Wetting of a liquid surface by another immiscible liquid in microgravity Original Research Article

The investigation of interfacial properties is essential to the development of new drugs either on earth or, particularly, in the absence of gravity. Under the reduced gravity conditions of parabolic flights, we have shown that, using an appropriate cell setup in order to control liquid surfaces, a liquid drop can be expanded onto and withdrawn from another immiscible liquid, which permits the measurement of the contact angle of this system. Surface energies of liquids being easily measurable, this technique allows a verification of numerous models used in interface science. During each parabola, 20 s of microgravity measurements permitted the acquisition of video pictures of these drops. Contact angles have been obtained from goniometric analysis of the recorded images. Generally, the drops obtained satisfied the equilibrium state predicted by Neumannʹs equations. However, unexpected long lasting metastable drops have also been observed on a curved unconfined liquid surface. The existence of a drop-sinking barrier, larger for a curved liquid surface, is proposed to explain this observation.