Patterns found in nature are most often astoundingly complex
while also being elegant and aesthetically pleasing. Snowflakes for example
consist of extremely delicate, symmetric structures of frozen ice and are
always different from each other. Yet these infinite patterns are influenced by
only two main controlling factors, temperature and humidity. A more obvious example of patterns in nature
are mud cracks. Although they are not as delicate and complex as snowflakes,
they also exhibit some patterns that are interesting to observe. What
follows are some facts I gathered from a small study about mud cracks and some photographs taken during my training period at
the Aruwakkalu limestone mine.
Mud cracks are formed quite obviously, when mud dries up. So the main factor controlling
mud crack formation is moisture content. Unlike in glass and many other
materials, cracks in mud do not originate at single point and then propagate.
If the rate of drying is uniform across a surface of a layer of mud, cracks
will start appearing simultaneously across the surface. Mud starts to dry from
the exposed surface. So when the moisture content of the surface layer starts
to drop, the mud particles come close to each other as the space occupied by
water is emptied. This results in the contraction of the surface layer. This
causes stresses to build up on the surface layer and ultimately results in the
formation of cracks. First the main cracks divide the exposed surface into a series of "islands" then, secondary cracks may occur within these islands.
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| primary and secondary cracks |
Since drying starts from the surface, the bottom layers still contain some water and still remain
expanded. Therefore the width of the crack is lesser in bottom layers, like a cut made
from the surface. As the moisture content further reduces and the bottom layers
also dry up, the crack propagates to the bottom layers and the gap widens.
If the moisture content in a single layer of mud differs at
the top of the layer and the bottom, the amount of contraction also differs.
The top portion will contract more than the bottom portion. This will result in
an upward curving of the layer like in a bimetallic strip. If the wet mud has
been standing for a long period, stratification of different layers will take
place. Stratified layers can be observed when a portion of the cracked mud is
viewed sideways. Stratified layers can have a bigger difference in contraction
rates, and if the layers are strongly adhered to each other, a more prominent
curving of the layers will be seen.
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| stratification and upward curving of layers |
If the layers are not strongly adhered, instead of curving
upwards, the layers will separately contract producing a step like effect of
the cracks.
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| step like contraction |
Although the mechanics of mud crack formation has no
immediate practical application, crack formation in other substances is studied
extensively. Crack formation on thin surface layers in electronic circuits can
lead to peeling off of printed components and cause problems. Therefore a lot
of effort is put into preventing the formation of such cracks. A very fine
network of small cracks can be seen on the surface of old paintings on close inspection.
This fine set of cracks is called "craquelure". The characteristics of these cracks depend on
the type of pigments, oil and also the ambient conditions at the time of
painting. For this reason, these cracks are used to authenticate old paintings.