6168
Langmuir 2003, 19, 6168-6177
Effect of Surface Wettability on Fast Transient Microboiling Behavior Owen C. Thomas, Richard E. Cavicchi,* and Michael J. Tarlov* Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received April 7, 2003. In Final Form: May 15, 2003 Microboiling events associated with the fast transient heating of a micrometer-scale metallic thin film heater immersed in water have been studied. The effect of surface properties on the microboiling transients was examined by modifying the heater surfaces with hydrophobic and hydrophilic alkanethiol self-assembled monolayers (SAMs). The microheaters are thin films of platinum or gold-plated platinum that are approximately tens of micrometers in width and hundreds of micrometers in length. The microheaters are immersed in water and rapidly heated with short (108 K/s), superheating occurs and nucleation temperatures as high as 296 °C have been measured in water. The surfaces of the gold-plated heaters were coated with a series of hydrophilic [HO(CH2)6SH, HO(CH2)11SH, and HO(CH2)16SH] and hydrophobic [CH3(CH2)7SH, CH3(CH2)11SH, and CH3(CH2)15SH] SAMs. Dramatic differences are observed in the temperature-time transients of the hydrophilic versus hydrophobic SAM-coated microheaters. Microheaters modified with hydrophobic SAMs exhibit lower boiling nucleation temperatures, more pronounced inflection points, and higher average temperatures during microboiling. These differences can be rationalized by considering simple models of surface wetting and surface vapor bubble formation.
In this paper, we describe experiments of fast transient microboiling from rapidly heated metal lines immersed in water. Using alkanethiol self-assembled monolayers (SAMs) to modify the heater surface, we demonstrate that microboiling is sensitive to the surface-wetting properties of the heater. The metal lines are Pt or Au-modified Pt thin films, approximately 200-nm thick and 3 × 200 µm in dimension, that act as fast transient microheaters when subjected to short (
