(285b) Determination of the Interaction Mechanism of 10 Um Oil-in-Water Emulsion Droplets Using Optical Tweezers

The stability of emulsions plays a crucial role in the production of many food, cosmetics, medicine, advanced materials. Emulsions can be stabilized using surfactants, polymer, nano-particles. These stabilizers readily adsorb onto the oil-water interfaces, resulting in an electrostatic double-layer repulsive force or a steric barrier that prevents the coalescence of emulsion drops. The stability of the emulsions relies on a well-developed understanding of dynamic interaction forces between single dispersed droplets. It is crucial to understand and measure the hydrodynamic interaction force and surface forces, such as van der Waals, electrostatic double layer, hydrophobic, steric interaction and so on.

Recently, developed experiment tools involving surface force apparatus (SFA) and atomic force microscope (AFM), have made such stability measurements possible by directly measuring the interaction forces between two deformable oil droplets under the various solution condition involving single ionic surfactant systems, polymers, polyelectrolytes, proteins, and salt solutions. The emulsion systems usually are chosen in the size range of 20-200 µm in the previous studies. Therefore, we focus on the deformable oil droplets diameter below 10 µm in the ionic surfactant and salt solution by optical tweezers in this work. For droplet diameter below 10 µm, the droplet internal pressure is large enough that equilibrium surface forces dominate the interaction over deformation and hydrodynamic effects. The internal pressure of droplet diameter below 10 µm probably has different effects on the deformation, surface force, and hydrodynamic interaction. Emulsion droplets with diameter below 10 µm have been widely used in various applications, such as toners, spacers for liquid crystal displays, and materials in biomedical and biochemical analysis. It is vital to summary the interaction mechanisms between two deformation oil droplets in the diameter below 10 µm.

Ionic surfactants readily dissociate in the aqueous solution and adsorb onto the oil-water interface, resulting in the electrostatic repulsive force that provides stability against coalescence. According to DLVO theory, higher surfactant concentrations bring out higher ion adsorption capacity. Therefore, droplets with more surfactants will cause higher repulsive effect. The concentration of salt solution makes a significant influence on the Debye length, while salt solution and surfactant have a common effect on the electrostatic double-layer force. Therefore, salt solution and surfactant have a profound influence on the measured force and the deformation of droplets during approach and separation process. In addition, the hydrodynamic pressure in the thin film of the continuous phase between two oil droplets can also make a major contribution to interfacial deformations and determine magnitude of the measured force at different velocities.

Optical tweezers were used to measure the interaction forces between two oil droplets in the diameter below 10 µm in this work. It is limited to find the quantitative measurement of dynamic interaction forces determined using optical tweezers in the previous studies. The magnitude of the interaction forces determined using optical tweezers was tens of pN (10^-12 N), providing higher force precision and more stable and precise force curve in the order of pN compared with other experiment tools. Optical tweezers have a unique advantage in deeper perspective of the dynamic interaction mechanism between tiny oil droplets. The experimental results of interaction forces between two oil droplets in salt solution in the presence of surfactant can be analyzed using DLVO theory and Reynolds lubrication theory. The magnitude of the measured force (pN) opened a new scale range of research of interaction mechanism between emulsion droplets in the work.

In this work, tetradecane was used as the model oil phase due to well - known physical and chemical properties and lots of research result in previous AFM studies, it is vital to compare this difference of the internal pressure, deformation, surface forces, and hydrodynamic interaction in different scale range. Optical tweezers (NanoTracker^TM 2) were used to measure the dynamic interaction forces between two tetradecane droplets with the diameter of 5.0 µm in different concentration of SDS and NaCl solutions. Optical tweezers are utilized by limiting the movement of particles, so it helps to accurately observe the dynamics of individual particles under precisely controlled conditions. The experimental results determined using optical tweezers could make a good agreement with DLVO theory and Reynolds lubrication theory. This work provides a useful approach to quantify the interaction behaviors of emulsified oil droplets with SDS adsorbed at the oil/water interface in the different salt solutions, and the results give insights into the stabilization mechanisms of the O/W emulsion droplets below 10 µm.