Freeze-fracture is a natural occurrence leading to processes like erosion of the earths crust or simply deterioration of food via freeze-thaw cycles.[1] [2] To investigate the process further freeze-fracture is artificially induced to view in detail the properties of materials. Fracture during freezing is often the result of crystallizing water which results in expansion. Crystallization is also a factor leading to chemical changes of a substance due to changes in the crystal surroundings called eutectic formation.[3]
Imaging the fractured surface of a frozen substance allows the interior of the structure to be investigated as illustrated by the picture of a fractured piece of glacier called an iceberg. By photographing at high magnifications more can be learnt about the fractured object's substructure and the changes in the object that occur during freezing. When imaging fractured surfaces in detail, changes occurring during and immediately after fracture as well as sample preparation, must be taken into account if trying to infer the unbroken material's structure.[4] [5] The often relatively cold temperatures needed to make an object solid enough to fracture, and the fracture process itself, stress and deform the material. Imaging of fine detail under sub-zero conditions is difficult. The material will start to warm again when removed to a position for photography. Ambient gases, often water vapor, will condense on the cold surfaces, reacting with them, obscuring detail and further warming the object allowing it to reshape.[6] [7]
Freezing of a substance is a relative term, often relative to ambient temperatures. Freezing something from liquid or gas phase to a solid allows fracture but has different effects depending on the material involved and how quickly it is frozen. Freezing things slowly allows the material time to re-arrange itself internally. In the example of water, ice forming slowly results in larger crystals leading to a clear glass like substance. If frozen quickly as with snow, the crystals are smaller and less organized, scattering light and appearing white.
Elastic solids generally become less elastic the cooler they get, making fracturing easier. For example, plastic hoses are flexible on hot days and less flexible and prone to cracking on cold days. Storage of critical items such as blood products in plastic bags must take into account the effect of freezing on the blood but also the changing plasticity of the storage bags.[8] While many synthetic and natural polymers become progressively less elastic with reducing temperature they do not usually crystallize, unless they also contain a free liquid such as water in plants and soils or plasticizers in plastics.
Liquids reduced in temperature will become solid enough to be fractured. The abundance of water on earth and particularly in living organisms and soils means frozen water often provides the rigidity needed for and otherwise less brittle object to fracture. While the water increases the rigidity of an object to allow it to be fractured, the formation of ice crystals within an object can also cause significant damage to structures which were previously intact. Changes in the eutectic around the forming crystals is also significant which can be disadvantageous, or used in the case of cooling solder advantageous.[3] Freeze-fracture can occur as part of the freezing process, particularly with liquids that expand as they crystallize such as water. Such fracture is termed pre-fracture.[5] To reduce damage from crystallization cryopreservatives which reduce ice crystal damage are often used but may themselves be toxic to living cells in the concentrations required.[9] For small objects, freezing of liquids can be rapid enough for limited or no crystallization. In the case of water, very rapid freezing leads to vitreous amorphous ice rather than crystalline ice resulting in no detectable damage.[10]
Perhaps counterintuitively, solids also change into different states when frozen. You may say they become more solid. For example, iron in its various forms will become more brittle at lower temperatures. Steels exhibit low temperature brittleness with a transition temperature from ductile to brittle fracture (TTDB) that varies from about −100 °C to about +100 °C depending on the alloy composition and processing. As the solids change with temperature so does the way they fracture.[11] [12]
Gases cooled sufficiently will become solid enough to fracture as well, such as with solid carbon dioxide also known as dry ice. Since gases have little structure when used under normal conditions there are currently no investigations into their solid phase structures using freeze-fracture. The requirement for freeze-fracture studies may increase with extra-planetary objects having surface temperatures cold enough for elements that are gases on earth to be naturally solid. Currently only the unfractured structures are being investigated such as solid carbon dioxide on the moon or solid methane and nitrogen on pluto.[13] [14] [15]
See main article: Stress (mechanics). To split a material into two pieces requires the material to be put under enough stress to break it. The amount of stress applied to an object prior to its fracture will determine the amount of energy available for the fracture to take place. Excessive stress results in multiple almost simultaneous fractures, as when shattering a sheet of glass with a hammer. Sufficient but not too much stress normally results in a single fracture. Even with a single fracture any slight excess in stress will lead to fracture that propagates more quickly with more energy and higher temperatures at the fracture face. The higher energy can also result distortions called plastic deformation or even in minute secondary fractures that break fragments off the main fracture face. If the stress is less focused a larger volume will be stressed leading to a slower propagation of the fracture with lower temperatures at the fracture face. Force in excess of that required for a single fracture plane to form is usually released as a combination of significant heating, plastic deformation and secondary fracture.[16]
Once cooled sufficiently to fracture, a sample is often cooled further. Stressing and fracturing a sample produces considerable heat, easily enough to thaw a sample again if the temperature is not well below the melting point.[17]
Here are a few visible examples of freeze-fracture being used directly in our daily lives. There are also less known applications of freeze-fracture knowledge. Better know examples relate to preventing freeze-fracture damage to water supply pipes or engine cooling systems in colder climates.
A very common requirement in many peoples' lives is using ice to keep things cool. Large blocks of ice were once the most common way of transporting ice.Once transported the ice would be later fractured into smaller pieces to make it practical to use. Today ice machines produced ready to use ice or ice cubes fractured by blending them into drinks such as a slushy or foods that contain fractured ice such as Ais kacang.
Tempered glass has its exterior surface rapidly cooled from the liquid state so that it is frozen solid while the center of the glass is still liquid. As a result, the glass becomes highly stressed. When a single fracture is initiated the considerable stored energy in the prestressing is released. The sudden release of the energy fractures the entire pane into small less damaging pieces, as with a car windscreen.
For practical purposes most devices manufactured have an "operating temperature". Often these relate to fragility induced by lowering temperatures increasing the likely-hood of prefracture or fracture.
Iron and its various alloys including steel, undergo changes in their resistance to fracturing with temperature. These changes can occur at higher temperatures as the steel solidifies during manufacture and also again at lower temperatures below 100 °C including below the freezing point of water at 0 °C. This has implications in the design and building of steel civil engineering structures such as bridges, buildings and pipes.
Freeze-thaw cycles of composite materials can weaken them.[18] Moisture within composite materials has been modeled to try and predict the effects of water freezing with composite materials.[19] As a widely used composite material, concrete is also an important material susceptible to freeze-fracture.[20]
Materials and colloid sciences use freeze-fracture techniques to investigate the nature of more complex substances. Even without the visualizing the atoms and molecules the shapes and textures of the interface between reacting substances will have an impact on how they behave and react with each other.[21]
Eutectic solders rely on the re-distribution of chemistry within the solder as they cool from liquid to solid. This can be taken advantage of to allow less toxic solders lower temperature solders that still bind well enough to prevent fracturing under operating conditions.
The idea of looking at the detail of biological cells and proteins in detail, without chemical fixation, plastic or wax embedding and chemical staining prior to sectioning resulted in extensive use of freeze-fracture microscopy in biology.
The first documented systematic visualization a frozen, fractured surface to look at the structure of the fracture face itself was done in the mid-1950s. Russel L. Steere [22] was looking at virus particles and became concerned that conventional preparation techniques for electron microscopy, which included dehydration and more, may be altering the virus structure. While acknowledging freezing the sample would also cause changes he considered using it would give a different view. A "planed surface" was created on the frozen material using a knife. Due to the conditions required for the transmission electron microscope at the time the rapidly frozen and fractured virus itself could not be viewed directly. Instead Steere made a carbon re-enforced chromium replica of the fractured surface based on a procedure devised by others.[23] Steere overcame the problem of ice crystals forming on the fractured viruses by etching them away as done by others in 1955 prior to making the copy for viewing.[24] The additional etching step adds additional variations to the appearance of the original fractured surface but is essential to get rid of the condensates from the surface.
With the principles of visualizing a freeze-fractured surface for electron microscopy established, Moore automated and commercialized the Freeze-Fracture-Etch-Replication method in 1961 calling it "Steer's freeze-etching method". Believing a sharp knife was required to achieve a controlled fracture he used a microtome in a vacuum chamber to fracture the specimens.[25] Much cheaper, non-commercial alternatives which did not rely on a microtome or etching to clean the fracture face were later established. The technique "Freeze-Fracture Replication", rather than Steere and Moore's Freeze-Fracture Etching replication began to be used for the first time in this context. Adequate shielding from contamination in the vacuum required for replication meant etching was not required to clean the fracture face.[26] For the Bullivant & Ames method cheaply modified standard coating machines already routinely used in electron microscope laboratories, initially using a Meccano set. For smaller electron microsopy labs these could be more easily used than a large, specialized, commercial piece of freeze-fracture etch replication equipment. The method also allowed for much greater variations to the way the specimens could be fractured.
During the 1960s-1980's it became apparent that the cell's lipid bilayer was shown to split into two halves revealing the interior when fractured under suitable conditions.[27] Freeze-fracture was the only method to give a planar view of the membrane interior so some effort was needed to establish what aspects of a membrane interior image were native to the biology and which were produced by the freezing, fracturing and replication processes. Adequate explanations as to why the two fracture halves were not always complementary and why the fracture plane sometimes went between the lipid bilayer and not other times were found through the use of additional freeze-fracture techniques. By 1989 the identity of "bumps" in the membrane as intramembrane protein particles was also established using a further modification of the basic freeze-fracture technique called freeze-fracture replica immunolabelling (FRIL).[28]