Table Of ContentSTATIC
ELECTRIFICATION
BY
LEONARD B. LOEB
PH. D. PROFESSOR OF PHYSICS
UNIVERSITY OF CALIFORNIA
BERKELEY, CAL./USA
WITH 63 FIGURES
S P RI N GER -VER LA G
BERLIN . GOTTINGEN . HEIDELBERG
1958
ISBN 978-3-642-88245-6 ISBN 978-3-642-88243-2 {eBook}
DOI 10.1007/978-3-642-88243-2
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Dedication
To Professor F. P. BOWDEN and his assoeiates, whose beautiful
pioneering investigations on the neglected classieal problems of
friction have done so mueh to clarify that important field and by its
clarifieation has aided so materially in the understanding of the
more baffling aspects of statie electrifieation, this book is humbly
dedieated.
October 5, 19S 7 LEONARD B. LOEB
Preface
In our preoccupation with thc dramatic developments in the numerous fields
of modern physics with their beautiful instrumentation and exciting revelations,
we tend to forget our profound ignorance of some of the longest known pheno
mena of physics. Among these were, until the middle nineteen hundred and thir
ties, ferromagnetism, friction, lightning stroke, the common electric spark, and
static electrification. The first two have now been pretty weIl clarified and the
und erst an ding of both of these phenomena have contributed greatly to our under
standing of the structure of matter and surface physics. The lightning stroke
and common spark are weH on their way to clarification. Strangely despite the
ever expanding importance of static electrification in industry affecting as it does,
a wide diversity of processes either as a useful tool or adversely and extending
even to the realms of meteorology, this field has awakened little curiosity and
stimulated little investigation in recent years except in so far as the immediate
industrial problems it invoked required an immediate and often make-shift
remedy.
Trained in his early years as a chemist, and brought into contact with some
aspects of colloidal chemistry involving electrokinetic potentials, cataphoresis,
and spray electrification, thc author had his curiosity aroused by a number of
these strange phenomena. Entering physics as a life career coincident with the
development of the earl)' studies in atomic structure, in part through his teacher,
R. A. MILLIKAN, the author became aware of the earlier confusions and contro
versies concerning elcctro-chemical potentials, the Volta potentials, the contact
potentials and work functions and their role in electrification. Thus as the years
have gone by while his chief interest focussed on the basic processes of gaseous
electronics and the related atomic physics, the author never could resist the urge
of his curiosity concerning various aspects of the several processes leading to
static electrification. In consequence, over the years, in addition to following
the progress in the fields of major interest, he and his students have, from time
to time, engaged in studies of various aspects of the static phenomena.
These led to aseries of rcsearches beginning in the middle nineteen hundred
and thirties on spray electrification of liquids which in the immediate prewar
years and subsequent to \Vorlct. War II, ended in a rat her long continued program
involving first the static electrification of dusts and subsequently of contact
charging of solids. With improving vacuum techniques, increasing knowledge
of the solid state and cspecially inspired by BOWDEN and TABoR's excellent book
on Friction and Lubrication, the studies took on a form such that much clarification
concerning the processes at work was achieved. Through the generosity of the
Office of Naval Research, beginning in 1947, grants became available so that it
was possible to support some excellent half time graduate assistants while they
worked for their doctorates whose curiosity also was aroused by this field. Thanks
VI Preface
to this support, the study of P. E. WAGNER, in some measure, has brought the
program to a degree of completion that makes a general summary of the work
seem desirable at this time. However, in addition to the integration and reporting
of the program completed in the author's laboratory, the crying need for a general
and critical summary of the whole lield 01 static electrilication processes and with
the encouragement of Professor BOWDEN, the author undertook the task which
resulted in this book at the end of 1955.
Since the processes of static electrification, and there are many competing and
confusing processes obscuring our understanding, have never been brought
together under one head before, the present attempt no doubt will have its
weaknesses and many flaws. It will, however, it is hoped, pave the way for more
and perhaps more properly directed and controlled researches and thus will, it
is hoped, lead to the publication of a bigger and bette r book when the time is
ripe.
The book is written for all those who are interested in, or curious about the
phenomena in their various aspects. It is aimed at explaining and clarifying the
basic physical principles underlying the processes. It does not attempt to solve
the many technical problems which industry requires. These, where solutions
exist, are buried in confidential files and in any case, present individual problems
that must each be solved separately, using knowledge of general principles.
The book is therefore written for the engineer or scientist whose professional
applied work requires a basic understanding of these elements in order to control
and apply them to the solution of his problems.
With the wide diversity of background of those interested, it has been neces
sary to develop the analysis of the various phenomena from the simplest principles
and the ground upward. Thus the introductory portions usually begin with the
simplest concepts and pictures and develop these further as the book progresses.
The book begins in Chapt. I dealing with electrolytic processes in static charging.
In so doing, it begins with the concept of electrode potentials of a single metal in
solution. Then it considers the application to charging mechanisms, where such
charging is to be found and through this, leads to the more complicated processes
involving electrokinetic potentials. These are first presented as explained by
PERRIN and then extended to the Debye-Rückel analysis. The chapter ends with
a presentation of the newly discovered freezing potentials of WORKMAN and
REYNOLDS. In such a chapter, examples of the electrolytic type of charging
action are drawn from the past literature in so far as the older studies were carried
out with adequate controls to insure that this action occurred.
In order to understand the more involved processes of solid-solid contact
charging, the second chapter presents a condensed and concise account of the
electron concept of metals, the Fermi bands and the nature of the surface barrier.
It shows how this is affected by external fields and adsorbed atoms. This sets
a pattern for the analysis and thought processes on contact charging of metal
inorganic insulator surfaces and is an essential introduction to what follows in the
later chapters. It also presents what evidence there is for static charging between
metals by virtue of this effect.
Chapter three discusses the electrification by spraying and bubbling of
liquids. It begins with an analysis of the significance of the work of COERN
Preface VII
eoneerning the potentials aeross liquid-gas or liquid-liquid surfaees leading to
the oft quoted COEHN'S law. While basic analysis gives a rational and simplified
interpretation of the law, it shows also that the derivation of the law under
COEHN'S experimental conditions presents a true enigma. Then follows the work
on cataphoresis of gas bubbles ending with the very important and significant
findings of ALTY on the true nature of the electrical double layer at liquid gas
interfaces which emphasizes the time element in the formation of the double
layers so vital to understanding the differences in the spray electrification data.
Against the background of ALTY's work, the studies of CHALMERS and PAS
QUILL, of FRUMKIN and of LENARD, but more especially of CHAPMAN on spray
electrification of liquids is unified and becomes comprehensible. The chapter closes
with the remarkable new spray electrification phenomenon observed by WOODCOCK
and BLANCHARD on the positive charge carried aloft on spray from sea water
bubbles eaused by breaking waves. This phenomena may have very important
meteorological implications certainly involving coastal haze, sometimes miscalled
smog (which has another origin), and perhaps on the maintenance of the atmos
pheric potential gradient. In this chapter, the studies of DODD on symmetrical
eharging of sprayed liquid droplets is presented and its import an ce on the falsi
fication of data on spray charging mechanisms is discussed.
Chapter four deals with the contact charging between solids. It emphasizes
the more recent studies of the controlled contact charging between simple in
organic insulators e.g., single crystals and clean metals. The techniques for such
studies are presented as weil as results and conclusions. The quest ion of the action
of extern al fields, the mechanics of rolling contact and charge transfer, ete., are
discussed in detail in terms of BOWDEN'S work. The study of electrification of
dusts and the nature of and conditions for, symmetrical and asymmetrical
charging, are discussed in detail. The chapter closes with a discussion of charging
by asymmetrical heating oi homogeneous substances, charging by common ion
exchange (the Henry model), by rupture of surface dipoles and the evidence
therefore.
Chapter five starts with the electrostatic charging pro ces ses incident on
ionization of gases. It indicates the nature of charging by flue gases, by unipolar
discharges, and allied matters. There is a short seetion devoted to the principles
of elimination of static in so far as this is possible and closes with a short seetion
on thunderstorm electrification and protection against lightning stroke.
One thing this book does not do. While enough references are cited to il
lustrate the principles and matters considered, the voluminous older literat ure
and findings are not quoted in extenso although sources are given. The reason
for this is that the more one considers the earlier investigations in the light of
the processes as we now know them, the more it is clear that controls were in
adequate to yield significant data. This is no reflection on the many excellent
earlier workers who could not have been endowed with the understanding which
modern techniques and knowledge imply.
In the hope that this initial step in the integration of past findings and analysis
of basic mechanisms of static electrification in the light of present day knowledge
will be of inspiration to the curious and of help to industry, the author presents
the book to the reader.
VIII Preface
Some months after the manuscript of this book had gone to press, an article
in arecent issue of Zeitschrift für Angewanate Physik, by Prof. Dr. P. BONING,135
brought to the author's attention the work on Static Electrification of that
investigator and indicated that he was author of a book on the subject published
in 1938. Dr. BONING is an engineer who became interested in the subject of
Static Electrification in connection with colloidal chemistry inspired in part by
Prof. Dr. WOLFGANG OSTWALD. Unfortunately, the extensive studies carried
on by Dr. BONING leading to his book and those that subsequently appeared,
were published in journals that are not often read by physicists and vice versa,
so that he appears to have been little acquainted with the work published in
the Physical Journals. The publication of this book shortly be fore the war and
its very nearly complete destruction during the war, resulted in its being little
known. So far, after nearly six months' search, only two copies have been dis
covered in the United States' libraries and the author has not been able to obtain
the loan of a copy. However, later papers by Dr. BONING have been sent the
author and he has maintained a correspondence with Dr. BONING. In principle,
Dr. BONING has, in consequence of his colloidal chemical viewpoint, regarded all
surfaces as being the seat of electrical double layers, having ions of one sign or
the other exposed on the surface. In some cases, the ions are inherent in the
nature of the substances; otherwise, the double layers are caused by ions adsorbed
on the surfaces, including, possibly, air ions. Thus, on contact between two
surfaces, transfer of ions from one surface to the other is inevitable, leading to
static electrification. Dealing with a wide range of complex organic solids, data
appear to substantiate these views. In principle and broadly speaking, as brought
out in the several chapters in this book, static electrification is, in all cases,
caused by transfer of carriers on contact between what could be loosely called
electrical double layers on surfaces, if this term includes the electron atmospheres
in metals, the electrical stratification at polar liquid-gas interfaces, and the electro
lytic double layers on moist surfaces. However, as amply exemplified by what
follows in the present volume, such generalization ignores the wide variety of
binding forces and their magnitudes, much of the present day knowledge of the solid
state and above all, the nature of surface interactions in the solid state as exem
plified by the frictional studies of BOWDEN, such that each case of charge transfer
must be analyzed separately with adequate experimental controls to separate it
and establish its specific mechanism. Thus it is believed that the author's book
and its analyses of the varied physical mechanisms and semi-quantitative illu
strations, fuHills adefinite need in the field of static electrification. However,
the important prior disclosures of Dr. BONING'S book cannot be ignored and the
author gladly lists under the reference numbers, all of those papers in regretting
that he came upon these too late to include a discussion of them at appropriate
points in this book.
Berkeley (Cal.), Nov. 25, 1957 LEONARD B. LOEB
Acknowledgements
The writing of this book has been made possible through the extended in
vestigations of a group of interested students whose studies were made possible
by grants from the Office of Naval Research, United States Navy, and the Ord·
nance Department of the U.S. Army. While the initial investigations of C. DVK,
Dr. SEVILLE CHAPMAN, Dr. DAVID DEBEAU, and Dr. ]. W. HANSEN were carried
out before the existence of this office, the extended studies of Dr. WULF B. KUN
KEL, Dr. E. E. DODD, Dr.]. W. PETERSON, and Dr. PETER E. WAGNER were
carried out under ONR Contract with the assistance of the Ordnance Department
of the United States Army. To this generous support and more to the excellent
work of this group of able young men, the author expresses his deep gratitude
and thanks.
The author's profound thanks are given to Mrs. BEATRICE GALBRETH, who
has carried out all of the typing and preparation of the manuscript.
He is also indebted to Messrs. WARREN WIESENFELD, BEN OSHINA, and
RICHARD FRANCIS for the drawings and to Mr. GENE WEBER for the photography
needed.
Table of Contents
Page
Dedication IU
Preface V
Acknowledgements IX
Introduction . . .
I. Static electrification by electrolytic process . 4
A. Basic principles in terms of galvanic action from metal surfaces 4
B. Nature of systems leading to charge separation and their limitations 8
1. Flow charging systems . . . . . . . . . . . . . . . . . . 9
2. Limited contact charging . . . . . . . . . . . . . . . . . 9
C. Do adequate aqueous surface films exist fer electrolytic charging ? 9
D. Past evidence of electrolytic charging . . . . . . . . . . . . 12
1. Electrolytic flow electrification. . . . . . . . . . . . . . 13
2. Electrolytic flow charging in impacts of liquid jets on surfaces 13
3. Electrolytic contact charging . . . .. ...... . 15
E. Electrical endosmosis, streaming potentials, and cataphoresis (Verified aspects
of electrolytic charging) . . 16
1. Electrical endosmosis . . 16
2. The streaming potential . 18
3· Cataphoresis. . . . . . 19
F. The more accurate calculation of the double layer characteristics including ions of
both signs .............. . 20
G. The Workman-Reynolds freezing potential. 25
H. Summary and conclusions . . . . . . . . 31
II. The contact potential difference or volta potential. 32
A. Introduction. . . . . . . . . . . . . . . . 32
B. The metallic state energy bands, the Fermi law and basic principles . 33
C. Surface structure and the work function in relation to the Fermi level 40
D. Influences modifying the work function . . . . . . . . . 42
E. The contact potential difference 44
F. The measurement of contact potential and/or work function 48
1. Thermionic emission . . . . . . . . . . 48
2. Calorimetric measurement of work function 49
3. The photoelectric studies . . . . . . . . 49
4. Contact potential difference measurements . 51
G. Discussion and conclusion . . . . . 56
UI. Static charging by spray electrification . 58
A. Introduction. . . . . . . . . . . 58
B. The Existence of an electrical double layer at gas-liquid interfaces 61
1. The studies of Coehn on double layers as related to dielectric constant 61
2. The electrical double layer in cataphoresis. . . . . . 65
3. Cataphoresis and the true nature of the double layer . 66
4. The total potential difference at the water-air interface 73
Taille of Contents XI
Page
C. Relative potentials in double layers of water in relation to surface tension, for salt,
acid and organic solutes . . . . . . . . 75
D. Tbe spray electrification pbenomena 80
1. Tbe studies of LENARD amI his scbool . 80
2. Tbe studies of CHAPMAN. . . . . . . R4
E. Spray electrification of water in relation to thunderstorm electrificatlOn 97
F. Homogeneous or symmetrical charging of liquid drop lets on dispersion. 98
1. The spray and dust electrical analyses of HANSEN and KUNKEL using the Hopper
and Laby technique. . . . . . . . . 100
2. The measurements of DODD on liquids ................. 109
3. Significance of DODIl'S results . . . . . . . . . . . . . . . . . . . . . 115
G. Asymmetrie charge distribution 01 drop lets of mieron size from bursting of small
air bubbles in concentratt'd solutions 119
H. Conclusions 122
IV. Mechanisms of electrificatioll on solid-solid contact 125
A. Introduction . . . . . . . . . . . . . . . . 125
B. Quantitative measuring techniques and the influence of electrical fields 126
1. Simple studies . . . . . . . . . . . . . . . . . . . . . . . . 126
2. The action of external fields on charging. . . . . . . . . . . . . 128
C. Initial investigation of controlled contact electrification of metal-inorganic insu-
lator systems .................... 134
D. Electrification of dusts on dispersion and impact on surfaces 136
1. Experimental techniqucs . . . . . . . . . . . . . . 136
2. Observations ................... 140
E. The contact charging by rolling of inorganic insulator spheres on metal surfaces 149
a) Design of apparatus. . . . . . . . 151
b) Experimental results on fused quartz 154
c) Eifect of gas pressurc 154
d) The charging rate. . . . . . 157
e) Borosilicate glass . . . . . . 158
f) Interpretation of charging data 160
g) Primary process of charge transfer 160
h) Secondary processes governing redistribution 01 charge 161
i) Surface conduction . . . . . . . . . . . . . 161
F. Extension of T'ETERSON'S investigations by WAGNER 164
1. Experimental techniques 164
2. Basic theory. . . . . . . 165
3. Experimental observations 167
a) Saturation charge . . . 167
b) Single crystal studies on quartz 168
c) Aluminum oxide, synthetic single crystals, (white sapphire) 169
d) Single crystals of alkali halides. . . . . . 171
e) Magnesium oxide single crystals (periclase) 172
f) Eifect of surface finish . . 173
g) High saturation charges. . 173
h) Charging and work function 174
i) Leakage currents. . . . . 175
j) Charging under ultra high vacuum with measured work function . 176
4. Interpretation of results. . . . . . . . . . . . 178
a) Surface and bulk conduetivity. . . . . . . . 178
b) Depcndence of charging rate on surface exposed 178
c) Maximum or saturation charge 180
IX) Effect of surfaee roughness . 180
ß) Effect of electrostatie forces 180
