Semiautomatic measurement of contact angles on cell layers by a modified axisymmetric drop shape analysis

• Published in: Colloids and surfaces Vol. 42; no. 3; pp. 391 - 403
• Main Authors: Duncan-Hewitt, W.C., Policova, Z., Cheng, P., Vargha-Butler, E.I., Neumann, A.W.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Amsterdam Elsevier B.V 15.12.1989; Elsevier
• Subjects: Biological and medical sciences; Diverse techniques; Fundamental and applied biological sciences. Psychology; Molecular and cellular biology
• ISSN: 0166-6622; 1873-4340
• DOI: 10.1016/0166-6622(89)80354-3

The contact angle between a sessile drop of liquid and a bed of biological particles such as bacterial cells provides a measure of the hydrophobicity or, more precisely, the surface tension of the cells. While this surface characteristic plays an important role in diverse processes such as cell adhesion and phagocytosis, it is difficult to obtain contact angles which are meaningful in a thermodynamic sense using traditional methods of measurement (e.g., goniometric techniques) because: (1) the contact angles, which are inherently small on biological substrates in their native hydrated state, are difficult to measure with precision; (2) the contact angle of the sessile drop invariably decreases as the liquid is absorbed into the layer of cells, and; (3) the heterogeneous and often rough surfaces produced by any method of cell deposition give rise to sessile drops which do not possess a perfectly circular perimeter. A modified axisymmetric drop shape analysis (ADSA-CD) permits the contact angle of a sessile drop to be calculated from the average diameter of the drop as viewed from above, and provides the means to circumvent these problems. The ADSA-CD procedure was used to measure the contact angle of water on layers of three different species of bacteria. The average contact angles for T. thiooxidans and Staph epidermidis were 16.9 and 20.6°, respectively. The contact angles for two different strains of T. ferrooxidans were 11.7 and 10.5° and were not statistically different. The 95% confidence intervals for these means, obtained from fewer than 17 independent measurements were less than or equal to ± 1.0°. We show that this novel strategy provides: (1) increased objectivity with respect to the goniometric method; (2) a precise estimate of the contact angle in spite of the fact that the contact angle is time dependent; (3) a simple means of assessing surface quality and the circularity of the drop periphery on biological surfaces.