Page 181 - Optofluidics Fundamentals, Devices, and Applications

156 Cha pte r Se v e n

40°

0

Transmission (dB) –15

–30
1.1 1.2 1.3 1.4 1.5
Wavelength (μm)
FIGURE 7-18 (top) The behavior of the fl uid meniscus in an untreated square
capillary (left) and a fl at meniscus in an organosilane treated capillary (right).
(bottom) The transmission of the device as a function of meniscus position
from completely outside the beam to optimally inserted. (C. Grillet, P.
Domachuk, V. Ta’eed, et al., “Compact tunable microfl uidic interferometer,”
Opt. Express, 12, 5440–5447 (2004).)

scattered off the curved surface. Thus a flat meniscus, one with a con-
tact angle of 90 degrees, is desirable. A flat water meniscus is attain-
able on silica using a silanization monolayer surface treatment [76].
This process involves the chemical addition of organosilanes to the
silica surface to form an atomic monolayer. If this chemistry is applied
all over the surface, the total surface energy of the silica is reduced by
this molecular monolayer. Such a monolayer is appropriate for
surface coatings in an optical environment since its thickness is
considerably subwavelength, as opposed to polymer surface treat-
ments that are significantly thicker. In this case, the organosilane used
is dodecyltrichlorosilane (C H SiCl )—an organic molecule with a
12 25 3
12-carbon chain attached to a chlorosilane group. Experimentally,