The affect of surface hydrophobicity on wind erosion probed by atomic force microscopy

Abstract

Several field and laboratory studies have shown indirectly the effects of atmospheric
humidity on soil particle adhesion through reduced resistance to movement by wind as
the humidity decreases. Similar studies have also shown that increased hydrophobicity
has the same effect on susceptibility of soil particles to movement by wind as reduced
humidity. Theoretical models of idealized soil particle cohesion by water menisci in
equilibrium with the atmospheric humidity have been used to explain these results.
However the interpretation of these phenomenological results has never been tested by
direct measurement of soil inter-particle cohesion as a function of humidity and
hydrophobicity.
To address this, adhesion force measurements were obtained using an atomic force
microscope (AFM) with colloidal glass tips of various sizes to represent one soil particle
coming in contact with another. The model surfaces used were Ottawa sand, representing
a soil particle with no organic coatings, and a quartz slide representing a more idealized
smooth surface. These surfaces were coated with various silanes to mimic fatty acids
coating the soil surface, giving the surface induced hydrophobicity.
Hexamethyldisilazane (HMDS) and dichlorodimethylsilane were used to represent shortchained
hydrocarbon lipids, and octadecilsilane and palmitic acid were used to represent
long-chain hydrocarbon lipids. In addition to these model surfaces, field soil samples
were also studied and the results compared to the model surfaces. These force curves
were taken at various values of relative humidity in order to measure the combined effect
of hydrophobicity and humidity.
The results of these experiments show that with increased relative humidity, there
was increased adhesion force between the tip and the surface for most surfaces. At high
relative humidity, there was an increase in adhesion force with an increase in tip size and
increased surface hydrophobicity. Also, with increased surface roughness (model quartz
surface versus model Ottawa sand and field soil samples) there was decreased adhesion
force at high relative humidity with the same surface treatments. Also, at high relative
humidity on rough surfaces, the measured adhesion force showed a bi-modal distribution
showing distinct wet (high adhesion force) and dry (low adhesion force) surfaces.
Comparing the data of the model surfaces with that from the field samples showed that,
depending on the surrounding foliage, the dichlorodimethylsilane and palmitic acid
surface treatments mimicked the relative humidity dependence seen in the field samples
in that the soil samples' measured adhesion force usually increased as the relative
humidity increased. By using the insights on the cohesive properties of these soils it is
possible to improve the prediction of the level of susceptibility of an area to wind erosion.

Date of Award	2011
Original language	American English
Awarding Institution	Eastern Illinois University
Supervisor	Jonathan Blitz (Supervisor)