Date of this Version
UCARE Poster session, University of Nebraska-Lincoln Research Fair, April 2016, Lincoln, NE.
In this work, an “Energy Barrier” was created to control the motion of Leidenfrost droplets. This barrier was created by functionalizing a portion of a mirror-polished stainless steel 304 surface with Femtosecond Laser Surface Processing (FLSP). FLSP results in superhydrophilic, hierarchical, micro- and nanostructures which are highly wetting and thus have an increased Leidenfrost temperature. Water droplets in the film boiling state were deposited, and propelled by gravity towards the Energy Barrier interface. Room temperature droplets were deposited over a range of surface temperatures beginning with the Leidenfrost temperature of mirror-polished stainless steel 304 and culminating at the Leidenfrost temperature of the FLSP region. High speed imaging was used to study the interactions between the droplets and FLSP interface. A custom Matlab program was developed to determine droplet velocities and accelerations. Video analysis revealed that droplet rejections (droplet reverses direction) at the interface were primarily the result of physical contact between the droplet and FLSP interface, resulting in wetting, violent boiling, and recoil. As a result, the percentage of droplets rejected by the processed region is greatest when the substrate temperature is lowest. As surface temperature was increased, the rejection percentage decreased until the Leidenfrost temperature of the FLSP region was reached. This technology will be useful in the development of microfluidic devices which utilize temperature gradients to direct Leidenfrost droplets.