Contactless measurements of salt-enhanced nonionic micelle-induced depletion forces by optical tweezers

Previous studies have found that ionic surfactant micelles can induce depletion forces between oil droplets, whereas the depletion forces induced by nonionic surfactant micelles cannot be measured. Nonionic surfactants are usually mixed with ionic surfactants to induce depletion forces. Here, we report that mixing of nonionic polymer micelles and inorganic salt is also an effective method for inducing depletion between oil droplets. Optical tweezers are used to measure the interaction forces between oil droplets in the aqueous solution of polyoxyethylene–polyoxypropylene–polyoxyethylene nonionic polymer. Without salts, there is no depletion force between oil droplets regardless of whether the concentration of polymer is above or below the critical micelle concentration (CMC). This result is consistent with literature that the oil drops adsorbed by the nonionic polymers were stable against coalescence. However, the depletion force between oil droplets generates when NaCl is added, and the concentration of Pluronic F127 is above or equal to the CMC. This suggests that salts can enhance the depletion force induced by nonionic polymer micelles. These findings have significant implications for the stability of emulsions in the food, oilfield, and medicine industries.