Hansen, Scott (2012-05). Experimental Study of Droplet Impingement Using Photo-Activated Nano-Coatings and Temperature Nano-Sensors Integrated with High Speed Visualization. Master's Thesis. Thesis uri icon

abstract

  • In this study nano-scale transport mechanisms are explored during liquid-vapor phase change phenomena. Surface temperature transients during droplet impingement cooling of a heated surface with a photo-activated nanocoating was measured using temperature nano-sensors integrated with a high speed digital data acquisition system and was synchronized with high speed digital image acquisition system. Control experiments were performed by repeating the experiments without the nanocoating. Photo-activation was achieved at different exposure levels by using an ultra-violet (UV) light source. Photo-activation caused a reduction in the contact angle by up to 20 degrees for a wafer surface with the nanocoatings (compared to that of an unexposed and uncoated wafer surface). Using microfabrication techniques (a combination of Physical Vapor Deposition/ "PVD" and "lift-off" process) a 2x3 array (300 micrometer pitch) of novel temperature nano-sensors called "Thin Film Thermocouples" (TFT) were designed and micro-fabricated on a silicon wafer. The wafer was subsequently sputter coated with an insulation layer (SiO2, 100 nm thick) followed by deposition of the photo-activated nanocoating layer (TiO2, 150 nm thick). After achieving steady state conditions, a single droplet of water was dispensed onto the heated surface, centered on the TFT array. The temperature transients recorded by the TFT array were used to estimate the spatial and temporal distribution of heat flux values (both local and global values) during evaporation and boiling of the individual droplets. High speed image acquisition (up to 1000 fps) was performed and synchronized with the high speed data acquisition system (~100 Hz). The transient profiles for temperature and heat flux along with the synchronized images of the droplet were combined into a single video file. These video images enabled the identification of several regimes of phase-change heat transfer during the droplet evaporation process. Significant improvement in heat flux (for both local and global average) values were observed for the nanocoatings, which were weakly affected by the UV illumination. This shows that the effect of enhanced surface roughness (nucleation site density enhancement) dominated over the effects associated with reduction in contact angle (higher bubble departure diameter and lower departure frequency). Large temporal variations (~102 degrees C/s) and spatial variations (~104 degrees C/m) in surface temperature (and therefore heat flux values at the surface) were observed to exist. These fluctuations in surface temperature were found to occur at time scales less than ~10 ms and length scales of ~300 micrometers.

publication date

  • May 2012
  • May 2012
  • May 2012