Surface Contamination. Proceedings of a Symposium Held at Gatlinburg, Tennessee, June 1964

397 Pages·1967·13.12 MB·English

by  B. R. Fish (Eds.)

SURFACE CONTAMINATION
Proceedings of a Symposium held at
Gatlinburg Tennessee
June 1964
Edited by
B. R. FISH
SYMPOSIUM PUBLICATIONS DIVISION
PERGAMON PRESS
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Copyright © 1967
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First edition 1967
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PREFACE
RECOGNITION of the potential hazard presented water, and other general environmental pollu-
by noxious contaminants on surfaces is by no tion. Each of the contaminants discussed in the
means new. Writing in 1890, Prudden(1) Symposium has its own peculiar properties and
described measurements made in hospital wards associated problems, and probably a separate
showing a definite relationship between person- international meeting could be justified for each,
nel movement, sweeping, etc., on airborne independent of the other. However, there are
bacteria. Prudden's remark concerning skeptics basic similarities in the needs for fundamental
is still applicable and is quoted as follows. research on the dynamics of contamination
control, and there are close parallels in the
Many usually very reasonable persons, when brought
problems of establishing and administering
face to face with such disagreeable facts—are disposed
to petulantly exclaim that they and their friends have contamination control programs. It was the
got along very well thus far with the dust which they purpose of the meeting to bring together
have encountered and that they don't want to be
administrative and technical persons in order to
worried with the possibilities of danger which may lurk
unseen about them. exchange information on surface contamination
control and to identify areas requiring future
Much later on, in the medical literature of the
research emphasis.
1930's, a typical comment regarding the signifi-
Four sessions of the Symposium were devoted
cance of bacterial contamination on surfaces
to fundamental research and development in the
said, in substance, that there are many more
fields of aerosol physics, surfaces, adhesion-
strong opinions on the subject than there are
redispersion, and transport-deposition. Other
results of research upon which to base any
administrative and technical problems were
opinion. Unfortunately, these statements remain
discussed in sessions on radioactive surface
substantially true today.
contamination control criteria, measurement
The first International Symposium on Surface
techniques, environmental control of surface
Contamination was convened in recognition of
contamination, dissemination of airborne micro-
the increasing importance of contamination in
organisms, radioactive contamination control
regard to the health and safety of man as well
applications, biological and chemical surface
as the integrity of his scientific and technical
contamination, insurance and economics, and
machinery and products which must meet the
decontamination. A review of the papers
exacting requirements of the space age. In scope,
presented shows a clear need for more intense
the meeting covered broad areas of interest
study in each of the areas covered. Especially
related to redispersible and evaporable con-
lacking, but which it is hoped may be forth-
tamination, including radioactive, biological,
coming in some future meeting, was the report
chemical, and abrasive contaminants; however,
of any significant results pertaining to surface
the subjects were confined primarily to con-
design, selection and pretreatment to control
tamination of limited areas such as in a room
the deposition and redispersion of contaminants.
or other semi-isolated environments and did
The aid and encouragement of the session
not include the very important subjects of air,
chairmen in conducting the meeting and their
valuable comments in informal panel discussions
(1) T. MITCHELL PRUDDEN, M.D., Dust and Its Dangers,
are appreciated and are gratefully acknowledged.
G. P. Putnam's Sons, New York (1905).
ix
χ PREFACE
Session chairmen were : Mr. Lawrence B. Hall, National Aeronautics
Dr. C. N. Davies, London School of Hygiene and Space Administration (Biological and
and Tropical Medicine (Aerosol Physics). Chemical Contamination).
Dr. Sydney Ross, Rensselaer Polytechnic Mr. R. G. McAllister, Liberty Mutual
Institute (Surfaces). Insurance Company (Insurance and Econ-
Dr. Morton Corn, University of Pittsburgh omics of Surface Contamination).
(Adhesion-Redispersion). Mr. P. Cerre, Service de Contrôle des Radia-
Dr. S. K. Friedlander, California Institute of tions et de Génie Radioactif, C.E.A.,
Technology (Transport-Deposition). Saclay (Decontamination).
Mr. H. J. Dunster, UKAEA-Harwell (Control The Symposium was sponsored jointly by the
Criteria for Radioactive Surface Con- American Association for Contamination Con-
tamination). trol, the Health Physics Society and the Oak
Mr. J. R. Prince, Oregon State University Ridge National Laboratory. These organi-
(Measurement Techniques). zations helped to disseminate information con-
Dr. J. L. Anderson, Space Research, Inc., cerning the Symposium and various individual
Orlando, Fla. (Environmental Control of members made valuable contributions in time
Contamination). and effort toward the conduct of the meeting.
Prof. T. W. Kethley, Georgia Institute of
Technology (Dissemination of Airborne BIRNEY R. FISH
Microorganisms). Oak Ridge, Tennessee
Mr. E. D. Graham, Argonne National (1966)
Laboratory (Radioactive Contamination
Control Applications).
AEROSOL PROPERTIES RELATED TO SURFACE
CONTAMINATION
C. Ν. DAVIES
London School of Hygiene and Tropical Medicine, London, W.C.I, England
1. DEPOSITION MECHANISMS be adjusted back to that of the aerosol, averaged
over a long time, by multiplying each size group
The deposition of aerosol particles on the
by the reciprocal of the square of the Stokes'
surfaces of a room can be effected in a number of
diameter, for sizes down to 1μ at unit density,
different ways, apart from their inertia giving
or to ρ~*μ, for density p. This is a valuable and
them a "stop-distance" along which they can be
little used technique for gauging airborne and
projected to encounter a surface. Inertia deposi-
surface contamination; allowance should be
tion is associated with moving air; only relatively
made for the difference between the Stokes' and
calm air, in enclosed spaces, will be considered
the observed diameters (DAVIES, 1962, 1964).
here.
It is not possible to classify deposition
3. BROWNIAN MOTION
mechanisms as belonging to the particle or to
the surface; mutual action is often involved. In addition to rate of fall, a property unique
Even gravity does not exclusively act on the to the particle is its Brownian motion. Assuming
particle since it may initiate convection of air and that no forces exist between the aerosol particles,
influence deposition by other processes than and that each has the same average kinetic
settlement. energy as a molecule of air, the mean square
displacement of a particle in any direction during
2. SEDIMENTATION DUE TO GRAVITY time, t is
9
In terms of weight of material, sedimentation
due to gravity is the most important deposition
process. It should not be disregarded as an index
of aerial contamination. Horizontal pipes and where \kT is the average kinetic energy in that
ducts, of circular section, invariably carry on the direction and Zndr\\F is the resistance to move-
upper surface a deposit graded according to the ment at unit velocity, including the Cunningham
cosine of the angle of inclination. The rate of slip factor, F.
fall of the particles is proportional to their The mean square displacement is very small,
density and to the square of their Stokes' as Table 1 shows; the last column gives the root
diameter, for sizes from lμ to 30μ at unit density. mean square displacement, due to Brownian
The air in most rooms is in random convective motion, in one day. Only those particles which
movement, as is evidenced by the uniformity of are within this distance of a surface will have a
deposit on upwards facing surfaces; similar chance of reaching it in one day. Considering
size-distributions are obtained from samples of that a horizontal surface collects, in one day,
deposits at different levels, under these circum- even particles as small as l μ diameter from a
stances, and the observed size-distribution can height of 300 cm, by settlement under gravity, it
ι
2 C. Ν. DAVIES
will be appreciated that Brownian deposition side possess a higher average velocity than those
is negligible for sizes above ΟΌΙμ diameter when coming from the cool side. The particle velocity
the air is at rest. is proportional to the temperature gradient and
to the reciprocal of the pressure of the gas; it is
Table 1. independent of particle size.
For particles exceeding ΙΌμ diameter the
•
Particle diameter, d Pit thermophoresis velocity is also independent of
their size but in ordinary air it is only about a
0 001 μ 1-02.10" ^m^sec 94 cm/day quarter as fast as the figure for very small
001 105.10- 3 9-5 particles. The velocity is approximately 0Ό7
01 1-36.10-5 108
cm/sec for a temperature gradient of 100°C/cm;
10 5-5 .10"7 0-22
this is about the same as the rate of fall of a 5μ
diameter particle. Thermal deposition can there-
fore considerably exceed sedimentation in the
4. GAS DIFFUSION PROCESSES OF DEPOSITION
presence of quite modest temperature gradients
Gaseous diffusion is enormously faster than
and is overwhelmingly the most important
particle diffusion due to Brownian motion, by the
mechanism for the deposition of sub-micron
order of the square root of the ratio of the weight
particles.
of the particle to the weight of the gas molecule,
Between ΙΌμ and 0Ό3μ diameter the velocity
a factor of 104 to 106. Gaseous self-diffusion
increases. No theoretical treatment of this range
goes on continuously with no net effect upon
of sizes, comparable with the mean free path of
suspended particles because it is the same in all
the gas molecules, has been attempted but a
directions. If, however, circumstances impose a
number of experimental observations have been
gradient of molecular velocity or molecular
made.
weight upon the space occupied by the gas,
The motion of particles smaller than the mean
directional forces are set in action upon sus-
free path of gas molecules (about 0·07μ in
pended particles; they are of a kinetic or fluid
ordinary air) is adequately accounted for by
mechanical character according as the particles
theory but there are difficulties in interpreting
are small or large in relation to the mean free
experimental data for large particles. These do
path of the gas molecules. The resulting particle
not respond to the differences in the impacts of
movements can be very much more rapid than
gas molecules coming from hot and cold regions
particle diffusion and play a significant part in
because they are too heavy. A radiometric
surface contamination.
force is produced, however, if the particle is a
good enough insulator to acquire a temperature
5. THERMOPHORESIS
gradient along its surface in the same direction
Gradients of gas-molecular velocity arise from as that in the gas.
temperature differences which may be imposed A tangential gas flow is set up, towards the
on the gas by the enclosing surfaces or may hotter region, with the maximum velocity
originate in the particles by their absorbing distant one mean free path from the particle
radiation. Thermal deposition may result when surface. The reaction on the particle drives it
the temperature gradient is imposed externally down the temperature gradient (FUCHS, 1964).
and causes thermophoresis of aerosol particles. Difficulties arise because the particles of high
For a particle smaller than about 0-03μ thermal conductivity are found experimentally
diameter, in air at one atmosphere, movement in to move nearly as fast as thermal insulators;
a gradient of temperature results because the this is 20-40 times faster than theory indicates.
molecules of gas which strike it from the warm The reason appears to be the neglect of the
AEROSOL PROPERTIES 3
distortion of the original distribution of gas- similar effects to the gradient of molecular
molecular velocities which is caused by the velocity, resulting from temperature difference,
presence of the particles. Allowance for this has but may be isothermal.
recently been made by DERYAGUIN and BAKANOV At least two gases must be present and a
(1962). concentration gradient is necessary; gas diffusion
A recent observation, so far unexplained proceeds along this direction and small particles
theoretically, is that particles from 6μ down to are impelled in the direction of the diffusion
1·5μ diameter, which move at a constant flow of the heavier gas by differential molec-
velocity in still air, show a rise in thermophoretic ular bombardment; this is diffusiophoresis
velocity with decreasing size when the thermal (WALDMANN, 1959).
motion takes place at right angles to a stream of Particles which are large compared with the
aerosol (DAVIES, 1964). mean free path experience a fluid-mechanical
Each of these departures from theory is in the force due to Stefan flow. This is a bulk gas
direction of enhanced deposition. movement which preserves constant pressure
when the components of a mixture of gases
6. PHOTOPHORESE; diffuse at unequal rates (DERYAGUIN and
DUKHIN, 1956).
When the temperature gradient results from
Imagine a vapour, not necessarily of higher
the absorption of radiation by a particle the
molecular weight than air, which is condensing
phenomenon is termed photophoresis. If the
upon a surface below the dew point temperature.
particle absorbs light and is a thermal insulator
Gaseous diffusion is essentially a process of
it becomes heated on the side which receives
interpénétration; hence air molecules diffuse
radiation, warms the adjacent gas and moves
away from the surface as vapour molecules
away from the source of radiation by one of the
diffuse towards it. The vapour molecules are
two mechanisms described for thermophoresis.
condensed at the surface, so there is no build-up
The resulting force is considerably greater than
of vapour, but neither is there a source of air at
radiation pressure.
the surface. Hence a drift of vapour-air mixture
Complicated situations arise with transparent
towards the surface is established, to avoid the
and selectively absorbing particles which may
creation of a pressure deficiency, and constitutes
become hotter on the side remote from the
the Stefan flow which exerts a hydrodynamic
source of radiation and move towards it;
drag on aerosol particles and impels them
magnetic particles may execute a helical motion
towards a surface upon which vapour is condens-
(ROHATSCHEK, 1956).
ing. Conversely they are repelled away from an
Although no specific instances of the deposi-
evaporating surface which is surrounded by a
tion of particles being occasioned by solar
dust-free space resembling that around a hot
radiation have ever been cited, there is little
object.
doubt that it is an appreciable factor, particu-
The effect can be estimated numerically from
larly since the force acts continuously and is
the equation of DERYAGUIN and DUKHIN (1956).
independent of the distance from the surface.
For water condensing on a cold wall it produces
It is a possible mechanism for the transport of
a deposition velocity for aerosol particles of the
cosmic dust into the troposphere.
order of magnitude of the rate of fall of 1μ
diameter particles; the effect is therefore
7. DIFFUSIOPHORESIS AND STEFAN FLOW
unlikely to be large, but, under certain circum-
The other way in which gas diffusion can stances, aerosol particles could be encouraged
induce a motion of particles is in the presence of to deposit on cold surfaces in a room by Stefan
a gradient of molecular weight. This produces flow.
4 C. N. DAVIES
8. ELECTRICAL DEPOSITION Deposition due to a cloud of similarly charged
particles expanding by mutual repulsion is
Aerosol particles will move in a uniform
unaffected by air movements since a uniform
electric field only if they are charged; in a non-
concentration is obtained at all points at any
uniform field, however, they become induced
instant. The total rate of deposition on the
dipoles and move towards the stronger field
containing walls is approximately
intensity unless the original charge on a particle
is large enough to impose an initial direction of dn/dt = -4nn2q2F/3^d
motion. where n is the number of particles per cm3 and q
The particles of an aerosol which carried a is the charge per particle in electrostatic units.
unipolar charge repel one another so that the The general theory has been worked out by
cloud expands and the particles deposit on the PICH (1962).
surfaces enclosing it. Using the above simple formula, the rate of
The velocity of a particle carrying a charge of deposition of aerosols carrying 10 electrons per
q electrostatic units in a field of 1 V/cm is particle has been calculated. The results are
shown in Table 3 as the aerosol concentration,
u = qF/900 πηα
in number of particles per cm3, which is necessary
Table 2. to result in deposition on the enclosing walls of
1/10 of the aerosol per hour.
Electrical mobility
Particle diameter, d (q = 4-8.10"10 e.s.u.) Table 3.
0-001/* 20 cm/sec (V/cm)
001 21.10-2 Concentration needed to cause
01 2-7.10-4 1/10 of the particles to deposit in
10 1-1.10-5 Particle diameter, d one hour
100 9-4.10-7
ΟΟΟΙμ 0 08 particles/cm3
001 7-5
01 570
In Table 2 the electrical mobility, or velocity 10 14,600
in a field of 1 V/cm when carrying a charge of
1 electron (4-8. 10"10 e.s.u.) is given for
The table shows that appreciable wall loss due
particles of various sizes. The table shows that
to homopolar charging occurs for sizes up to
deposition velocities exceeding 1 cm/sec (which
01 μ or more and that the rate is very large
corresponds to the settlement of 15-20μ dia-
for ultrafine aerosols. For a charge of only one
meter particles) are available for particles
electron per particle the last column needs
below 01 μ diameter, even if they carry a
multiplying by 100.
charge of only a few electrons, in fields of
1000 V/cm or less. As long as the particles have a FOSTER (1959) has performed experiments
upon the deposition of homopolar smokes.
slight charge, very fine sizes are quickly removed
When the charges are of the same sign but differ
by moderate electric fields; above 0·1μ really
in magnitude the expansion of the cloud is
powerful fields and large charges on the particles
decreased by the induction of dipoles in the more
are necessary. Plastic surfaces are often very
weakly charged particles.
good insulators and hold static charges for a
long time, but only in exceptional circumstances
is contamination by particles greater than 9. CONCLUSIONS
01 μ diameter likely to result from adventitious It is difficult to see what other aerosol
electrical deposition. properties could be associated with the deposition
AEROSOL PROPERTIES 5
of surface contamination in enclosed spaces; the account of their high mobility due to the
loss or the retention of contamination is not an Cunningham slip factor. The question then arises
aerosol property and is considered by other as to how small a particle has to be for its
authors in this symposium, as is the influence thermal energy to cause it to evaporate from a
of the nature of the surface. surface soon after deposition.
The deliberate enhancement of the rate of The work of CORN (1961) suggests that
deposition by working on the aerosol with particles of 50μ diameter adhere to a dry surface
acoustic radiation, or some other means, also with a force of the order of 01 dyne. The
seems irrelevant to the accidental contamination molecular attractive force can then be taken as
which has been the subject of this paper. about 0Ό02</ dynes and the work of removal as
μ
Summarizing the findings we conclude: 0·002</. IO"8 ergs.
μ
(i) Gravitational deposition is very effective The thermal energy is of the order of
for particles down to about 1μ, unit 3fcT/2«4. 10"14 ergs. The critical particle size
density spheres. below which spontaneous evaporation might be
(ii) Brownian motion is negligible for sizes expected is thus
over 0-01 μ.
ά = 4 . 10" 14/2 . 10" 11 «0 002/1
(iii) Thermophoresis is very important for μ
sizes below 5μ and is the main mechanism This is getting down to aggregates of compara-
around 01 μ. tively few molecules. It seems as though mole-
(iv) Photophoresis could be important in the cular adhesion can be relied upon to overcome
presence of sunlight over a similar range the tendency towards thermal evaporation as
of sizes. long as the concept "particle" is thermo-
(v) Diffusiophoresis and Stefan flow are dynamically valid.
unlikely to be major factors but could
cause a rather slow precipitation of
aerosol particles along with moisture REFERENCES
condensing on cold surfaces.
CORN, M.,J. Air Poll Contr. Assoc. 11, 523, 1961.
(vi) In the presence of static charge on
DAVIES, C. N., Nature, 195 (4843), 768, 1962; lb. 201
surfaces, electrical deposition of particles (4922), 905, 1964; Recent Advances in Aerosol Re-
below 0·1μ is increasingly rapid as the size search. Pergamon Press, Oxford, 1964.
DERYAGUIN, Β. V. and BAKANOV, S. P., Dokl. Acad.
decreases. Very strong fields would be
Nauk SSSR. 147, 139. Nature 196, 669, 1962.
needed for larger particles. Homopolar
DERYAGUIN, Β. V. and DUKHIN, S., Dokl. Acad. Nauk
aerosols can deposit by mutual repulsion SSSR. Ill, 613, 1956.
of the particles at a high rate for ultrafine FOSTER, W. W„ Brit. J. Appi. Physics. 10 (5), 206, 1959.
aerosols. Above 0·5μ, however, the effect FUCHS, Ν. Α., The Mechanics of Aerosols. Eni. edn.
Pergamon Press, Oxford, 1964.
is small unless the charging is high.
PICH, J., Staub. 22 (1), 15, 1962.
It is worth noting that, apart from gravity,
ROHATSCHEK, H., Acta Phys. Austriaca. 10 (3), 227, 267,
the processess considered tend to favour the
1956.
deposition of very fine particles; this is on WALDMANN, L., Zeits. Naturforsch. 14a (7), 589, 1959.
LIGHT SCATTERING INSTRUMENTATION FOR COUNTING
AND SIZING PARTICLES
CARL V. SEGELSTROM, Jr.
Royco Instruments, Inc., Menlo Park, California
1. INTRODUCTION window illuminates it. While an individual
Counting and sizing of particulate matter has particle of cigarette smoke is too small to be
been accomplished for many years by manual detected by the unaided eye, when light strikes
techniques. That is, particles are collected by billions of them relatively close together the
any one of several techniques and then either a resultant scattered light is readily observed.
total or statistical count is made through a Photometric devices utilizing this principle have
microscope using a graduated reticule to deter- been in use for many years. These devices, while
mine size. As individual components of manu- useful for relating one environment to another
factured precision equipment have become or for comparisons of environmental changes
smaller and smaller and clearances between with time, do not provide size information or a
moving surfaces have decreased, the require- numerical count. To accomplish this it is neces-
ments placed on measuring techniques and sary to thin out or dilute the concentration so
equipment have increased. Occasionally it that the particles may be examined one at a time.
becomes necessary to increase the counting Obviously where the concentration of particles
frequency to provide a continuous count; in is low, such as in a well designed and maintained
some cases 100 per cent of the gas or liquid must clean room, dilution would not be necessary.
be examined, such as the gas in gas bearing The Royco Particle Counter operates on the
gyros. The atmosphere in clean fabrication and principle of right angle light scattering (Fig. 1).
assembly areas must be monitored at frequent Energy from a well regulated white light source
intervals. Small particle sizes must be measured, of high intensity is collected in a pair of lenses
at times below the reliable limits of manual and focused on a 1 mm χ 2 mm aperture in a
techniques. In certain situations it is necessary to shim stock knife edge. A pair of projection
know immediately when contamination levels lenses, in turn, focuses the energy onto an
exceed preset limits. For these and other illuminated volume 1x2x2 mm which is en-
reasons, it was necessary to evolve better closed by a black body. The light source used is a
techniques for counting and sizing particulate G.E. 2331 lamp which is operated at a filament
matter. The light scattering technique provides a temperature of about 2850°K. This produces
useful and accepted tool to meet this need. a good white light and also results in about
400 hr bulb life. Care has been taken in the lens
2. COUNTING AND SIZING AIRBORNE system to minimize random rays of light
PARTICLES traveling down the barrel and causing excessive
A. The light scattering technique stray light in the viewing area. Stray light in the
We have all observed cigarette smoke in a still viewing area must be minimized since a high
room on a sunny afternoon where the smoke level of background light will produce a high
settles in layers and the sun streaming in a noise output at the photomultiplier thereby
Β