Electrical Phenomena During Freezing of Water and Soils

Permafrost can be found on land as well as below the ocean floor. It is found in areas where temperatures rarely rise above freezing. With construction activities increasing in the northern regions underlain by permafrost, for excavations for minerals, oil and gas and building new communities and improving the existing facilities for the people who have been living there for the past several centuries, it is important to study the behavior of frozen ground and how it will be affected by these construction activities. In permafrost areas where the ground goes through yearly freezing and thawing cycles, electrical potentials of small magnitudes ranging from a few millivolts to several volts, have been observed and measured across the freeze–thaw interface. This chapter also presents an overview of the key concepts discussed in this book.

Electrical freezing potentials are developed when water and dilute solutions freeze. As early as the beginning of 1800s, some curious scientifically oriented minds were interested in finding a scientific explanation of thunderstorm production in nature and resulting lightning, producing sparks. Atmospheric Lightning and Thunderstorm Electricity fascinated early observers, which led to scientific investigations to understand the phenomena, as it was speculated that these could cause static charging of aircraft flying through clouds containing supercooled water in upper atmosphere. The generally accepted concept for the development of thunderstorm charge is the physical separation of oppositely charged particles (dipoles) within the cloud. Electrical potentials could be established between the freezing front, such as a pipeline, and this could also cause stress and corrosion problems. The amount of electricity present in thunderstorms, its magnitude, capture, storage, usefulness, and cost of saving it, is discussed in this chapter.

According to Drake of Imperial College, London, the first observations of electrical charges produced during the melting of ice were reported by Dinger and Gunn. The charge separation effect during freezing and thawing of water attracted the attention of Blanchard and Day, who interpreted them as arising from an electric double layer at the freezing interface. Simpson, in a lecture delivered to the Royal Meteorological Society on January 28, 1942, described the electricity of clouds and rain. A cloud should be thought of as a mass of non-conducting air in which are suspended various particulates, spheres of water droplets, and ice crystals, which carry the electrical charges. The first observations of these charges carried by atmospheric precipitation were made around 1887 by Elster and Geitel. They captured the precipitation during thunderclouds in an insulated vessel connected to an electrometer and measured the charges.

In 1940, Findstein in Germany reported that water drops on impact on ice, gave a positive charge to ice and the negative charges were carried away by the droplets splashing away from the ice surface. Meinhold for the first time reported observing strong negative charges to be produced on the surface of an aircraft flying through a supercooled cloud with “glazing” taking place. The Signals Corps of the US Army funded the study of electrical phenomena in the 1940s. With funding from the old NACA, detailed investigation of atmospheric electrification was authorized in the RD division of the New Mexico School of Mines in Socorro, New Mexico. Equipment to measure electrical potentials in freezing dilute solutions was developed, and some of the earliest reports on the research carried out there were reported by Wokman, Reynolds, and others.

This chapter describes some early measurements of EFP developed during freezing of water and solutions. Gill and Alfrey of the Clarendon Laboratory of the Department of Physics in Oxford University, England, immersed a copper block cooled to liquid air temperatures into liquid water and measured freezing potentials (ice positive) up to 50V between the copper block and a wire electrode in water. They found higher potentials with faster rates of cooling or freezing. After these initial experiments, several investigators studied the development of electrical potentials developed during the freezing of pure water and in dilute aqueous solutions containing different ions. The highest values of EFP measured by Cobb and Gross and Gross et al., were in dilute solutions (10 to 15 micromolar) of ammonium salts (chloride, carbonate, acetate). Korkina describes the measurement of EFP in freezing dilute solutions containing ions of Fe, Ca, Na, and ammonia.

Krorkina's measurements in pure double-distilled water are given, a maximum potential of +12 V. An early SIPRE (Snow, Ice, and Permafrost Research Establishment, the predecessor of CRREL) translation of an article by Arabadzhi reported that collision of water drops with ice crystals in clouds causes the electricity in lightning and laboratory measurements of contact potentials of 150 to 200 mV between water and ice were measured during freezing. More recently, Murphy meticulously measured the potentials generated during freezing of double-distilled water in a special cell from which air was pumped out, to avoid CO₂ absorption by water. In general, Murphy found that in pure water and most dilute solutions (NH₄Cl, Na Cl, K Cl, LiCl, NaI, CaCl₂, MgCl₂, KMnO₄, K-dichromate, HCl, HNO₃, NaOH, ethyl alcohol, Nabarbiturate and phenol-phthalein. At high concentrations (exceeding 1 micromolar) most of these solutions showed ice-negative or zero potentials.

Early observations of thunderstorm electrification in the atmosphere were attributed to the large electrical potentials developing between the frozen and unfrozen regions of water and aqueous solutions subjected to freezing temperatures. Contact potentials between solids and liquids are usually of the order of 1 V and hence, some further fact must be operated to achieve 100 V. The theory suggested is that, whereas in the usual case the charges producing the potentials are adjacent to the boundary, in the case of freezing ice the charges extend to a small distance from this boundary. To bring out the uncertainties in the measurements by various authors, Dennis Kelsh and Susan Taylor investigated the freezing potential phenomenon in a short study conducted in CRREL in 1988. They were quite critical about the techniques of measurements and uncertainties and the difficulty in using EFP as a measurement technique for studying corrosion and water migration in freezing moist soils.

Reports on the observations of freezing potentials in soils and rocks were meager and inconclusive. As early as 1958, Jumikis had reported measurements of freezing potentials of 40 to 120 mV in Dunellen Clay (a glacial till) and suggested enhanced moisture transport under such potentials. Korkina studied the freezing potentials in clay suspensions containing microparticles, saturated with ion. Korkina's results showed that at low concentrations, the potentials were ice positive (as observed in pure water). At higher densities, the potentials were reversed, with ice attaining negative charges. In sandy soils, where redistribution of moisture is not noticeable, EFP arises by redistribution of ions at the phase boundary, creating a zone of excess positive charges that prevent migration of moisture to the freezing front.

In cold regions, airlines incur huge expenses in de-icing aircraft wings and fuselage before take-off from airfields in the cold regions. If the development of contact electrical potentials is reversible, as observed by Gill, Alfrey, Borovitskii, and others, one could think of a method of deicing the metallic surfaces by applying a small reverse current or potential to the body, opposite to the freezing potentials. In the warming season, when the active layer thaws from top downwards, water migrates through the active layer down to the top of the permafrost table and forms thick ice beneath the thawing plane. Thus, in the warm season, moisture moves downward, whereas in the cold season, moisture moves upward. Geo-electrical fields generated in freezing ground could transport water to the freezing front and cause heaving of the ground, an important aspect to be considered in construction in cold regions.

Laboratory investigations on the development of EFP in water and soils in the laboratory arose as a corollary to the main project of studying pile foundations in permafrost in the late 1970s and early 1980s, in the Division of Building Research (later known as the Institute for Research in Construction and now known as NRC-Construction Research Center), National Research Council Canada in Ottawa. Thus, the study of electrical freezing potentials arose as a serendipity. A lot of laboratory and field experiments were carried out by Parameswaran et al., in collaboration with universities. Special probes containing electrodes and thermistors were designed and built for the laboratory and for field measurements. These studies will now be presented with the results obtained in the laboratory and field installations. Electrical potential [probes and thermistors were installed in freezing lakes, in the active layer, inside the core of Pingos, among others], in Inuvik and Tuktoyaktuk in the Northwest territories.

This chapter provides information on freeze-thaw studies at Carleton University, Department of Geography and Environmental Sciences. A cylindrical PVC vessel, closed at one end with an aluminum base, was used to contain the solution. The cylinder was insulated with Styrofoam rings and was placed in a cooling chamber through which a cold fluid could be circulated. A Plexiglas rod containing 6 gold-plated copper strip electrodes, was placed at the center of the cylinder. As the freezing front touches each electrode, an abrupt change in potential occurs due to the freezing potential associated with the phase change. The potentials measured have been ice-negative. As the freezing front advances beyond the uppermost electrode, the voltage-time curve shows a spike caused by a shorting of the system. The magnitude of this "shorting potential" peak may be of the same order but of the opposite sign, as the first freezing potential observed at that electrode.

Borovitskii measured the inherent electric fields developed in the active layer in USSR. The measured values did not exceed 50 mV, but varied with depth in the ground, due to differences in moisture migration. One area chosen was in the middle of an artificially drained lake (Illisarvik, meaning "a place of learning" in Inuvialuktun) on the Richards Island, NWT, where the permafrost was aggrading from top downward. An electrode probe, about 10 m long, consisting of 5 plexiglas tubes connected together, was installed in the center of a drained lake in June 1981. The probe had 22 copper electrodes set 150 mm apart, in the bottom 3 m length, to monitor the downward movement of the active layer or the freezing front in the lake freezing downward. A parallel cable containing thermistors was also installed to measure the temperature at each electrode location.

As the Illisarvik site was not accessible easily, frequent readings could not be taken at regular intervals. The first readings obtained were those taken on March 23, 1982, June 8, 1982, and August 12, 1982. The soil at Inuvik site was a silty clay with particle size 50% finer than 0.002 mm, with a specific surface area, about 120 m²g. At the levels of 0.51 m and 0.96 m, the EFP measured remained at 600 to 700 mV, after the first rise of EFP, even after the temperature dropped below 0 degree Celsius. All these measurements suggest that EFPs do develop at the freezing front in soils as ground freezes and electrical probes to measure these potentials could be developed to detect the advance/retreat of the freezing front and the technique could be refined to form a geophysical method to study in permafrost.

In the winter of 1996-1997, the author, in collaboration with the Department of Geography and Environmental Sciences of Carleton University, Ottawa, designed equipment to study the development of electrical potentials developed during the freezing of large bodies of water, lakes, in a natural environment in a permafrost area. Two lakes in Inuvik area were chosen that had contrasting hydrologic regime and were within 10 km of each other. Inuvik is on the east side of Mackenzie Delta, about 40 km south of the tree line and in the continuous permafrost zone, with a mean monthly air temperature below 0 degree Celsius, through October to May. The electrical potential differences between the bottom electrode and each of the others above that measured, using a multimeter of high impedence. Measurements were done every week from mid-November 1996, when freezing starts from the top downward, until early March 1997, when thawing begins from the surface.

The electrical freezing potentials observed at the Delta Lake are larger than at the Upland Lake, probably due to the large ionic concentrations in the former. The readings showed larger fluctuations at the Delta Lake than in the Upland Lake, again, probably due to larger ion concentrations and influx from the Mackenzie River. The magnitude of the freezing potentials was 300 to 500 mV in general and went as high as 700 mV, the potentials being ice-negative. The conclusions from these measurements of electrical freezing potentials in natural lakes during the development of ice cover, showed that there is a charge separation and potential difference at the freeze—thaw interface.

Electrical potentials develop during freezing of water, aqueous solutions, and moist soils, due to the polar nature of water molecules and the separation and accumulation of charges of opposite signs into the liquid and solid phase of water, at the freezing interface. The experiments described in this book show that this phenomenon can be used as another geophysical method to locate the freezing boundary in freezing ground. In remote areas under harsh weather conditions, it may be difficult to directly monitor and measure the small potentials developed during freezing. It has been established and confirmed that electrical potentials do develop during freezing of solutions and soils. The magnitude, sign, effects of cooling rates, and so on need to be studied further, to make any use of the potentials.