Chemistry Reference
In-Depth Information
species [414-416]. The discharge is initiated in the gas bubble, so that no energy has
to be consumed to form the bubbles in the liquid. Operating at electric field strengths
of about 30 kV/cm limits the discharges to the gaseous bubbles. It is also possible
to create a discharge above the water surface by placing one electrode outside the
water [410,411], in which case the discharge occurs mostly in the gas phase above
the water.
Different mechanisms are possible for creating an electrical discharge in the gas
bubbles. Investigations using mesh and wire electrodes report discharges that initiate
at points of contact between the wire, the solid insulation, air, and possibly water and
proceed along the surface of the bubble [415]. Some experiments have used bubbles
passing through the water relatively far from the electrodes. In other arrangements,
bubbles spend long periods of time attached to the electrodes and the bubble may
be in contact with one or both electrodes. In cases where the gas bubble is in direct
contact with both electrodes, the discharge forms via an electron avalanche, and a
transition to an arc must be prevented by the outside electrical circuit or by using short
voltage pulses. If water covers one or both electrodes, the discharge current is limited,
particularly for short-voltage pulses. As a consequence, DBDs or corona discharges
may be expected [405,410,411,413-417]. The study of discharges in bubbled water
using wire-to-plane, wire-to-wire, or mesh electrode configurations is complicated
by the flow of bubbles [418,419]. The discharge can occur in various bubbles at the
same time and hence the field in the bubbles is difficult to estimate and the properties
of the discharge are difficult to study. Single-bubble systems allow detailed studies
of the discharge process in the bubble, which is stationary and can be subjected to
repeated voltage pulses.
8.3.2 Experimental Systems
Two experimental systems will be discussed here briefly. The first system allows
a discharge to be created in a single stationary bubble in water [416]. The system
consists of a water chamber constructed from PVC to reduce the impedance, a
Blumlein pulse-forming network, a gas delivery system, and various diagnostic tools.
The chamber has three ports (for the fiber-optic cable, the needle electrode with
gas inlet, and the movable disk electrode) and two windows at 90
◦
to each other
(see Figure 8.58a). A gas flow controller delivers gas to a microsyringe, which
is used to form a gas bubble at the tip of a hypodermic needle inserted into the
water chamber. The amount of gas injected can be controlled up to a microliter,
and a pressure release valve ensures that the overall pressure remains at about one
atmosphere. The syringe is used to control the size of the bubble. A digital camera
is used to record the size of the bubble and a UV transparent fiber is used to collect
optical emissions from the bubble. Argon and oxygen bubbles have also been used
in this study.
This experimental arrangement allows the study of a single stationary bubble with
just the tip of the needle extending into the bubble (see Figure 8.58b). The hypodermic
needle, which is mostly covered by the insulating material of the chamber with only
a 0.5 mm tip protruding into the bubble, serves as the negative electrode. The second
