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  The measurement of contact angles with the captive bubble method could also be useful in the surface analysis of the reverse osmosis membrane in the study of membrane performances. Through the analysis of contact angles, properties of membranes, such as roughness, can be determined. The roughness of membranes, which indicates the effective surface area, can further lead to the investigation in the hydrophilic and hydrophobic properties of the surface. Through studies, a higher contact angle may correspond to a more hydrophobic surface in membrane analysis. In the performance of the captive bubble method in membrane analysis, several factors can have influence on the contact angle, including the bubble volume, liquid types and tensions.
  In the apparatus of the captive bubble method in membrane analysis, the membrane sample is attached to a piece of glass, while a J-shaped needle is inserted from below the glass in the liquid to release air bubbles. In this case, the contact angle between the air bubble and the glass surface is recorded as a function of time to investigate the effect of measurement time.