A study of captive bubbles with axisymmetric drop shape analysis

Abstract

Of the many methodologies used in surface thermodynamic experimentation, Axisymmetric Drop Shape Analysis (ADSA) has proven to be one of the most accurate and versatile. Until recently, however, ADSA has had two main limitations: (1) the inability to analyse drops with near-zero curvature at the apex; and (2) problems with drop edge detection when the contrast between the drop and the background is not adequately sharp. In this paper, ‘second-generation’ ADSA algorithms and new image analysis schemes are introduced as solutions to these shortcomings. To illustrate these improvements, an inverted sessile air bubble in a pulmonary surfactant suspension is used. This ‘captive bubble’ geometry was chosen to overcome the problems of film leakage at low surface tensions. The modified experimental apparatus, called ADSA-captive bubble (CB), in conjunction with the new software, proves to be a powerful technique to investigate these types of systems. ADSA-CB is fully automated, requiring less user intervention, enabling the collection of large amounts of data, and thereby providing intricate details of surface behaviour. The so-called ‘squeeze-out’ and film collapse phenomena are examined closely. To demonstrate the problems of film leakage, pendant (hanging) drop experimental results are also presented.