Electrokinetic phenomena of electrically induced Janus droplets and their applications
Janus droplets refer to droplets comprised of two hemispheres with different properties. Among the Janus droplets, electrically anisotropic Janus droplets with two sides carrying opposite signs of surface charges are unique. Due to their specific properties, electrokinetic phenomena of the electrically anisotropic Janus droplets are quite different from homogeneous ones and are not covered by the classical electrokinetic theory. The electrically heterogeneous Janus droplets have great potential in many fields, such as biotechnology, materials science, pharmaceutical science, food analysis as well as chemistry. Although various techniques have been developed to form the Janus droplets with different anisotropic properties, the techniques for generating the electrically anisotropic Janus droplets are seldom reported. Restricted by the fabrication methods, the studies of electrokinetic phenomena of electrically anisotropic Janus droplets and the applications are limited. This thesis systematically studies the electrokinetic phenomena of electrically induced Janus droplets (EIJDs), as well as their corresponding applications in microfluidic systems. The initial stage of the thesis is focused on developing simple and controllable methods for generating EIJDs and droplets with multiple heterogeneous surface strips with nanoparticles. Both sessile and suspended EIJDs are formed by partially covering the oil droplets with Al2O3 nanoparticles under an electric field. Because the Al2O3 nanoparticles and the oil-water interface carrying surface charges with opposite signs, the EIJDs are electrically anisotropic. The nanoparticle coverage of the EIJDs is controllable using the concentration of the nanoparticle suspension and the electric field strength. The droplets with multiple heterogeneous surface strips are prepared in a microfluidic chip under an electric field. By controlling the delivery of nanoparticles in the microfluidic chip, different nanoparticles, Al2O3, MgO and ZnO, accumulate on the surfaces of the oil droplets to form desired strips. In fundamental part, the studies of the electrokinetic phenomena of the EIJDs are conducted, including electroosmosis, electrokinetic motion and wall-induced dielectrophoresis. Electroosmotic flow fields around sessile EIJDs are visualized with the particle tracing method. Because two sides of the Janus droplets carry opposite surface charges, vortices can be generated around the dipoles under electric field. To understand the evolution of these vortices, the effects of the electric field strength and nanoparticle coverage of the EIJDs on the vortices are studied. The comparisons between the experimental results and the numerical results indicate good agreement. The Electrokinetic motions of the suspended EIJDs in a straight microchannel under both a relatively weak electric field and a relatively high electric field are investigated, respectively. In this study of the electrokinetic motion, the effects of the electric field strength, the nanoparticle coverage of the EIJDs, the droplet size and the electrolyte concentration on the electrokinetic velocity of the EIJDs are studied systematically. The results indicate that, under weak electric field, nonlinear electrokinetic motion of the EIJDs is observed due to the variation of the nanoparticle coverage with electric field. Finally, the wall-induced dielectrophoretic lateral migration of the EIJDs in a microchannel is studied theoretically and experimentally. The lateral migration of the EIJDs is compared with that of the oil droplets, and it is shown that separation of target EIJDs is accomplishable with wall-induced dielectrophoresis. Two applications of the Janus droplets are introduced in this thesis: microvalve and micromotor. The EIJD-based microvalve is controllable using electric field. By testing the performance of the microvalve systematically, the capability of such an EIJD-based microvalve in sealing, switching time and flow rate control is confirmed. The micromotor moves spontaneously in an alkaline solution through the propulsion of gas bubbles generated on the particle-coated side of the Janus droplet. The factors affecting the motion of the microvalve include: time, pH value of the buffer solution, particle coverage and surfactant. The experimental results verify that the directional motion of the micromotor can be accomplished using an externally applied electric field. This thesis develops simple methods for fabricating EIJDs and droplets with multiple heterogeneous surface strips. The fundamental research in this thesis extends the understanding in the electrokinetic phenomena. The microvalve and the micromotor fabricated from the Janus droplets offer great potential in various microfluidic devices and applications.
Cite this version of the work
Mengqi Li (2018). Electrokinetic phenomena of electrically induced Janus droplets and their applications. UWSpace. http://hdl.handle.net/10012/14197