Tension gradient-driven oil/water interface rapid particle self-assembly and its application in microdroplet motion control

Hypothesis: A binary mixture was used during injection with one water-miscible component and the other water-immiscible, which can help particles to migrate toward and then self-assemble at the inter face. Experiments: The ethanol-tetrachloromethane binary mixture was used to verify the self-assembly method, with the diameter of droplets being about 1 mm. As the ethanol diffused into the colloidal solution, the colloidal particles efficiently moved towards and self-assembled on the oil/water interface, while a colloidal particle film with high-coverage was able to rapidly form on the droplet surface even in an ultra-low concentration colloidal solution. The effects of ethanol concentration and particle concentration on self-assembly were investigated. Findings: The driving force for self-assembly originated from the tension gradient generated by ethanol's concentration gradient at the particle/liquid interfaces, where the concentrations of ethanol and the colloidal solution had significant effects on self-assembly. The simulation and calculations results aligned well with experiments, providing the theoretical basis for this self-assembly method. Further, as-prepared magnetic particle-coated droplets transformed into a non-wetting soft solid, which had long lifetimes and could be precisely moved, coalesced, and transferred in various two-dimensional and threedimensional liquid environments. Thus, wider applications are facilitated, such as droplet transfer, microreactor and other potential fields. (c) 2020 Elsevier Inc. All rights reserved.

Automated summary

The use of a binary mixture during injection facilitated particles to migrate and self-assemble at an interface, with significant effects from ethanol and particle concentration. The self-assembly method allowed for high-coverage colloidal particle film formation at low concentrations, providing a theoretical basis for experimental and simulation studies.