that the wetting angle of the liquid on a flat surface of the same material is > 90 degree . The mechanism that produces the effect is based on the fact that the liquid only wets the tops of the hills and gas (air) is trapped within the valleys. Simple analytical functions have been developed for different periodic, textured surfaces to show that the wetting angle is related to the area fraction of wetted 'hills' and the wetting angle for a flat surface.

The lotus leaf is a natural example of a super-hydrophobic surface. In 1997, micron size wax bumps were discovered to produce this effect. Water drops on the lotus leaf are nearly spherical (q ~ 170 degree) and easily roll around collecting dust particles to produce a self-cleansing effect that has made the lotus plant revered for its purity. Synthetic super-hydrophobic surfaces have been produced by coating a surface with a 'rumbled' thin film and with a micromolding method. These surfaces are promising for practical applications that include rain-repellent surfaces, surfaces designed to decrease the resistance to fluid flow, and surfaces designed for selective liquid condensation.

Recently, we demonstrated that super-hydrophobic surfaces could be produced by simply dip-coating a substrate into a slurry containing small, dispersed particles. The particles were attracted to the substrate by to their opposite surface charge, relative to the substrate. We demonstrated that the area fraction and particle size can be systematically controlled and that surfaces can be textured with commerically available particles in the range of 5 nm to 300 nm. By reacting the surface with fluoroalkyltrichlorosilane molecules, a flat surface is rendered hydrophobic, and super-hydrophobic, when textured.

We observed that the super-hydrophobic effect was related to the area fraction of adsorbed silica particles and that the super-hydrophobic effect disappears when the average spacing between the spherical particles exceeds a critical value. When the particles are very small the water droplet shows absolute adherence to the surface. Both of these latter two effects can be predicted with the Laplace equation, which relates the equilibrium curvature of a meniscus to the pressure exerted by the water drop.

Systematic experiments are planned with nano-textured surfaces to study these two new phenomena in relation to specific functions derived with the Laplace equation. This will be accomplished using glass substrates that will be coated with colloidal silica particles that are commercially available in the size range of 5 nm to 300 nm. The prepared nano-textured surfaces will consist of randomly distributed silica sphere on a glass substrate. The silica particles will be fixed to the substrate by sintering. The nano-textured substrates will then be reacted with fluoroalkyltrichlorosilane molecules to ensure optimum hydrophobicity. All experiments will be conducted with deionized water on nano-textured surfaces treated with the same fluoroalkyltrichlorosilane molecules. With these constrains, the surface energy per unit area, g, and the contact angle of a flat surface, q, will be keep constant.

Spontaneous Wetting Experiments: Spontaneous wetting will occur when the Laplace pressure exceeds a critical value to produce an instability; spontaneous wetting is expected to be a strong function of both the area fraction and particle size. Experiments will be designed to determine the validity of this expected result that can be formalized as an analytical equation. These experiments will include contact angle measurements vs. drop size, area coverage and particle size (5 nm to 300 nm).

Adhesion Experiments : Tilting experiments will be carried out to measure the a) advancing and receding contacts angles and b) the critical angle for drop movement, all as a function of the drop size and the characteristics of the textured surface (area fraction and particle size). Experiments will be carried out to determine the critical drop size that can still adhere to the surface when the substrate is held up-side down; these results will be related to an analytical equation that describes the critical pressure for spontaneous de-wetting.

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