Electromechanical Phenomena in Ice
VICTOR F. PETRENKO
ice friction (Petrenko 1994a). These phenomena illus-
INTRODUCTION
trate the important role that ice electrical properties
Among the numerous interesting properties of ice,
plays in ice friction. It is most likely that these effects
there is a group of physical phenomena in which elec-
result from the special structure and electrical proper-
trical and mechanical properties correlate. That corre-
ties of the ice surface. Since the physical properties of
lation can manifest itself in three ways:
the ice surface were discussed in detail in a previous
1. Electromagnetic fields are generated by means
report (Petrenko 1994b), we will only briefly review
of mechanical actions, such as elastic stress, plastic
the information necessary to the following discussion.
2. Mechanical properties, such as plasticity, elas-
Structure and electrical properties
ticity and friction, are modified by the application of
of the ice surface
electrical fields.
The physical properties of the ice surface differ sig-
nificantly from those of bulk ice. In the temperature
are changed by strain.
range from 0C to approximately 4C, there is a thin
We will refer to such effects as electromechanical
liquid or liquid-like film on the ice surface, which has
phenomena (EMP). Most of the experimental results
optical properties very similar to those of ordinary
described in this report were discovered recently, dur-
water (Furukawa et al. 1987, Elbaum et al. 1993). The
ing the last decade. Of course, ice is not unique in
film's thickness depends on temperature, surface crys-
exhibiting electromechanical effects. Similar phenom-
tallographic orientation and ice purity. The film is also
ena were previously found in semiconductors, ferro-
sensitive to environmental conditions and it is about 50
electric and piezoelectric materials, where EMP may
nm at 1C. A special layer remains on the ice surface
have a much greater intensity--electrical fields are
even at temperatures down to 100C. This layer exhi-
much stronger, for instance. Nevertheless, EMP in nat-
ural ice may have quite a large total magnitude, such as
structure and viscosity that are quite distinct from
electrical potential difference, owing to the large size
those of bulk ice (see Maeno 1973, Barer et al. 1977,
typical of ice sheets. In addition, since ice is one of the
Golecki and Jaccard 1978, Mizuno and Hanafuza
most widespread materials on Earth, the electrome-
1987).
chanical effects described below may have important
One of the best-known theoretical models devel-
practical applications.
oped to explain ice surface structure and properties
was invented by Fletcher (1962, 1963, 1968). The key
idea in Fletcher's model is the assumption that at the
ELECTRICAL PHENOMENA
ice surface a significant fraction of molecules (denoted
IN ICE FRICTION
as αο) is oriented with their dipole moments (i.e., pro-
This section describes two recently discovered
tons) pointing outwards, as opposed to the bulk where
effects in friction on ice and snow. The first one is a
molecules are oriented randomly. This would result in
strong frictional electrification (Petrenko and Colbeck
1995), and the second is an effect of electrical fields on
the surface. Fletcher came to his conclusion about