WO
2011023746
CLEANING APPARATUS AND METHOD, AND MONITORING THEREOF
Publication date: 2011-03-03
Inventor(s): LEIGHTON TIMOTHY GRANT [GB]; VIAN
CHRISTOPHER JAMES BRADSHAW [GB]; BIRKIN PETER ROBERT [GB] +
(LEIGHTON, TIMOTHY, GRANT, ; VIAN, CHRISTOPHER, JAMES, BRADSHAW, ;
BIRKIN, PETER, ROBERT)
Applicant(s): UNIV SOUTHAMPTON [GB]; LEIGHTON
TIMOTHY GRANT [GB]; VIAN CHRISTOPHER JAMES BRADSHAW [GB]; BIRKIN
PETER ROBERT [GB] + (UNIVERSITY OF SOUTHAMPTON, ; LEIGHTON,
TIMOTHY, GRANT, ; VIAN, CHRISTOPHER, JAMES, BRADSHAW, ; BIRKIN,
PETER, ROBERT)
Classification: - international: B08B3/10 - European: B08B3/10;
B08B3/12
Also published as: WO 2011023746 // GB 2472998 (A)
Abstract -- An apparatus
for cleaning a surface, the apparatus comprising a body defining a
chamber, an inlet for liquid flow into the chamber, an outlet for
liquid flow from the chamber, a nozzle connected to the outlet for
generating an output flow of liquid for cleaning a surface, an
acoustic transducer associated with the body to introduce acoustic
energy into the liquid within the chamber whereby the acoustic
energy is present in the liquid flowing out of the nozzle, and a
gas bubble generator for generating gas bubbles within the liquid
flowing out of the nozzle.
Description
The present invention relates to a cleaning apparatus and to a
cleaning method, and to a method of monitoring cleaning.
Cleaning is an essential part of many research, commercial and
public service processes, three obvious examples being
manufacturing, healthcare, and laboratory work. Cleaning is often
not simple: the object to be cleaned may be complicated, with many
inaccessible crevices or chambers, and the potential contaminant
very hazardous (a good example being the biopsy endoscope). The
object to be cleaned may also be delicate (a good example of this
being salad and vegetable matter, microchips, flesh, forensic
material etc.) or tolerate minimal levels of scratching and damage
(such as optical lenses, jewelry, prestige watch glasses and faces
or prestige car finishes). Often the time available for cleaning
is limited, as there is an imperative to move the object along to
the next stage of processing or usage after it is cleaned (either
because the number of units available for use is limited - as with
the endoscope -, or because retardation of through-put cuts
profile - as in the salad example).
Cleaning uses up huge amounts of water, even for 'natural'
products: the production of 1 tonne of wool currently requires use
of around 500 tonnes of water. When one considers the biohazardous
waste of a hospital or abattoir, or the cleaning associated with
chemical and nuclear plants, water conservation becomes a very
major concern. The requirement for thorough cleaning is often in
conflict with the requirements not to damage the target to be
cleaned, not to use excessive water, not to contaminate the
environment with chemical run-off, and not to use excessive energy
or manpower or time.
Ultrasonic cleaning has been known in the art for many years, by
the use of 'ultrasonic cleaning baths', whereby inertial
cavitation and the generation of high speed liquid jets through
bubble involution causes the removal of surface contaminants. The
exploitation of cavitation in ultrasonic cleaning baths has for
decades provided ultrasonic cleaning facilities that are suitable
for those applications which have had robust objects to be cleaned
(i.e. where cavitation erosion damage is not an issue), and where
the size of the object to be cleaned is small enough to be
immersed, and where the cleaning lacks the urgency which would
necessitate a portable decontamination unit to supply on-the-spot
cleaning resulting from, say, accidental contamination. In many
instances of such cleaning, samples are either cleaned prior to
further processing or dispersed within a suitable media as part of
a larger methodology. Cleaning or processing is then facilitated
by the employment of an ultrasonic bath. This invariably involves
the immersion of a suitable container within the bath.
The cleaning action is often attributed to the generation of
violent cavitation within the vessel itself and the interaction of
these phenomena with the walls of the object in question. Cleaning
action is attributed to cavitation events where the inertia of the
liquid has had a dominant effect on the bubble dynamics, e.g. when
a high-speed liquid jet passes through the bubble as a result of
involution of the bubble wall and generates a blast wave on impact
with liquid or solid; when bubbles collapse with almost spherical
symmetry in 'transient' or 'inertiaP cavitation events, generating
shock waves in the liquid and highly reactive chemical species
such as free radicals; and when clouds of bubbles collapse in a
concerted manner to magnify these effects to become greater than
would be expected without the cloud effect. Hence the exact
mechanism is often associated with 'transient cavitation' or more
precisely inertial cavitation where the violent collapse phase
results in the local generation of these extreme conditions.
However, such ultrasonic cleaning systems may suffer from one or
more problems of surface damage, poor cleaning, particularly of
three dimensional surfaces, e.g. crevices, and an inability to
clean larger objects or surfaces. Furthermore the insertion of the
object to be cleaned into an ultrasonic cleaning bath may disturb
the sound field in a manner which degrades its ability to cause
cleaning.
The present invention aims at least partially to overcome these
problems of known surface cleaning processes, particularly
ultrasonic cleaning.
The present invention provides an apparatus for cleaning a
surface, the apparatus comprising a body defining a chamber, an
inlet for liquid flow into the chamber, an outlet for liquid flow
from the chamber, a nozzle connected to the outlet for generating
an output flow of liquid for cleaning a surface, an acoustic
transducer associated with the body to introduce acoustic energy
into the liquid within the chamber whereby the acoustic energy is
present in the liquid flowing out of the nozzle, and a gas bubble
generator for generating gas bubbles within the liquid flowing out
of the nozzle.
Preferably, the bubble generator comprises electrodes which are
adapted to generate gas bubbles electrolytically within the
liquid. Typically, the electrode comprises an array of
electrically conductive wires extending across a direction of
liquid flow.
Optionally, the gas bubble generator is located within the nozzle.
The apparatus may further comprise a first controller for the gas
bubble generator which is adapted to control the gas bubble
generator to generate pulses of gas bubbles.
The apparatus may further comprise a second controller for the
acoustic transducer which is adapted to control the acoustic
transducer to generate pulses of acoustic energy. Preferably, the
second controller is adapted to switch the acoustic transducer on
and off intermittently to produce pulses of acoustic energy. The
apparatus may further comprise a modulator to provide an amplitude
or frequency modulation of the pulses of acoustic energy.
Preferably, the first and second controllers are coordinated so
that gas bubbles and pulses of acoustic energy are generated with
a mutually controlled time relationship.
This coordination ensures that sound and bubbles can occur at the
same time at a surface location to be cleaned. Their relative
on/off timings at the nozzle can be varied to achieve this
occurrence by taking into account the different travel times of
sound and bubbles down the liquid, which may be a stream, and this
depends on the length of liquid between the nozzle and the surface
to be cleaned. The sound travels through the liquid at a velocity
of over 1 km/second whereas the bubble velocity is related to the
flow rate of the liquid. Once the sound is on, it can then also be
amplitude or frequency modulated. Accordingly, in accordance with
this particular preferred implemenation, it is possible to
coordinate in timing of the two effects: the sound field is turned
off entirely while the bubble swarm is generated and transferred
to the surface. Once the swarm has reached the surface the sound
field is activated. This sound field can be continuous or
amplitude or frequency modulated.
The sound field is off whilst the bubbles travel down the stream
in order to prevent colaescene of bubbles whilst they travel. This
is the simplest implemenation to control the sound. However, for
some applications (e.g. with small volumes of water and the
correct higher levels of surfactant) it may not be necessary to
turn the sound field off entirely whilst the bubbles travel down
the stream, and other implemetations may be used, e.g. switching
the sound to much higher frequencies.
Preferably, the body includes a rear wall on which the acoustic
transducer is mounted and a substantially conical element
extending forwardly therefrom to form a relatively small radius
end thereof communicating with the outlet, the rear wall and the
substantially conical element defining a substantially conical
chamber of decreasing radius extending from the transducer towards
the outlet. The substantially conical element may be geometrically
conical, or alternatively may have a non-geometric shape, such as
being horn-shaped or bell shaped. The substantially conical
element may be formed, for example, of cellular foam or rubber.
Other materials may be employed. The choice of material is
determined by the requirement to match (as closely as practicable)
the acoustic wall boundary conditions within the cone to those in
the nozzle and liquid stream once it leaves the nozzle, so as to
avoid sharp impedance mismatches between cone, nozzle and liquid
stream that would hinder the passage of acoustic energy along the
stream.
Furthermore, a design principle employed by the chamber and nozzle
used in the preferred embodiments of the present invention is that
the acoustic boundary condition on the inner wall of the chamber
and nozzle should match the acoustic boundary condition that will
occur in the stream of liquid once it leaves the nozzle. The
embodiments disclosed herein produce a stream of liquid in free
air, and hence the inner wall of the chamber needs to be
pressure-release, and so a pressure-release material such as
cellular foam or rubber has been used to provide such a
pressure-release boundary. If, however, in accordance with an
alternative embodiment of the present invention a cleaning jet of
liquid (e.g. water) was not directed into air but instead squirted
into another article to be cleaned, for example up a pipe, e.g.
for cleaning an endoscope of narrow internal diameter, that
embodiment would use a chamber with an internal wall condition
matched to the respective acoustic boundary condition of the
article, and a pressure-release characteristic may not be
required.
The apparatus may further comprise an inlet manifold which
comprises a plurality of inlet passages each connected at an inlet
end to the inlet and at an outlet end to the body and/or an
acoustic isolation device in the inlet.
The apparatus may also include a device for adding surfactant to
the liquid.
The present invention further provides a method of cleaning a
surface, the method comprising the step of: directing towards the
surface a liquid flow from a nozzle, the liquid flow including
acoustic energy and entrained gas bubbles within the liquid
flowing out of the nozzle.
The surface may be an external surface or an internal surface, for
example of a cavity. The liquid flow may be directed against the
surface or into the vicinity of the surface, for example by
squirting the liquid up the inside of a tube (e.g. an endoscope)
or pipe, such as a drinks dispenser nozzle.
The method may further comprise the step of generating gas bubbles
electrolytically within the liquid.
Preferably, the gas bubbles are generated within or at a distance
from a free end of the nozzle. For example, it was found that if
the gas bubble generator, e.g. electrolytic wires generating the
gas bubbles, was positioned a small distance, such as about 1 cm,
from the nozzle tip, the stability of the fluid stream was
increased.
Preferably, the gas bubbles are generated intermittently.
The method may further comprise the step of generating pulses of
the acoustic energy. Furthermore, the acoustic energy within the
pulses may be frequency or amplitude modulated.
Preferably, the liquid flow impacts the surface with waves of
bubbles and pulses of acoustic energy which substantially
simultaneously reach the surface.
Preferably, the acoustic energy is introduced into the liquid by
an acoustic transducer as the liquid flows through a substantially
conical chamber of decreasing radius extending from the transducer
towards the nozzle.
Such a conical chamber is not essential, and other chamber shapes,
for example of constant cross-section, formed by a body such as a
cylindrical pipe, may be used in certain applications.
Preferably, a liquid input flow into the chamber is divided into a
plurality of parallel flows by an inlet manifold which comprises a
plurality of inlet passages each connected at an inlet end to the
inlet and at an outlet end to the chamber.
The method may further comprise acoustically isolating an inlet
conduit of the chamber from the acoustic transducer.
The present invention further provides a method of cleaning a
surface, the method comprising the step of providing gas bubbles
at the surface and employing modulated acoustic energy to generate
surface waves in the bubbles to cause non-inertial collapse of the
bubbles.
Preferably, the bubbles and acoustic energy are in a liquid flow
directed towards the surface.
Typically, the surface includes at least one cavity, recess or
pore and the bubbles are dimensioned to be able to enter at least
one cavity, recess or pore. Preferably, the acoustic energy
excites the surface of the bubbles when the bubbles are located in
the at least one cavity, recess or pore.
Preferably, the bubbles and acoustic energy are directed to the
surface as pulses so that the pulses of bubbles and acoustic
energy are incident on the surface substantially simultaneously.
The present invention is at least partly predicated on the finding
by the present inventors that when ultrasonic cleaning is carried
out, it is not necessary for cleaning that inertial cavitation is
generated. The preferred embodiments of the present invention
provide a cleaning apparatus adapted to achieve surface cleaning
(decontamination) by the employment of bubble action on a surface
(or within a crevice within a surface) driven by acoustic
stimulation. This avoids inertial collapse at the interface and
hence the associated parasitic erosion mechanisms of known
ultrasonic cleaning systems and methods. However, it is possible
optionally to generate inertial cavitation in accordance with some
embodiments of the present invention if the surface to be cleaned
is sufficiently robust.
Without being bound by theory, it is believed that in accordance
with the preferred aspects of the present invention, the motion of
the bubble process is dominated by the pressure within the gas
phase which results in non-inertial cavitation, rather than the
converging inertia of the liquid which results in inertial
collapse. The cleaning can be further enhanced by the
establishment of surface waves on the bubble wall (also sometimes
referred to as bubble shape oscillations). Therefore the apparatus
and method of the present invention are preferably adapted to
produce bubbles remote from, but close to, the solid/liquid
interface of the object to be cleaned and then to drive them
against that surface with an appropriate sound wave sufficient to
produce non-inertial collapse and, if applicable, surface waves on
the bubble wall.
However, as described above, for some particularly robust surfaces
inertial collapse may additionally be achieved at the surface
which may provide enhanced cleaning without excessive damage to
the surface. A further feature of the preferred embodiments of the
present invention is to deliver such cleaning ability, using
non-inertial cavitation, through a liquid stream or hosepipe/tap
output, which avoids the need for immersion, and so makes the
apparatus portable. This may be achieved by a suitable adaptation
of exisiting cleaning systems which currently use hosepipes or
taps to deliver a flow of cleaning fluid. Such an apparatus of the
preferred embodiments of the present invention system may also
conserve water and/or power compared to a known immersion system.
The preferred embodiments of the present invention can provide
apparatus and methods which employ a novel application of the
excitation of gas bubbles within liquids with the ultimate aim of
surface decontamination. Substantially any surface may be cleaned
in accordance with the invention, ranging from internal or
external surfaces, hard or soft surfaces, inorganic objects (e.g.
an endoscope), organic or living bodies, including foodstuffs,
(e.g. wrinkles on a lettuce), the human skin (e.g. under the
fingernails of a surgeon), using portable or fixed hoses and taps
(e.g. for forensic, autopsy, archaeological examinations). The
surface for decontamination might include buildings, facilities,
infrastructure (e.g. abattoirs, hospital wards, surgeries), and
associated objects contained within those (personnel, keyboards,
telephones etc.), or used outside. In particular, those apparatus
and methods employ targeted excitation of bubbles at the surface
of an interface or within a pore, cavity, recess, crevice, pipe,
tube or chamber. These bubbles have been shown to do useful work
including the cleaning the surface, pore, cavity, recess or
crevice within the surface, or cleaning in a pipe, tube or
chamber. As such this represents a new and powerful method to
clean a wide variety of surfaces.
In particular, the present invention is at least partly based on
the findings by the present inventors that surface cleaning may be
achieved through the generation of bubble oscillation (including
surface waves) driven by appropriate acoustic excitation. Also,
crevice cleaning may be achieved through bubble capture into pores
and other surface features, including, but not restricted to,
capture through processes of flow, hydrodynamic effects, or
acoustic radiation forces. These bubbles oscillate and remove
material from the crevice. The acoustic excitation of these events
may be achieved along a flowing stream of liquid. Bubble
population effects may be harnessed to allow transmission of sound
down through the liquid to the surface to be cleaned. The flow
apparatus, geometry, materials and acoustic characteristics of the
bubble population may allow efficient acoustic transfer to the
surface to be cleaned. Relatively low flow rates may be deployed,
minimising cleaning solution wastage.
As an additional preferred mechanism to generate bubbles,
electrochemical bubble seeding technology has been developed and
exploited. Pulsed bubble generation (creating a bubble swarm) in
tandem with pulsed acoustic excitation may generate 'active'
bubbles on the surface to be cleaned. An amplitude or frequency
modulated sound field, coupled with the acoustic energy optionally
being switched on and off, may be employed to maximise the
acoustic pressure delivered by the apparatus to the interface in
the presence of a suitable bubble swarm. Pulsed bubble generation
and pulsed acoustic excitation may be independently controlled so
that at the nozzle bubble generation is independent of the
generation of a pulse of the acoustic excitation, and such
independent control can vary the bubble pulses and the acoustic
energy pulses independently so that at the surface to be cleaned
the bubbles and the acoustic energy pulse can be incident on, or
in the vicinity of, the surface substantially simultaneously to
enable efficient cleaning of the substrate by the acoustic energy
causing non-inertial cavitation of the bubbles at or in the
vicinity of the surface.
Such pulsing of the acoustic energy does not need necessarily to
turn the sound field off between pulses, but instead may modulate
the acoustic energy, by amplitude or frequency modulation, it to
provide high energy acoustic pulses separated by low energy
background.
In some embodiments, the sound is turned off as the bubble swarm
travels down the stream (to prevent acoustically-induced bubble
coalescence), and then the sound is turned on to provide a
modulated acoustic energy pulse once the bubble swarm reaches the
surface to be cleaned. Once these bubbles have undertaken some
cleaning and started to disperse in the flow, the sound is turned
off and another swarm of bubbles is generated at the nozzle and
the process is repeated. The independent control can be achieved
by taking into account the fact that sound travels down the liquid
stream at a different speed to the bubbles. The timing of the
current supplies used to generate bubbles and sound is such as to
ensure both bubble swarm and ultrasound arrive at the surface at
the same time. Given this criterion, the different transit times
of bubbles and sound down the tube dictate the timing for the
activation of the currents which generate sound and bubbles, such
that their activations may be staggered if the timing so dictates.
The underlying technical concept is to utilise their different
transit times down the liquid stream to ensure that the bubbles
and acoustic energy occur at the same time at the surface which is
to be cleaned.
In addition, novel electrochemical techniques may be used to
monitor the degree of in situ cleaning as a result of fluid flow
and bubble action on the surface. The invention may also include
apparatus for monitoring the efficacy of the cleaning through the
use of sensors close to the location where the surface to be
cleaned is to be placed, or embedded in that surface.
Accordingly, the cleaning apparatus may further comprise a device
for monitoring the cleaning of the surface, the device comprising
first and second electrodes, forming an electrochemical cell,
adapted to be respectively located at a portion of the surface and
interconnected by a resistance measuring apparatus.
The present invention further provides a method of monitoring the
cleaning of a surface, the apparatus comprising locating first and
second electrodes, forming an electrochemical cell, at
respectively portions of a surface to be cleaned and measuring the
resistance therebetween.
Typically, the first electrode is located in a cavity, recess or
pore to be cleaned and the second electrode is located on an
external portion of the surface.
Preferably, the method comprises determining a decrease in the
resistance to indicate cleaning of the cavity, recess or pore. For
the apparatus of the preferred embodiments of the present
invention, the nozzle material and shape, and the driving acoustic
frequency, may be chosen such that at least one mode is not
evanescent in the liquid stream. The nozzle may be designed to
prevent a strong impedance mismatch between the sound field in the
conical body and the sound field in the liquid stream. For some
applications (for example if the liquid stream is surrounded by
gas once it leaves the nozzle) a specific (but not exclusive)
preferred manifestation of this is in use of materials which are
exactly (or nearly) pressure-release in the construction of the
nozzle and/or conical body. The flow rate and nozzle design may be
chosen so that the liquid stream does not lose integrity before it
reaches the target surface to be cleaned (e.g. break up into
drops, entrain unwanted bubbles, etc.) to the extent that it
hinders the transmission of sound from the nozzle to the target.
The shape of the conical body may be designed to assist the
transmission of sound from the cone to the liquid stream and
subsequently through the nozzle. An amplitude or frequency
modulated sound field may dramatically improve pressure
transmission within the fluid flowing through the apparatus to the
target substrate.
Embodiments of the present invention will now be described by way
of example only, with reference to the accompanying drawings, in
which:
Figure 1 is a schematic side view
of a cleaning apparatus in accordance with a first embodiment of
the present invention;
Figure 2 is a schematic side view
of a cleaning apparatus in accordance with a second embodiment
of the present invention;
Figure 3 is a schematic
perspective view of a cleaning apparatus in accordance with a
third embodiment of the present invention;
Figure 4 is a schematic view of
an alternative shape for the cone for use in any of the
embodiments of the cleaning apparatus of the present invention;
Figure 5 is a schematic representation, as a side view, of a
sequence of steps in a cleaning cycle showning the generation of
acoustic energy pulses and gas bubble pulses producible by any
of the embodiments of the cleaning apparatus of the present
invention;
Figure 6 shows the phase
relationship between (a) acoustic energy (sound) generation and
(b) bubble generation, to provide the pulses, with respect to
time at the nozzle in any of the embodiments of the cleaning
apparatus of the present invention, and additionally shows the
acoustic energy modulation;
Figure 7 shows the acoustic
pressure signal recorded at the target surface for a modulated
and unmodulated pressure sequence using the duty cycle shown in
Figure 6;
Figure 8 shows the relationship
between pressure and time, measured at a hydrophone, generated
by the acoustic energy, either in continuous or modulated mode,
in the cleaning apparatus of any of any of the embodiments of
the present invention; and
Figure 9 shows the relationship
between resistance and time, measured at a surface, either clean
or unclean, for use in a method of monitoring the cleaning of a
surface according to an embodinment of the present invention.
Referring to Figure 1, there is shown a cleaning apparatus in
accordance with a first embodiment of the present invention.
The cleaning apparatus, designated generally as 2, comprises a
hollow body 4 defining a central chamber 6. The body 4 has a rear
wall 8 and a substantially conical element 10 extending forwardly
away therefrom which terminates in a forwardly-located orifice 12.
Typically, both the element 10 and the wall 8 are rotationally
symmetric, i.e. circular, although other geometric shapes may be
employed. In this specification the term substantially conical"
should be interpreted broadly to encompass structures which are
not only geometrically conical, but also structures which for
example are bell-like, having a concave inner wall as seen from
inside, as shown in Figure 1, or have a constant half-angle as
shown in Figure 2, or are horn-like as shown in Figure 4 (i.e. has
a convex inner wall as seen from the inside). Accordingly, the
element 10 forms a conical body such as a bell- or horn-like
structure, and for brevity may be referred to hereinafter simply
as a ,,horn".
A nozzle 14 extends forwardly from the orifice 12 and defines a
liquid outlet 16. A liquid inlet 18 is located at or adjacent to
the rear wall 8. A liquid supply conduit 20, typically in the form
of a flexible hose, communicates with the inlet 18. An acoustic
transducer 22 is mounted on the rear wall 8. A controller 23
controls the operation of the transducer 22. Typically, the
transducer 22 is mounted on an outer surface of the wall 8 and
extends over a substantial proportion of the surface area of the
wall 8. Alternatively, the transducer may be embedded into the
chamber on or through the rear wall. Indeed to achieve a
pressure-release condition in the chamber walls (e.g. the stream
was to be squirted into a cavity, such as into the internal bore
of an endoscope) then the appratus may employ either a rear-wall
transducer or alternatively make the inner surface of the horn
comprise a transducer. It should be noted that the transducer
could be mounted elsewhere such as on the walls of the horn or the
nozzle, providing that it was not required, for the particular
application, to match to a pressure-release boundary condition
once the stream has left the nozzle (e.g. if the stream was being
used to clean the inside of an endoscope).
In use, liquid flows continuously through the supply conduit 20
into the central chamber 6 and then outwardly through the outlet
16 of the nozzle 14 to form a stream 24 of liquid which is
directed against the surface 26 of a substrate 28 to be cleaned.
The surface 26 may, in particular, be provided with three
dimensional surface features, such as a crevice 30 shown in an
exaggerated form in Figure 1.
A bubble generator 32 is located within the nozzle 14 upstream, in
the direction of fluid flow, from the outlet 16. The bubble
generator 32 generates gas bubbles within the liquid stream so
that the liquid stream impacting on the substrate surface 26
includes not only acoustic energy from the transducer 22 but also
gas bubbles.
There are several options for seeding gas bubbles into the liquid
flow, including gas injection and in situ electrochemical gas
bubble generation by electrochemical decomposition of water in the
liquid. For in situ electrochemical gas bubble generation, the
incorporation of electrodes into the structure allows controlled
seeding, and is preferably achieved by threading 50-100 [mu]m
diameter Pt wires through the nozzle of the jet (~ 1 cm before the
exit). Other options including use of one or more electrodes in
the liquid flow, or in the wall of the nozzle.
Referring to Figure 2, in this modified embodiment, the rear wall
40 consists of a plate, for example of plastic or a metal such as
aluminium or stainless steel, having the liquid inlet 42 therein
and the acoustic transducer 44, which may itself incorporate a
truncated cone 46 is bonded or otherwise held on to the rear
surface of the plate 40. The substantially conical element 48,
comprising a horn, extends forwardly of the plate 40 and forms an
integral nozzle 50 at which the bubble generator 52 is located.
In the embodiment of Figure 1 the hollow body 4 may be made
integral and made of a single material, with the rear wall 8 being
integral with the horn 10. In this embodiment, the horn is
composed of material that can function as a pressure release
interface when fluid is directed thereagainst, so that acoustic
energy in the material of the horn is effectively and efficiently
transmitted into the flowing liquid at the inner surface of the
horn. The aim of the apparatus of this embodiment is to introduce
acoustic energy into the flowing fluid stream and then to direct
that stream through the outlet onto the surface to be treated by
using the conical shape of the horn to concentrate both the
acoustic energy and the fluid flow while minimising acoustic
losses or frictional loses against the conical surface.
The embodiment discussed above is directed to the specific
application of introducing sound energy into the liquid stream
when the liquid is surrounded by air after leaving the nozzle.
That is applicable, for example, for cleaning under a surgeon's
fingernails or cleaning a letttuce. However for other applications
in which the liquid stream is directed into the article to be
cleaned (such as cleaning up an endoscope) the liquid stream on
leaving the nozzle may not have a pressure-release boundary
condition, and for that embodiment a horn made of different,
non-pressure release, material may be employed.
The nozzle and the outlet are shaped and dimensioned to allow for
acoustic transmission along the fluid stream. It is advantageous
to form a smooth flow of the stream. It is well within the
abilities of a person skilled in the art to produce a suitable
combination of shape and dimensions for the horn, the outlet and
the inlet to achieve the desired smooth flow of liquid containing
acoustic energy from the transducer.
As described above, novel electrochemical techniques may be used
to monitor the degree of in situ cleaning as a result of fluid
flow and bubble action on the surface.
In this embodiment, the cleaning apparatus further comprises a
device for monitoring the cleaning of the surface, the device
comprising first and second electrodes 200, 202, forming an
electrochemical cell, adapted to be respectively located at a
portion of the surface and interconnected by a resistance
measuring apparatus 204. In the method of monitoring the cleaning
of the surface, the first electrode 200 is located on an external
portion of the surface and the second electrode 202 is located in
a cavity or recess to be cleaned. The resistance therebetween is
measured using the resistance measuring apparatus 204 to determine
a decrease in the resistance to indicate cleaning of the cavity or
recess. As show in Figure 9, an initial resistance value A changes
according to value B by decreasing significantly when the
monitored surface portion has been cleaned.
In this aspect of the invention, electrochemical cleaning
measurements under the tip of the ultrasonic horn, using either
flat, recessed or cannular electrodes, made use of an
electrochemical cell consisting of two or more electrodes. The
cleaning liquid acted as an electrolyte. The resistance of the
fouled working electrode was monitored such that when the cavity
or recess was unclean, the working electrode would have no or only
poor electrical contact with the liquid electrolyte and so a
relatively high resistance would exist between the two electrodes,
whereas when the cavity or recess was cleaned and therefore the
working electrode came into contact with the solution, a at low
resistance would be measured. This provides a very effective
method for quantifiably measuring the cleaning time. This method
relies on the measurement of the uncompensated resistance of the
system.
Therefore, if the uncompensated resistance of the system can be
monitored as a function of time, it is possible to use this method
to detect the cleaning of the cavity, recess or pore. This method
is able to give quantifiable data on the cleaning of the cavity,
recess or pore. In order to remove the effect of capacitance of
the electrode a low amplitude high frequency (100 kHz, 200 mV zero
to peak amplitude) alternating voltage signal was applied between
the working electrode (for example a 0.5 mm diameter Pt recessed
electrode) and reference electrode (normally a large piece of
metallic material, for example a Cu plate). The current passed can
then be used directly to determine the resistance of the system as
a function of time. Using this system the only chemical required
is a supporting electrolyte to make the solution conducting such
as potassium chloride (KCl) or common salt (NaCl) both of which
are relatively cheap and easy to dispose of. This method is used
to monitor the cleaning of a surface with a variety of features
designed to investigate the efficiency of targeted ultrasonic
cleaning.
If the monitoring component is not needed for a particular
application when carrying out the cleaning method of the
invention, and only cleaning is required, then an electrolyte such
as KCl or NaCl or equivalent is not necessary within the cleaning
liquid.
Referring to Figure 3, in an alternative embodiment, the inlet
conduit 60, in the form of a hose, is connected to a manifold 62
which divides the inlet liquid flow into a plurality of different
inlet flow passages each in a respective secondary inlet conduit
64, each secondary inlet conduit 64 connecting the inlet conduit
60 to a respective inlet 66 on the rear wall 68 conected to the
horn 70. The assembly of the rear wall 68 and the horn 70, and the
manifold 62, may be connected together within a common housing 72.
The horn 70 is connected to an outlet 74.
Referring to Figure 4, an alternative shape for the horn 80 is
disclosed, in which the horn 80 has a cylindrical downstream
portion 82 and a hyperboloidal (or some other horn- shape, such as
parabolic, catenoidal, etc.) outwardly flaring upstream portion
84.
It may be seen from all of Figures 1 to 4 that various different
shapes of configurations for the horn may be employed, provided
that the horn is shaped to provide a constant fluid flow outlet
minimising both loss of acoustic energy and frictional losses.
This would provide optimal acoustic and flow properties in the
stream of fluid which impacts the surface to be cleaned.
Furthermore, such a horn-shaped structure is not essential for
some applications and the chamber may have any other shape, and be
defined by any material of the body, that permits acoustic energy
to be introduced into the fluid flow and exit from a nozzle.
If the liquid stream is to flow through gas on leaving the nozzle,
and therefore the inner walls of the horn need to be
pressure-release, then a particularly preferred material for the
horn is a cellular foam material or a rubber which can avoid an
impedance mismatch between the sound field in the horn and the
sound field in the liquid stream flowing therethrough. The flow
rate and nozzle design are selected so that the liquid stream does
not lose integrity before it reaches the surface to be cleaned.
The shape of the horn is designed to assist the transmission of
sound from the horn to the liquid stream flowing through the
nozzle. For example, when the horn is a cellular foam material the
horn is formed by moulding a conical cavity within a solid foam
block (although other manufacturing processes, such as cutting
from a block, may be used).
Most typically, the horn and nozzle are rotationally symmetric.
In any of the embodiments, the inlet may be provided with an
acoustic isolation device which prevents acoustic energy being
transmitted back along the liquid supply conduit 20. The acoustic
isolation device, shown schematically as 25 in Figure 1, may
comprise an acoustic filter, optionally having a selected
frequency range, and/or a venturi narrowing in the conduit 20,
and/or an expansion chamber, and/or by control of the diameter of
the conduit to provide that the driving frequency is below the
cut-off frequency of all modes for the inlet (as would happen for
sufficiently small-bore manifold inlets made of pressure-release
material).
In these embodiments, the apparatus size can be varied to provide
varying volumes of the liquid stream. Smaller or larger volumes
can be achieved by scaling the flow rate, nozzle size and the
driving acoustic frequency, in line with the provision that at
least one mode is not evanescent in the liquid stream, thereby to
provide a cleaning solution stream impacted onto the surface
accompanied by a suitable sound field and active bubbles. This
mode may be the plane wave mode if the acoustic boundary
conditions at the wall allow. In order to achieve the required
volumetric flow rate, as well as enabling the flow to project to a
sufficient distance beyond the free end of the nozzle, a small
outlet aperture is required. Except for the plane wave mode (if
the acoustic boundary conditions permit it to propagate, which is
not the case if the liquid stream flows through air), then for
each mode the sound transmission down the liquid jet will be
undesirably restricted below a characteristic "cut-off frequency
(Fco). If the stream were to be passed into a solid tube, such
that solid wall would surround the stream, then the lowest
frequency mode would be plane wave and there would be no cut-off
frequency for that mode, although higher order modes would have
their own cut-off frequencies. In the particular case where the
liquid stream flows through a gas space on leaving the nozzle,
then the boundary condition at the curved walls of the stream
would be pressure-release, and for such a condition the cut-off
frequency (Fco) for the lowest mode is calculated according to the
equation
Fco = 2.4048c (Equation 1)
2[pi]a where c represents the velocity of sound in the fluid and a
represents the liquid stream radius. For example, for a flow
outlet of around 10 mm internal diameter, and assuming a speed of
sound in the liquid of 1500 m/sec, the cut-off frequency of the
liquid stream for the lowest mode would be on the order of 114 kHz
(modes of higher order would have higher cut-off frequencies).
However, fluid properties and any entrapment of bubbles, as
discussed hereinafter, would affect this cut-off frequency.
Bubbles, for example, may reduce the sound speed in the liquid,
and hence reduce the cut-off frequency of the mode.
The bubble generator 32 is adapted to generate gas bubbles which
are then acoustically excited and impact on the surface to be
cleaned. The bubbles are driven into oscillation by the acoustic
energy and can get into crevices and pores on the substrate to be
cleaned, so that they effectively clean the substrate surface.
The bubble generator 32 may act directly to inject gaseous bubbles
into the fluid flow, for example through a needle, the needle
optionally vibrating. Other options for bubble generation include
through use of cavitation (hydrodynamic or acoustic) or
free-surface bubble entrainment, or chemical gas production, or by
a more preferred route of electrochemical in situ generation of
gas bubbles by electolytic decomposition of the water in the
liquid flow. The bubble generator 32 adapted for electrochemical
bubble generation comprises an electrode comprising an array of
electrically conductive wires, for example platinum wise having a
diameter of 50 [mu]m, extending across the outlet. The electrode
is connected to a source of electrical energy (not shown) and,
when electrically powered, the electrical energy electrolytically
decomposes water in the fluid flow to generate streams of bubbles
of both oxygen and hydrogen gas which are entrained in the flowing
fluid and directed towards the target surface to be cleaned.
Figure 5 shows a sequence of steps in a cleaning cycle for a
respective bubble swarm.
As shown in Figure 5 (a), the bubble generator is controlled by a
controller 98 so that bubbles are formed intermittently to form
intermittent swarms 100 (or waves) of bubbles which successively
impact against the surface 102 to be cleaned. When the bubbles
impact the surface 102 to be cleaned, the bubbles are driven to
oscillate by the acoustic energy, thereby penetrating crevices
which are cleaned by the acoustic energy.
As also shown in Figure 5, the amplitude or frequency modulated
acoustic energy from the transducer is pulsed intermittently. This
produces pulses of acoustic energy, which interact with the
intermittent bubble swarms 100 described above, in a concerted
manner.
Figure 5(b) shows that when the acoutic transducer is switched
off, the bubble swarm 100 travels downstream together with the
liquid flow directed towards the surface 102. The bubble swarm 100
reaches the surface 102, as shown in Figure 5(c). Figure 5(d)
shows that as the bubble swarm 100 reaches the surface 102, the
acoustic transducer is switched on, to generate a sound field
pulse, optionally amlitude or frequency modulated, which is
transmitted towards the surface 102 at the speed of sound through
the liquid. The acoustic energy of the pulse activates the bubbles
of the swarm at the surface 102 to effect enhanced cleaning, by
non-inertial collapse of the bubbles at the surface, and
optionally generating surface waves in the bubbles, and/or
optionally causing higher energy cleaning events (e.g. inertial
collapse of bubbles, jetting etc.). This completes a cleaning
cycle for a single bubble swarm. A next cleaning cycle for a
subsequent bubble swarm is then initiated by generation of the
subsequent bubble swarm as shown in Figure 5 (a).
As shown in Figure 6, at the nozzle there is a particular phase
relationship between the generation of the sound pulse and the
generation of the pulse of bubbles. The phase relationship changes
as the sound and bubbles are transmitted away from the nozzle
through the liquid since the acoustic energy and the bubbles are
transmited at different velocities through the liquid towards the
surface to be cleaned. The aim is to provide a phase relationship,
which typically involves a delay time td betwen bubble generation
and generation of the pulse of the acoustic energy, so that the
acoustic energy and the bubbles reach the surface to be cleaned in
phase and at the same time. In the example illustrated, a delay
time td is provided which would vary with flow rate and distance
to the target.
In this embodiment the sound is turned off during bubble
generation and bubble transfer to the surface to be cleaned. The
excitation of these bubbles is intermittent, and in synchronism
with the intermittent on/off nature of the electrochemical bubble
generation. In the embodiment of Figure 6, the bubbles may be
generated for a generation period, typically 10 milliseconds, with
a periodicity of 100 milliseconds. After the termination of each
bubble generation, there is a delay, typically 30 milliseconds,
after which the sound is turned on (or, in other embodiments,
modulated to provide a high energy pulse) for a period of 60
milliseconds. The sound is then turned off and simultaneously the
bubbles are turned on, in a subsequent cleaning cycle.
These values apply to one particular device, but would be longer
or shorter if the device was larger or smaller in size
respectively. This delay is flow rate and distance dependant. It
can be variable and, for example, if a long pipe (endoscope) is
the cleaning target, the delay can be varied to achieve cleaning
at different positions along the liquid flow direction.
Within each acoustic energy pulse, the acoustic energy is
amplitude or frequency modulated, as also shown in Figure 6
(amplitude modulation being exemplified by varying the driving
voltage of the transducer). The modulation period, which is
frequency dependent, is typically 1 millisecond.
As shown in Figure 7, the pulsed generation of such bubble swarms
causes modulated pressure to be applied to the cleaned surface by
each bubble swarm when the respective bubble swarm impacts the
surface to be cleaned. Such modulated presssure typically occurs
every 100 milliseconds. As explained earlier, each bubble swarm is
oscillated by the acoustic energy which produces a cleaning
effect.
Figure 8 shows pressure at a hydrophone for a constant drive
acoustic field, either in continuous mode or modulated mode. As
shown by Figure 8, when a constant acoustic energy impacts on the
surface, the pressure generated at the surface is relatively low
and constant, whereas when modulated waves of acoustic energy
impact the surface, the maximum energy released at the surface by
each wave is significantly greater.
Therefore by employing pulsed bubble generation and pulsed
generation of acoustic energy in a coordinated manner, bubbles are
excited at the surface so that bubbles are present at the surface
when the acoutic energy is also at the surface, and furthermore
the cleaning impact achieved by both the bubbles and the acoustic
energy is increased by additionally providing that the acoustic
energy is amplitude or frequency modulated at a higher frequency
that the pulses, greatly improving cleaning efficacy. The presence
of a bubble swarm formed between a pair of acoustic energy pulses
separates those acoustic energy pulses. Each bubble swarm is
independently impacted on the surface to be treated and
independently excited by the acoustic energy of the succeeding
acoustic energy pulse.
In accordance with a further aspect of the apparatus and method of
the present invention, it has been found that the addition of a
surfactant to the liquid can afffect the bubble size achievable
without bubble coalescence. Sufficient surfactant may be added, if
necessary, to prevent coalescence of bubbles as they flow down the
stream if, without surfactant, such coleascence produces bubbles
too large for approprite cleaning; but not so much surfactant that
the bubbles are too small for cleaning when they reach the site.
The following Table 1 shows how the bubble size (estimated from
high-speed camera experiments) is affected by the surfactant
loading, and how the activity, defined as erratic bubble motion
across a surface, which is indicative of bubble oscillation by
acoustic energy, varies with bubble diameter.
Table 1
