Emulsion characterization via microfluidic devices : A review on interfacial tension and stability to coalescence

Emulsions have gained significant importance in many industries including foods, pharmaceuticals, cosmetics, health care formulations, paintings, polymer blends and oils. During emulsion generation, collisions can occur between newly-generated droplets, which may lead to coalescence between the droplets. The extent of coalescence is driven by properties of dispersed and continuous phases, e.g. density, viscosity, ion strength and pH, and system conditions, e.g. temperature, pressure or any external applied forces. In addition, the diffusion and adsorption behaviors of emulsifiers which govern the dynamic interfacial tension of the forming droplets, the surface potential, and the duration and frequency of the droplet collisions, contribute to the overall rate of coalescence. An understanding of these complex behaviors, particularly those of interfacial tension and droplet coalescence during emulsion generation, is critical for the design of an emulsion with desirable properties and the optimization of the processing conditions. However, in many cases, the time scales over which these phenomena occur are extremely short, typically a fraction of a second, which makes their accurate determination by conventional analytical methods extremely challenging. In the past few years, with advances in microfluidic technology, many attempts have demonstrated that microfluidic systems, characterized by micrometer-size channels, can be successfully employed to precisely characterize these properties of emulsions. In this review, current applications of microfluidic devices to determine the equilibrium and dynamic interfacial tension during the droplet formation, and to investigate the coalescence stability of dispersed droplets applicable to the processing and storage of emulsions, are discussed.Peer reviewe

Design and fabrication of novel microfluidic systems for microsphere generation

In this thesis, a study of the rational design and fabrication of microfluidic systems for microsphere generation is presented. The required function of microfluidic systems is to produce microspheres with the following attributes: (i) the microsphere size being around one micron or less, (ii) the size uniformity (in particular coefficient of variation (CV)) being less than 5%, and (iii) the size range being adjustable as widely as possible. Micro-electro-mechanical system (MEMS) technology, largely referring to various micro-fabrication techniques in the context of this thesis, has been applied for decades to develop microfluidic systems that can fulfill the foregoing required function of microsphere generation; however, this goal has yet to be achieved. To change this situation was a motivation of the study presented in this thesis. The philosophy behind this study stands on combining an effective design theory and methodology called Axiomatic Design Theory (ADT) with advanced micro-fabrication techniques for the microfluidic systems development. Both theoretical developments and experimental validations were carried out in this study. Consequently, the study has led to the following conclusions: (i) Existing micro-fluidic systems are coupled designs according to ADT, which is responsible for a limited achievement of the required function; (ii) Existing micro-fabrication techniques, especially for pattern transfer, have difficulty in producing a typical feature of micro-fluidic systems - that is, a large overall size (~ mm) of the device but a small channel size (~nm); and (iii) Contemporary micro-fabrication techniques to the silicon-based microfluidic system may have reached a size limit for microspheres, i.e., ~1 micron. Through this study, the following contributions to the field of the microfluidic system technology have been made: (i) Producing three rational designs of microfluidic systems, device 1 (perforated silicon membrane), device 2 (integration of hydrodynamic flow focusing and crossflow principles), and device 3 (liquid chopper using a piezoelectric actuator), with each having a distinct advantage over the others and together having achieved the requirements, size uniformity (CV ≤ 5%) and size controllability (1-186 µm); (ii) Proposing a new pattern transfer technique which combines a photolithography process with a direct writing lithography process (e.g., focused ion beam process); (iii) Proposing a decoupled design principle for micro-fluidic systems, which is effective in improving microfluidic systems for microsphere generation and is likely applicable to microfluidic systems for other applications; and (iv) Developing the mathematical models for the foregoing three devices, which can be used to further optimize the design and the microsphere generation process

Expanding the applications of poly(dimethylsiloxane) in biomicrofluidics

This work aims to create novel applications for poly(dimethylsiloxane) (PDMS) in the field of biomicrofluidics through oxidative stress detection, doping of the polymer for intentional leaching into microdevices, and the development of low-cost implements for fabricating PDMS microfluidic devices. PDMS has become the polymer of choice for research in microfluidics due to its optical clarity, ease of fabrication, flexibility in design, good mechanical properties, and the ability to chemically modify the surface. Biomicrofluidics enables the rapid throughput and analysis of small biological samples requiring less time investment and reagent use than traditional macroscale laboratory techniques. Polymer devices are inexpensive, easily fabricated using rapid prototyping techniques, and lend themselves well to surface chemistry modifications. A new chemical surface modification has been developed that allows the selective capture of carbonylated proteins on a PDMS microchannel. PDMS can be doped with small molecules prior to curing of the prepolymer mixture, and these small molecules can subsequently leach into cell culture media or a microfluidic flow. By quantifying the leaching amount over time, this research lays the groundwork for tunable doped microfluidic devices that can deliver a steady low concentration dose of certain molecules into a cell culture or microdevice without human interference or risk of contamination. PDMS soft lithography traditionally relies on cleanroom techniques such as photolithography for creation of mold masters for PDMS devices. Such methods require significant investment into specialized equipment and environments to develop molds that may not be suitable for the desired applications. This research employs computational fluid dynamics (CFD) and rapid prototyping techniques in the development of novel microfluidic designs. CFD provides verification of the flow rate and pressure drop in a microfluidic channel, ensuring that the resulting flow speeds allow the captured proteins or attached cells in culture to remain attached to the microchannel. A 3D printer and an Arduino microcontroller were used to create a spin table for coating silicon wafers in photoresist, and a UV LED light source was designed for exposing the photoresist. This approach reduces the equipment cost involved in creating microfluidic molds and allows the creation of a variety of new microfluidic devices

Synthesis of anisotropic microparticles and capsules via droplet microfluidics

We have developed simplified microfluidic droplet generators and employed them to fabricate anisotropic polymer particles and capsules in the size range of 100–500 μm. We used cheap and generally available materials and equipment to design and assemble microfluidic devices. All our devices were made of standard wall borosilicate capillaries (OD 1.0mm, ID 0.58mm), steel dispensing needles without bevel (30 G, 32 G), microscopy glass slides, fast-curing epoxy glue (Araldite-80805) and diamond scribe to process the glass. We designed four different geometries for each device, which can be separated for two groups: single and double droplet generators. The performance of the devices was validated using computational fluid dynamics and laboratory experiments. First of all, we tried to fabricate intricate single emulsion droplets and then moved on to double emulsion droplets. The range of the fabricated particles and capsules includes anisotropically-shaped amphiphilic polymer “microbuckets”, biphasic particles, capsules with various fillers and stimuli responsive polymer vesicles. To produce such objects we employed different functional monomers, for instance “clickable” glycidyl methacrylate or hydrophilic 2-hydroxyethyl methacrylate. We also utilized several chemical and physical phenomena such as internal phase separation, wettability or polymer chain cross-linking to tune the properties of the synthesized particles. We investigated properties of the above mentioned particles and capsules. For example, “microbuckets” which are hydrophilic at the exterior surface, but hydrophobic inside the cavity, were able to withdraw oil droplets from an aqueous phase and “arrest” them inside the cavity

Bioinspired Design of Wetting and Anti-Wetting Surfaces via Thiol-ene Photopolymerization

Surface wettability is known to have a profound influence in both academic study and industrial application of materials. Superhydrophobic surfaces, with a static contact angle higher than 150° and a contact angle hysteresis lower than 10°, have received continued attention for their broad applications, such as self-cleaning, antifogging and frosting, and drag reduction. The continuous development of materials and approaches that used to create superhydrophobic surfaces has led to further exploration of coatings with other desirable properties such as superamphiphobicity, mechanical robustness and thermal stability. In this work, coatings with super wetting and super anti-wetting properties were designed and fabricated by tailoring the chemical composition and the morphology of the surface in an effort to expand the application and to improve the mechanical property of the coatings. In the first study, a superamphiphobic coating was prepared by spray deposition and followed up UV-polymerization of a hybrid organic-inorganic thiol-ene precursor. The combination of dual-scale roughness and low surface energy materials led to surfaces with strong water/ oil repellency and self-cleaning properties. In the second study, a superhydrophilic and superoleophobic membrane for oil/water separation applications was developed. The textured membrane morphology enhanced the hydrophilic and oleophobic properties of the surface. The efficiency of the superhydrophilic/superoleophobic membrane on oil/water separation was demonstrated by emulsion and dye contained emulsion separation studies. In the third study, a superhydrophobic surface was prepared with porogen leaching approach in an effort to reduce the loading level of NPs. The microphase separation and porogen leaching process resulted in microscale roughness. NPs migration from bulk to interphase led to the formation of nanoscale roughness. The combination of micro- and nano-scale feature provides the surface with superhydrophobicity with 50 wt.% reduced NPs loading level

Functionalized silicone composites: omniphobic coatings, microspheres and plastic explosives

Silicones are ubiquitous polymers containing a silicon-oxygen backbone and a variety of functional groups that can be tailored to very specific applications. Their flexibility, biocompatibility and relative inertness make them the ideal choice in materials as diverse as cosmetics, defoaming agents in food and medical implants. This thesis will focus on three separate projects, each one a silicone-based composite. Chapter 1 is an overview that includes a brief history, background, synthesis, applications, chemical structure, and any other relevant information regarding silicones. Chapter 2 describes the successful fabrication of a sprayable omniphobic coating that contains a polydimethylsiloxane binder and nanoparticle ZnO. A coating, or any other surface, is considered omniphobic if it is both water-repellent (i.e. hydrophobic) and oil-repellant (i.e. oleophobic). The coating herein was sprayed on a variety of different surfaces, such as metal mesh, filter paper and bare aluminum, rending them resistant to liquid contamination. The desired application of this coating is to promote efficient heat transfer in condensing pipes by preventing insulating oily films from forming on their interior. By keeping the surface free of films, more heat may be available for transport to the ambient environment. Chapter 3 describes the synthesis of silicone microspheres via ultrasonic spray pyrolysis. A viable route to silicone microspheres has eluded researchers for many years, in large part due to the very low surface energy of silicone polymers. This prevents a simple emulsion route; the surface energy promotes agglomeration, a problem which cannot be combatted effectively by common surfactants. Microfluidic devices are expensive and afford only low yield and even lower throughput. Thus, we have developed a simple route which nebulizes silicone precursors into micron-sized aerosol droplets and flows the droplets through a furnace tube, where curing and solvent evaporation take place. Since each droplet is its own micro-reactor, each produces a well-formed microsphere with ano observable agglomeration. Furthermore, we can tune the size and composition of these microspheres simply by altering the concentration and components of the precursor. Chapter 4 describes a series of experiments on silicone-based plastic explosives. There is a paucity of literature regarding the controlled shock impact and subsequent detonation of commonly used explosives. What reports exist rely on computer-modelling and idealized assumptions to make conclusions about the thermomechanical and chemical nature of these events. When an actual explosive is used, it is often loosely packed powder which is of low density and contains many pores and defects. We have devised a method that uses mild-impact sources to generate explosions in a very small amount of explosive material. We incorporate instrumentation that allows us to see, with nanosecond resolution, the temperature and spectral emission of this explosive, under real-life impact conditions

MULTIPHASE MICROFLUIDIC FABRICATION OF POLYMERIC MICRO-CONTAINERS FOR BIO-ENCAPSULATION

Ph.DDOCTOR OF PHILOSOPH

Electrohydrodynamic Manipulation Of Liquid Droplet Emulsions In A Microfluidic Channel

This work specifically aims to provide a fundamental framework, with some experimental validation, for understanding droplet emulsion dynamics in a microfluidic channel with an applied electric field. Electrification of fluids can result in several different modes of electrohydrodynamics (EHD). Several studies to date have provided theoretical, experimental, and numerical results for stationary droplet deformations and some flowing droplet configurations, but none have reported a method by which droplets of different diameters can be separated, binned and routed through the use of electric fields. It is therefore the goal of this work to fill that void and report a comprehensive understanding of how the electric field can affect flowing droplet dynamics. This work deals with two primary models used in electrohydrodynamics: the leaky dielectric model and the perfect dielectric model. The perfect dielectric model assumes that fluids with low conductivities do not react to any effects from the small amount of free charge they contain, and can be assumed as dielectrics, or electrical insulators. The leaky dielectric model suggests that even though the free charge is minimal in fluids with low conductivities, it is still is enough to affect droplet deformations. Finite element numerical results of stationary droplet deformations, implemented using the level set method, compare well both qualitatively (prolate/oblate and vortex directions), and quantitatively with results published by other researchers. Errors of less than 7.5% are found when comparing three-dimensional (3D) numerical results of this study to results predicted by the 3D leaky dielectric model, for a stationary high conductivity drop suspended in a slightly lower conductivity suspending medium. Droplet formations in a T-junction with no applied electric field are adequately predicted numerically using the level set finite element technique, as demonstrated by other researchers and verified in this study. For 3D models, droplet size is within 6%, and droplet production frequency is within 2.4% of experimental values found in the microfluidic Tjunction device. In order to reduce computational complexity, a larger scale model was solved first iii to obtain electrical potential distributions localized at the channel walls for the electrode placement configurations. Droplet deceleration and pinning is demonstrated, both experimentally and numerically, by applying steep gradients of electrical potential to the microchannel walls. As droplets flow over these electrical potential “steps,” they are pinned to the channel walls if the resulting electric forces are large enough to overcome the hydrodynamic forces. A balance between four dimensionless force ratios, the electric Euler number (Eue – ratio of inertial to electric forces), Mason number (M a – ratio of viscous to electric forces), electric pressure (P s – ratio of upstream pressure forces to electric forces), and the electric capillary number (Cae – ratio of electric to capillary forces) are used to quantify the magnitudes of each of these forces required to pin a droplet, and is consistent with a cubic dependency on the drop diameter. For larger drop diameters, effects of hydrodynamic forces become more prominent, and for smaller droplets, a greater electric forces is required due to the proximity of the droplet boundary with reference to the electrified channel wall. Droplet deceleration and pinning can be exploited to route droplets into different branches of a microfluidic T-junction. In addition, using steep electrical potential gradients placed strategically along a microchannel, droplets can even be passively binned by size into separate branches of the microfluidic device. These characteristics have been identified and demonstrated in this work

Dielectrophoretic characterization of particles and erythrocytes

Medical lab work, such as blood testing, will one day be near instantaneous and inexpensive via capabilities enabled by the fast growing world of microtechnology. In this research study, sorting and separation of different ABO blood types have been investigated by applying alternating and direct electric fields using class=SpellE\u3edielectrophoresis in microdevices. Poly(dimethylsiloxane) (PDMS) microdevices, fabricated by standard photolithography techniques have been used. Embedded perpendicular platinum (Pt) electrodes to generate forces in AC dielectrophoresis were used to successfully distinguish positive ABO blood types, with O+ distinguishable from other blood types at \u3e95% confidence. This is an important foundation for exploring DC dielectrophoretic sorting of blood types. The expansion of red blood cell sorting employing direct current insulative class=SpellE\u3edielectrophoresis (DC-iDEP) is novel. Here Pt electrodes were remotely situated in the inlet and outlet ports of the microdevice and an insulating obstacle generates the required dielectrophoretic force. The presence of ABO antigens on the red blood cell were found to affect the class=SpellE\u3edielectrophoretic deflection around the insulating obstacle thus sorting cells by type. To optimize the placement of insulating obstacle in the microchannel, COMSOL Multiphysics® simulations were performed. Microdevice dimensions were optimized by evaluating the behaviors of fluorescent polystyrene particles of three different sizes roughly corresponding to the three main components of blood: platelets (2-4 µm), erythrocytes (6-8 µm) and leukocytes (10-15 µm). This work provided the operating conditions for successfully performing size dependent blood cell insulator based DC dielectrophoresis in PDMS microdevices. In subsequent studies, the optimized microdevice geometry was then used for continuous separation of erythrocytes. The class=SpellE\u3emicrodevice design enabled erythrocyte collection into specific channels based on the cell’s deflection from the high field density region of the obstacle. The channel with the highest concentration of cells is indicative of the ABO blood type of the sample. DC resistance measurement system for quantification of erythrocytes was developed with single PDMS class=SpellE\u3emicrochannel system to be integrated with the DC- class=SpellE\u3eiDEP device developed in this research. This lab-on-a-chip technology application could be applied to emergency situations and naturalcalamities for accurate, fast, and portable blood typing with minimal error