Dr. James R. Carter, Professor Emeritus
Illinois State University, Normal IL 61790-4400
The process of Ice Segregation takes place in a porous medium when supercooled water moves to the presence of ice and adds to the ice through freezing. In this case the ice grows away from the ice/water interface. It is logical to assume the name derives from the fact that where water starts off widely dispersed within the porous medium it becomes segregated into discrete pieces of ice in the form of lenses, ribbons, needles, layers or strands of ice.
In nature Ice Segregation occurs at or near the surface of the Earth in the form of Needle Ice in soil, Ice Flowers on plant stems, Hair Ice on dead wood, and Pebble Ice on small rocks. Such ice segregation products may be quite attractive and people post photos of such ice formations. In some cases needle ice disturbs the soil surface which has been shown to accelerate soil erosion.
In the subsurface in some soils ice segregation forms distinct layers of ice that can cause frost heaving. Within the crust of the Earth ice segregation forms wedges and layers of ice that are found in tundra and periglacial environments. In subsurface environments ice segregation may disturb soil structure and wedge apart rock formations. The process leads to so much upheaval in some places that it produces unique landforms.
Ice segregation occurs where above freezing and below freezing temperatures are juxtaposed. At the Earth’s surface this is most common in fall at night as the air cools to below freezing while the land surface stays relatively warm. However, micro-meteorological events may produce these conditions at other times. And, as freezing takes place heat is liberated into the immediate environment through the latent heat of fusion. The process continues as long as rate of cooling is balanced by the rate of warming with the latent heat of fusion. When these get out of balance everything freezes up or it becomes warm enough to stop freezing.
Ozawa and Kinosita, 1989, conducted controlled experiments on ice segregation where they were able to monitor and test the formation and growth of ice with great precision, away from the vagaries of nature. In their paper they provide explanations for the processes involved in ice segregation. They used a microporous filter as the barrier between supercooled water below and ice on top of the filter. A small piece of ice was placed on the filter to start the process. When they had the right temperatures water was drawn up from below and froze to the base of the ice, causing it to grow up and away from the filter. At that freezing front they confirmed there was a thin layer of water between the filter and the ice.
They recognize that ice segregation occurs at the surface producing things like needle ice and at depth leading to things like frost heave, but also noted ice segregation may produce ice on the surface of thin biomembranes leading to dehydration of constitutive cells in a process called extracellular freezing (113). Thus, ice segregation is a significant force in the natural environment.
Ozawa and Kinosita recognize that it is known that ice segregation occurs in soils only when particle sizes are less than 0.1 mm (silt and clay) so they selected filters with micropores in the range of 0.015 – 0.2 um diameters. Pores of this size permit water to pass through the filter but ice cannot. They found that when cold enough the supercooled water below froze showing that ice segregation takes place within a critical range of temperatures. By making many observations they got insights into the critical degree of supercooling, that temperature below which ice formed in the supercooled water. They found that this temperature was slightly higher with increasing pore size showing that finer textured media can experience ice segregation through a larger range of temperatures (all within a fraction of 1 degree C).
In this study they did not answer all questions about ice segregation but they demonstrated how the process occurs with precise measurements that could not be taken in nature. Indeed ice segregation is the process that accounts for ice flowers on some plant stems, needle ice in fine textured soils, pebble ice on a few small rocks, hair ice on some pieces of dead wood, frost heaving in some soils and ice wedges and more in tundra and periglacial environments.
Ozawa, Hisashi and Seiiti Kinosita, 1989, “Segregated Ice Growth on a Microporous Filter,” Journal of Colloid and Interface Science, Vol. 132, No. 1, pp. 113-124.
This page is one of a number I have on ice in nature. Ten years ago I knew about icicles and patches of ice on streets and walkways, and that was about it. Now when it is above freezing part of the day and below freezing part of the day I look for ice in many places, and often find it. I have learned much by observing these many forms of ice and now realize that much of the time I am observing products of ice segregation.
What we do know is that this ice from ice segregation is not a form of frost. Frost comes about when the air becomes saturated and water vapor is deposited on a surface as an ice crystal. If the air temperatures are above freezing we get dew, but when it is below freezing moisture is deposited out of the air as frost. In most cases the conditions that are appropriate to the growth of ice flowers, needle ice, hair ice and pebble ice are also appropriate to the formation of frost. For this reason people may give the name frost to any formation of ice on anything at the surface. But, such ice is not related to frost although it make take a deposit of an ice crystal to start the Ice Segregation process.
Thank goodness for the Internet and digital cameras for they let us exchange information about these attractive ice formations. Please look for interesting ice when the freeze/thaw processes are underway. For additional perspectives on ice see http://my.ilstu.edu/~jrcarter/ice/
Feel free to contact me at email@example.com to share your photos of ice of this nature from your outings or freezer experiments.
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