molecule orientational ordering by analyzing the inter-
ing point and at high normal pressure, the pressure
action between dipole and quadruple moments of wa-
melting of ice can also contribute to the formation of a
ter molecules close to the surface. Such an orientation
liquid film between the ice and a slider.
of water molecules implies a high density surface elec-
The study of the surface potential of ice and electri-
trical charge λs. Though this charge was predicted by
cal charges at the ice surface has quite a long history
Fletcher many years ago, its existence was proven
and continues to attract the attention of numerous
experimentally only recently during experiments on
scholars. Such interest in the electrostatic properties of
the frictional electrification of ice and snow.
the surface of ice is stimulated by both the fundamen-
All other predictions of Fletcher's model are conse-
tal problems associated with it and certain practical is-
quences of the molecular ordering in the surface layer.
sues. Fundamental questions, which the study of the
In fact, since in the bulk there is no ordering between
surface potential and ice surface might help to resolve,
the surface and the bulk, there should be a transitional
include the issue of the structure of the surface of ice.
layer with a large concentration of hydrogen bond de-
For instance, if all the molecules at the ice surface are
fects allowing molecules to reorient. This transitional
oriented "proton-outwards" as Fletcher's model sug-
disordered layer, according to Fletcher, is the liquid-
gests, this must lead to a positive charge at the surface
λs and a positive potential of the ice surface ϕs. Both λs
like layer. Besides the Fletcher model, there have been
and ϕs are determined by the thickness of the surface
several others designed to explain the unique physical
layer ∆ and the fraction of the oriented water mole-
properties of the ice surface. No one of them is able to
cules contained in this layer. Thus, measurement of λs
account for all of its known properties.
and ϕs in conjunction with other data (∆ for example)
One important surface property is ice friction. The
bibliography on ice and snow friction is vast (see re-
might provide valuable information on the micro-
views by Hobbs [1974] and Colbeck [1992]). Here, we
scopic structure and nature of the ice surface layer.
can examine only the major ideas available in the liter-
The practical issues stimulating the study of the
ature on this subject. In doing so, we will underscore
electrical properties of the ice surface, include, first of
the facts that make ice different from most other mate-
all, the problem of atmospheric electricity (see review
rials. In static friction, the strong and universal adhe-
in Hobbs 1974) as well as the problem of adhesion and
sion of ice to almost any solid is one such difference.
friction of ice and snow (Petrenko 1994a,b).
Depending on temperature, ice/slider interface rough-
Among physical mechanisms suggested to explain
ness and slider materials, it takes from a fraction of a
ice surface charge and thunderstorm electricity were
second to hours to produce mechanically a very strong
surface electronic states (Buser and Jaccard 1978), a
ice/solid interface. Intensive mass transport via the liq-
thermoelectrical effect (Latham and Mason 1961,
uid-like surface film accelerates this process at temper-
Latham 1963), charge separation on freezing (Work-
atures above 10C. The nature of the strong bonding
man and Reynolds 1949, 1950), and the motion of
between ice and solids is not quite understood yet and
charged dislocations produced by rubbing one piece of
perhaps originates from the special arrangement of
ice against another (Takahashi 1969a).
water molecules on the ice surface mentioned above.
The coefficient of kinetic friction of ice in the tem-
Early works on asymmetrical rubbing
perature interval from 3 to 40C strongly depends
Until recently, asymmetrical rubbing of ice on ice
on sliding velocity v, varying from 0.9 at v = 105 m/s
was the only phenomenon studied in connection with
to 0.05 at v > 101 m/s (Barnes et al. 1971, Evans et al.
ice frictional electrification. Chalmers (1952) mea-
1976, Jones et al. 1991). For granite-on-ice friction,
sured an electrical charge from small snow fragments
Barnes et al. (1971) distinguish three ranges of sliding
when two handfuls of snow were rubbed together,
finding a negative electrical charge of 28 1010 C on
velocity characterized by the different mechanisms
the ice fragments. Yoshida (1944), Reynolds et al.
(1957), Latham and Mason (1961) and Latham (1963)
m/s; and frictional melting, when v > 103 m/s, T =
studied charge separation during asymmetrical rub-
11.75C. The velocity at which the transition to fric-
bing of ice on ice. In such an experiment, the constant
tional melting takes place may depend on the initial ice
rubbing point becomes warmer than the variable rub-
temperature, normal pressure and ice and slider rough-
bing point. The electrical potential differences gener-
ness, but the occurrence of melting and liquid lubrica-
ated in such experiments were very small and did not
tion of the ice friction have been proven many times.
exceed tens of millivolts. Such weak electrification is
Yet, for a precise quantitative description of such fric-
attributable to the rubbing piece being heated more
tion, we need more data on both the film thickness and
than the rubbed one, with the potential difference aris-
the real contact area. At temperatures close to the melt-
ing from the thermoelectrical effect. Since the thermo-
2
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