Page 212 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

Contamination and Industrial Systems 205

Liquid
(a)
feed
Detector

Fiber-coupled
source Hanging droplet

(b)

Optical
path
Meniscus supported
in perforated disc
Figure 9.13 Small liquid samples may be supported by
surface tension and probed in transmission without use of
wetted windows, either using hanging droplets or a thin film.

pump. Both of these pumped and immersed free-surface geometries can be used
for fluorescence detection and Raman spectroscopy, with similar nonfouling
advantages. They are also useful for reliable long-term video-based observation
of particles and flocs in industrial process streams.

9.5.2 Transmission measurements and
the bubble interface
The nonfouling characteristic of the liquid-air interface is equally attractive
for transmission-geometry measurements, although it is less obvious how to
arrange this. A good starting point is the hanging droplet of Fig. 9.13a. By eject-
ing a small droplet from a capillary, an unbounded sample is formed which can
be probed for optical absorption, fluorescence, etc. When small droplets (<2mm
diameter) are used they can be accurately spherical, and useful as a lens to focus
light from a small source or optical fiber onto a detector. Bigger drops become
more pendulous, leading to severe aberrations. Hence we are limited to rather
small path lengths and poor sensitivity. Total internal reflection can be used to
increase the path length, but this is difficult to control (Liu et al., 1995). Figure
9.13b shows another approach, more suitable for high viscosity liquids like oils
and gelatinous film emulsions. A perforated disk is dipped into a bath of the
sample and removed, leaving a meniscus supported in the hole. This can be used
for windowless optical measurements free from gross fouling. As with the
droplet, the path-length is rather short.
A more promising technique is the “holey-cup” of Fig. 9.14. Here we have
drilled a 0.5mm diameter hole in the base of a plastic cup, subsequently filled
with water. As long as the hole is not too large, and the water head not too high,
water will not flow through the hole, but will be retained by surface tension
around the hole periphery. The 0.5mm hole will support a 40mm depth of clean

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