Interactions between micro-scale oil droplets in aqueous surfactant solution determined using optical tweezers

An Chen; Shao-Wei Li; Fu-Ning Sang; Hong-Bo Zeng; Jian-Hong Xu

doi:10.1016/j.jcis.2018.07.116

Interactions between micro-scale oil droplets in aqueous surfactant solution determined using optical tweezers

J Colloid Interface Sci. 2018 Dec 15:532:128-135. doi: 10.1016/j.jcis.2018.07.116. Epub 2018 Jul 29.

Authors

An Chen¹, Shao-Wei Li², Fu-Ning Sang¹, Hong-Bo Zeng³, Jian-Hong Xu⁴

Affiliations

¹ The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China.
² Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China. Electronic address: lsw@tsinghua.edu.cn.
³ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada.
⁴ The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China. Electronic address: xujianhong@tsinghua.edu.cn.

PMID: 30077826
DOI: 10.1016/j.jcis.2018.07.116

Abstract

Hypothesis: The stability of the emulsions is crucial, which relies on a well-developed understanding of dynamic interaction forces between single dispersed droplets. In the previous studies, many interests focus on the oil droplets of size range of 20-200 µm. However, emulsion droplets with diameter below 10 µm are rarely mentioned, which is the size scale of real emulsion droplets in various applications, such as toners, spacers for liquid crystal displays, and materials in biomedical and biochemical analysis. The micro-scale droplets have many differences on the deformation, internal pressure and hydrodynamic effects. It is necessary to understand the interaction mechanisms between two real size scales of oil droplets for guiding practical production and application.

Experiments: In this work, tetradecane was chosen as the model oil phase in all experiments. The interaction forces of two tetradecane droplets with the diameter of 5.0 µm in water in the presence of surfactant and salt solution were directly measured using optical tweezers. The force-distance curves were established, and the zeta potential of tetradecane droplets was studied using Zetasizer Nano ZSP.

Findings: The absolute value of zeta potential of tetradecane droplets was found to decrease with the increase of salt concentration and increase with the increase of surfactant concentration. The repulsive force between two tetradecane droplets was found to decrease with the increase of salt concentration because the electrostatic double-layer force was suppressed gradually with the increase of salt concentration. The "hydrodynamic suction" effect during the process of retraction becomes more pronounced due to the corresponding increase in the hydrodynamic force with the increase of the approaching velocity between the tetradecane droplets. Furthermore, we found the existing model for the measurement of large droplets by atomic force microscope (AFM) is invalid for the measurement of micro-scale droplets by optical tweezers. The deformation of colliding micro-scale droplets can be safely ignored, which is quite different from the large droplets. Our results provide a useful method to study the interaction forces between micro-scale emulsion droplets with pN force resolution, and gives a deep insight of the stabilization mechanism of real size scale of O/W emulsions. These findings have significant implications on the stability of emulsions in many food, cosmetics, medicine, and advanced materials.

Keywords: Interaction forces; Ionic surfactant; Micro-scale droplets; Optical tweezers; Stabilization mechanism; Tetradecane.