Transport phenomena of immiscible fluids in microchannels

• Main Author: Yin, Shuai
• Other Authors: Wong Teck Neng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Mechanical engineering::Fluid mechanics
• Online Access: https://hdl.handle.net/10356/146705
• Institution: Nanyang Technological University

Due to the prevalence of miniaturization in the past two decades, microdroplets have been widely applied to the fields of chemistry, biology, and material science. Particularly, microdroplet has recently emerged as an excellent platform for diverse applications because of its incomparable controllability in handling small volumes of fluids. The precise formation of the microdroplet in microfluidic devices has drawn tremendous attention from researchers. Coupled with external control, active droplet formation is proposed to further improve the flexibility of the technique and hence the overall performance of the microfluidic devices. The electric field has been proven to be an effective active technique in microfluidic devices for precise manipulation of microdroplets. The potentials of complex droplets, specifically oil in water in oil (O/W/O) and water in oil in water (W/O/W) double emulsions, are extended by investigating the effectiveness of AC electric fields on the formation and the manipulation of the two droplet systems. The double-flow-focusing geometry together with the non-contact type of electrodes is adopted to achieve the hydrodynamic generation of the double emulsions under the AC electric field. The results show that the AC electric field is capable of controlling the size of the outer droplet in the O/W/O system hence achieving the control of the inner core droplets. The electric capillary number Ca_E was adopted to analyze the effectiveness of the AC electric field applied at high frequencies, which offered a guideline for practical applications. Moreover, both the one-step and the two-step merging of the core droplets in the W/O/W droplet system have been achieved within 100 milliseconds, which are by far the fastest merging in double emulsions ever reported. To reveal the mechanism behind the electric-field-controlled droplet formation, the water in oil (W/O) droplet formation was investigated in a flow-focusing microchannel under AC electric field experimentally and numerically. A three-dimensional numerical model was built combining the Volume of Fraction (VOF) method and the leaky dielectric model, which reveals the droplet formation mechanism under the effects of an electric field. Due to the Maxwell stress induced by the electric field, the sinusoidal waveform electric field induces the oscillation at the liquid interfaces, which accelerates/stimulates the breakup of the disperse phase and thus tunes the droplet size. The phenomena were analyzed by the electric capillary number Ca_E evaluated according to the numerical results. The dominating effect of the pressure difference between the disperse phase and the continuous phase shifts from the initial hydrodynamic pressure pattern to an electric-field-induced one during the evolution of the electric voltage. The numerical results show that the surge in the magnitude of the electric body force tends to stretch the disperse phase at liquid interfaces, which leads to the transition of droplet formation from dripping to jetting under the electric field. This study will lead to a better control strategy for droplet formations with applied electric fields. In addition, a universal and simple method is developed to actively generate oil in water (O/W) droplet in a Poly(dimethylsiloxane) (PDMS) based microfluidic device by using both DC and AC electric fields to cut a stable layered flow of disperse phase into dispersed droplets in a precise manner. The system owns the feature of ultra-fast response and precise control of O/W droplet generation. The square waveform electric field was proven to be effective in terms of precise cutting and tuning the size of the generated droplet. Controlled by the sinusoidal AC electric fields, the breakup of the disperse phase can be cataloged into three stages, namely, non-breakup, transition, and continuous breakup. The voltage boundaries in between these stages were also located. To demonstrate the generality of the proposed method, the control strategy was applied to four typical oil fluids, all of which had gone through thoughtful tests with success considering different hydrodynamic characteristics. The biggest advantage of the proposed approach is that it removes the necessity of surface treatment for O/W droplet formation in PMDS microchannels, breaking the limits brought by the surface wettability for the first time and rendering itself to be a universal method for O/W droplet active generation with outstanding controllability. Our findings would widen the applications of the PDMS based microchips. In the end, a novel magnetic Janus particle as a steerable micro-scale suspension was proposed, consisting of a liquid oil compartment and a solidified compartment with magnetic nanoparticles inside. The formation of the Janus droplet in the microfluidic device was studied and hence the Janus particle is fabricated by accurate UV curing of the magnetic compartment within the Janus droplet. The size and the ratio between the two compartments of the Janus droplet can be controlled by hydrodynamical conditions. A custom-made external magnetic field was applied to steer the Janus droplet. Rapid rotation of the Janus particle under the magnetic field was studied and the relationship between the angular velocity of the Janus particle and the frequency of the applied magnetic field was revealed. The results demonstrate the potential of the magnetic Janus particle for micro-scale vortex generation and substance steerable carrier.