| Preliminary experiments of the behavior of aqueous drops on superhydrophobic surfaces showed some unique properties. These experiments suggested that if a novel architecture combining superhydrophobic surfaces, drop manipulation, and optical sensing methods could be developed, it might lead to new analytical and medical diagnostic devices requiring smaller sample volumes and with higher sensitivity than comparable existing devices. Thus, an examination, in four phases, of the effects and uses of superhydrophobicity was performed. The first hypothesis was that an inexpensive superhydrophobic could be created from low density polyethylene. It was found that using xylene as solvent and methyl ethyl ketene (MEK) as nonsolvent in the ratio of 55%:45%, such a surface would have a contact angle of 150 - 160°. The second hypothesis was that an automated stage could be constructed that would move such discrete drops at relatively high speed. Using computer control, it was found that such a stage could be constructed that would move discrete drops at an average speed of 2.8 cm/sec over a pathlength of 1m. The next hypothesis was that a forward-scattering nephelometer could be constructed for a drop wherein the scattering signal would not be swamped by the incident light. After construction, the device was tested using a custom agglutination immunoassay for bovine serum albumin (BSA), giving a signal increase when BSA was present of>300%. When a commercial assay for C-reactive protein (CRP) was modified and tested, the signal increase when CRP was present was >1100%. Mathematical simulations confirmed the observed signal increase. The final phase used particle image velocimetry (PIV) to image the particles of the CRP assay in a drop. It was hypothesized that, with proper choice of dilution and signal processing, PIV would show a difference between positive and negative controls. Using a 1:50 v/v dilution and fast Fourier transform (FFT) with a 0.5 Hz ideal low-pass filter, the PIV system did show a higher pixel density for the positive control after approximately 6 min elapsed time. The results as a whole demonstrate that drop-based analytical and diagnostic equipment and tests are viable and have important advantages over cuvette-based devices and techniques. |
