Partial Wetting of Non-Smooth Surfaces and Shaped Microchannels
Abstract
Increasing attention is being given to partial wetting of micro-channels in MEMS micro-fluidic devices, such as pressure sensors and accelerometers. The behavior of liquids that have contact angles with a solid surface greater than zero but less then 180 degrees is also relevant to many other technologies. Moreover, when the solid surface is not perfectly smooth, liquid contact with that surface is poorly understood and the subject of considerable misconception. When the contact angle is between zero and 90 degrees, the liquid is traditionally said to "wet" the capillary wall and it will enter the capillary on its own accord. Pressure is then needed to expel it from the capillary. If the contact angle is between 90 degrees and 180 degrees, the fluid is termed "non-wetting" and pressure is needed to force it into the capillary. When capillary geometry is allowed to vary, the traditional concepts of wetting and nonwetting must be carefully examined. The authors conducted an experiment to clarify this point. A drop of mercury was placed directly over the juncture of two horizontal glass plates. The plates were mounted in an apparatus designed to keep their upper edges firmly together during rotation, thereby maintaining a sharp interstice. The mercury on the two plates formed a symmetric sessile drop having a contact angle of 140 degrees. When the right plate was rotated so that the angle between the two plates was 135 degrees, the outer surface of the mercury drop assumed a modified sessile-drop curvature, but the juncture between the two plates was still filled. When rotation continued until the included angle was 60 degrees, the outer surface was still asymmetric, but the interstice was no longer filled. The experiment was repeated with a mercury drop on a nonsmooth surface having a 110 degree included angle and with water on a surface having 50 degree and 8 degree included angles. (7 figures, 22 refs.)
Document Details
- Document Type
- Technical Report
- Publication Date
- Apr 13, 2001
- Accession Number
- ADA410545
Entities
People
- Phillip G. Wapner
- Wesley P. Hoffman
Organizations
- Air Force Research Laboratory