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|Ultralyophobic and Superhydrophobic Properties|
C. W. Extrand of Entregris studies the
nature of super-repellency1. In the referenced paper, Extrand
shows that in addition to lyophobicity (solvent hating) and an adequate
topography (such as the one shown below made up of arrays of columns),
two additional criteria must be met to have an ultralyophobic surface:
the surface forces must be great enough to suspend the liquid against
the forces of gravity (which he calls the contact line density
criterion); and the topographic features must be tall enough that the
liquid does not reach the underlying surface (which he calls the
asperity height criterion).
The diagram above illustrates the topography of one possible ultralyophobic surface and is referenced in Extrand's US Patent 6923216.
Another recent development in superhydrophobicity is an experimental material called nanopin film. A water droplet on this film forms a nearly perfect sphere producing contact angles as high as 178°. Cassie's law is used to describe the contact angle of a droplet on a composite surface, such as nanopin film. This law helps explain how a rough surface can increase the apparent contact angle.
In nature there are numerous examples of hydrophobicity. The most stunning example is the lotus leaf (see image below). In fact, the lotus effect describes the self-cleaning nature of the lotus plant which, ironically, typically grows in swamps and muddy rivers. The leaf never really gets wet due to its microscopic structure and is continually studied by nanotechnologist who seek to understand this amazing phenomenon.
1Criteria for Ultralyophobic Surfaces, Contact Angle, Wettability and Adhesion, Vol. 4, pp295-305.
What makes our DROPimage software great are the improvements we make based on feedback from our customers. Last month we highlighted a new tool we added to DROPimage Advanced which allows for the measurement of relaxation (see http://www.ramehart.com/goniometers/newsletters/2007-09_news.htm). Another tool that was developed at the request of our customers and is now part of is a "Measure Distance" command. This feature is now included in both DROPimage Advanced and Standard v2.2. Once the command is started, the user picks two points on the image window. A red crosshair indicates the selection points. The Measure Distance command windows reports the two pixel locations and then, using the pixel dimension as defined by the most recent calibration is able to measure the distance between the two points in mm. This feature, as illustrated below, is particularly useful for extracting exact dimensional values on pendant drops, sessile drops, as well as substrate geometry.