Figure 1. Advancing contact angle of different surfaces
Peter Rieke & Liang Liang
This project focuses on developing scientific knowledge of self-assembled smart surfaces in micro-channels and applying molecular and material sciences to enhance the functionality of micro-structures by incorporating molecular and polymeric smart surfaces. The central scientific issue is to determine strategies that exploit highly functional surfaces to develop new concepts in fluid handling for nanoscale chemical reactors and heat exchangers. Various materials will be evaluated, in particular, thermal sensitive polymers. New materials will be designed that can be actuated by novel thermal, optical, electrical, magnetic, or electrochemical means. Methods will be developed for reversible actuation of the active elements.
Thermally-sensitive surfaces were prepared on the surface of silicone wafers (substrates) two methods of reaction of a polymer. After applying the first method, ellipsometry measurement revealed a thin grafting layer (~44 Å) consisting of a single polymer chain on the surface of the silicone wafers. X-ray photoelectron spectroscopy confirmed that the grafting layer was composed of the desired structure. The properties of the grafting layer can be adjusted and manipulated by varying the reaction time and the concentration of the monomer, which increases both the thickness of the grafting layer and the wettability of the surface.
The characteristics of the temperature-sensitive surface were investigated by dynamic contact angle as a function of temperature. The remarkable change on advancing the contact angle can be observed around 32°C (see Figure 1). Compared with the substrate grafted by previous efforts, this chain exhibited a lower transition temperature and a narrower change range of transition temperature.
After applying the second method,the modified were characterized by scanning electron microscopy, x-ray photo-electron spectroscopy, and attenuated total reflectance-Fourier transform infrared spectroscopy. The reversible hydrophilic/hydrophobic properties of the surface were evaluated by dynamic contact angle. The morphology of the surface can be turned from "sea-island" to "mountain-valley" by changing reactant molar and by varying the polymerization time. A completely hydro-philic surface (advancing contact angle = 0°) was observed below 25°C, and the surface became extremely hydro-phobic (advancing contact angle = 92°) above 40°C (see Figure 2). The sensitivity of surfaces to temperature change can be improved by increasing the cross-linking density of the polymer layer and varying the polymeriza-tion time. The water meniscus height in a capillary tube, whose wall was coated by the polymer, went up or down as the temperature changed. The differences in the water meniscus height are 10 and 5 mm for a capillary tube with a diameter of 2 and 3 mm, respectively, corresponding to a change of temperature from 23°C to 50°C. The temperature-sensitive characteristics, which produce remarkable and rapid changes of surface properties, make this technology applicable to actuators, modulators, sensors, and switches.
Liang L, PC Rieke, GE Fryxell, J Liu, M Engelhard, and KL Alford. 1999. "Temperature-Sensitive Surfaces Prepared by UV Photo, Grafting Reaction of Photo-sensitizer and N-isopropylacrylamide." J. Phy. Chem. B. (to be submitted).
Liang L, PC Rieke, J Liu, GE Fryxell, JS Young, MH Engelhard, and KL Alford. 1999. "Surfaces with Reversible Characteristics on Hydrophilicity/ Hydrophobicity from Crosslinked Poly(N-isopropylacrylamide) Hydrogel." Langmuir (to be submitted).
We are thankful for the help of M. H. Engelhard and J. S. Young on the measurements of SEM and XPS.
Figure 1. Advancing contact angle of different surfaces
|
