Method and apparatus for determining the interfacial tension between two liquids

A method for determining the dynamic interfacial tension of a surfactant between two liquids, comprising the following steps:

• flowing a first liquid, the internal liquid (Li), in an internal flow member (2);
• flowing a second liquid, the external liquid (Le), in an external flow member (4);
• and,
• adding a surfactant to the one and/or the other of the first and second liquids;
• wherein the respective internal and external flow members are coaxial and the internal member opens into the internal volume of the external flow member;
• forming droplets (G) of the internal liquid, in a carrier phase (P) formed by the external liquid, downstream of the outlet (2′) of the internal flow member in the external flow member, under conditions set for a plurality of values of flow rate (Qe), the flow rate of the external liquid;
• and determining the value of time (t) of formation of these droplets;
• subsequently varying the internal flow rate (Qi);
• and
• after identifying a transition value of the internal flow rate above which a continuous jet of the internal liquid is then formed in the external liquid, deducing therefrom a value of the interfacial tension (γ) between these two liquids;
• and
• producing a curve representing the variation of the interfacial tension value as a function of the droplet formation time;
• and
• identifying a characteristic time (tk) of the surfactant, corresponding to the transition between a first zone (I) in which the value of the interfacial tension is approximately constant as a function of the droplet formation time and a second zone (II) in which this value of the interfacial tension increases as this formation time decreases.