attension Theta, User Manual

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Assignment of baseline: One of the major factors limiting the reproducibility of contact angle
measurements is the accurate assignment of a baseline for the image analyzed. Two methods are
commonly employed. One is to make the surface of the solid horizontal with respect to the camera. The
solid thus appears as a sharp-edged, flat object with no three dimensional aspect. The image does not
show the surface of the solid receding above the front edge of the sample. Place the drop near the front
edge of the sample and use the sharp front edge on the image as the baseline.
The other approach is to align the camera so that it is pointing slightly downwards towards the sample
(less than 5°). The more distant surface of the sample is seen receding above the front of the sample. In
this case the reflection of the drop onto the solid surface may be observed. The baseline is assigned at
the point where the curve of the drop and its reflection meet.
Analysis of Porous Solids: When a solid has a porous structure (paper, packed powder, etc.) the liquid
may imbibe into the solid causing the contact angle to decrease over time. Researchers generally report
graphs of contact angle, drop volume, baseline length or height of droplet as a function of time. These
graphs are used to characterize the wetting behavior. Use of Theta on-screen trigger and fast recording
mode are recommended in situations where the imbibing is rapid.