Abstract—Effective generation of biochemically active species
inside a liquid can be performed by an electric discharge in small
gas bubbles admixed from outside into the liquid. In such a case,
active species are produced by a gas discharge plasma inside
bubbles and then transported due to diffusion into the liquid. This
paper presents experimental data of electric discharges in small
bubbles inserted into a thin dielectric tube filled with water. A
low-frequency regime of periodic electrical breakdowns is revealed
and described.
Index Terms—Gas discharges, plasma generation, plasma
materials-processing applications.
APROMISING approach that allows us to generate radicals
in a liquid is an electric discharge in the water filled with
gas bubbles. However, in normal situations, gas bubbles travel
chaotically in a liquid and rise to the top due to Archimedes’
force which hampers experimental observations of electrical
breakdown in a gas bubble. In addition, breakdown voltage
Ub = Ebl across a bubble surrounded by a liquid can be reached
only under pulsed superposition of high voltage (HV) U0 on
the liquid. Here, Eb is the gas breakdown electric field strength,
l is the length of the bubble, and U0 > Ub is for fast breakdown.
The rise time τ of the pulse has to meet the condition
τ < εε0/σ, where the displacement current in a liquid has to
exceed the conduction current. Here, ε and ε0 are dielectric permittivities
of the liquid and vacuum. Thus, at high magnitudes
in σ and Ub, pulsed generators having nanosecond and many
kilovolt pulses are required.
To avoid bubble immersion and using HV nanosecond
pulses, we have localized the bubble at fixed points. A thin
(inner diameter is 2.5 mm) and long (100-mm) quartz tube
oriented horizontally was used (Fig. 1). This tube was filled
with a water solution having different electrical conductivities
(20 µS/cm for distilled water and 12 mS/cm for a physiological
solution). An initial bubble of 4–5 mm in diameter was
filled with ambient air at atmospheric pressure. HV pulses of up to 20 kV were applied. Typical rise times and duration
of the pulses are 0.1 µs and 3 s, respectively. Current and
voltage waveforms were recorded by a TDS520D oscilloscope.
Images of the bubbles were taken by a Panasonic NV-GS500
video frame camera under 25 frames/s at an exposure time of
20 ms. Another video camera with the same or better parameters
could be used in the experiments. HV pulse was
applied in 2 s after camera triggering. The first frame in the
sequence of video frames, which records the discharge image,
corresponds to the first current pulse in periodic breakdown of a
gas bubble.
We recorded that electrical breakdown in a single gas bubble
inserted in narrow dielectric tube filled with a conductive
liquid occurs in a filamentary mode (Fig. 2). The individual
thickness of these numerous filaments is smaller than the
inner diameter of the tube. The filaments are nonstationary
in space and time, twist like snakes, and, therefore, nonuniformly
occupy the bulk of the bubble. We suppose that the
high-frequency noisy component of electric current seen in
Fig. 3 is associated with the nonstationarity of the current
filaments. If we neglect the noisy current component, one can
say that electrical breakdown in a single gas bubble forms
almost periodic current pulses of trapezoidal shape, even if
applied voltage is practically constant or slowly diminishing.
The typical period of such breakdown pulsations is rather
long—about 0.5 s.
The voltage drop across gas bubbles increases with diminishing
current over each period of pulsations (Fig. 3). When the electric current diminishes to a critical value, it nearly
extinguishes. The voltage drop across the bubble at this moment
quickly increases to the breakdown magnitude, and breakdown
occurs again. These pulses are correlated with periodic extension
and shrinking of gas bubble in its length (Fig. 3). For a
bubble located near the anode, the bubble shrinks to its initial
size. A bubble located in the middle of the tube does not
shrink to initial size. The initial bubble can be fragmented after
breakdown into several smaller bubbles [Fig. 2(d)]. The average
current (i.e., averaged over high-frequency noise) decreases
with an increase in length L of the bubble. The average voltage
drop U across the bubble grows with increasing length of the
bubble.
The free-running pulsations in the length of gas bubble
likely occur due to an electrical breakdown and gas discharge
inside the bubble. After electrical breakdown, evaporation of
liquid begins that results in a growing gas pressure inside
the bubble and its expansion. The water vapor is an electronattaching
gas. Therefore, the increase in length of a bubble
(i.e., increase in total intensity of electron-attaching processes)
results in decreasing the electron number density, and the
current drops. On extinction of the discharge inside the bubble,
water vapor condenses rapidly. The bubble length diminishes
again to its initial size, and a new breakdown in sequence
repeats.