Regular papers

High resolution 2-D fluorescence imaging of the mass boundary layer thickness at free water surfaces

¹ Universit¨at Heidelberg, Institut für Umweltphysik, INF 229, 69120 Heidelberg, Germany
² Universit¨at Heidelberg, Heidelberg Collaboratory for Image Processing (HCI), Heidelberg, Germany

^* christine.kraeuter@iup.uni-heidelberg.de

Received: 15 November 2013
Revised: 19 February 2014

Abstract

A novel 2-D fluorescence imaging technique has been developed to visualize the thickness of the aqueous mass boundary layer at a free water surface. Fluorescence is stimulated by high-power LEDs and is observed from above with a low noise, high resolution and high-speed camera. The invasion of ammonia into water leads to an increase in pH (from a starting value of 4), which is visualized with the fluorescent dye pyranine. The flux of ammonia can be controlled by controlling its air side concentration. A higher flux leads to basic pH values (pH > 7) in a thicker layer at the water surface from which fluorescent light is emitted. This allows the investigation of processes affecting the transport of gases in different depths in the aqueous mass boundary layer. In this paper, the chemical system and optical components of the measurement method are presented and its applicability to a wind-wave tank experiment is demonstrated.