16 research outputs found

    Microvortices In Droplets: Generation & Applications

    Get PDF
    The emerging field of droplet microfluidics deals with the manipulation of nL-fL droplets encapsulated within an immiscible carrier phase. The droplets are used as reaction containers for biochemical assays, enabling drastic reduction in assay volumes needed for modern life sciences research. To achieve this, basic laboratory processes such as mixing, detection, and metering must be emulated in the droplet format. Three important unit operations relevant to high throughput screening include 1) the concentration of particles and species within droplets, which is necessary for heterogeneous assays; 2) sensing the biochemical contents of a droplet; and 3) the sorting of droplets based on physical or chemical properties, which is important for single cell and proteomic assays. Currently, particle concentration in droplets requires active components, such as on-chip electrodes or magnets, along with charged or magnetic particles. Similarly, sensing and sorting droplets by chemical composition is based on flow cytometry, which also requires on-chip electrodes, feedback control, and chemical labeling. It is desirable to avoid active field techniques due to complexity, size, and cost constraints, and replace them with more simple and passive techniques. In this thesis, we utilize microvortices, the rotational motion of fluid, to enhance the capabilities of droplet microfluidics in the above three areas. The microvortices are generated using two methods: (i) hydrodynamic recirculation drag and (ii) tensiophoresis. In the first method, species concentration is accomplished by exploiting the shear-induced vortices that occur naturally inside a droplet/plug as it moves through a microchannel. Prior studies utilized these flows for enhancing mixing or interphase mass transfer. This work exploits microvortices together with two other independent phenomena--sedimentation of particles and interfacial adsorption of proteins--to concentrate both types of species at the rear of the droplet, where they can be extracted from the drop. In the latter case, the protein localization at the rear of drop reduces the interfacial tension locally resulting in an asymmetry in the drop shape. Under laminar flow, the shape deformation is deterministic and can serve as a sensitive, label-free indicator of protein concentration in proteomic screening. In the second method, label-free sorting of droplets is accomplished by a novel droplet actuation technique termed Tensiophoresis. A microchemical gradient across the droplet is transduced into a microvortex flow which propels the droplets up the chemical gradient. Using laminar flow to precisely control the gradient, droplets can be sorted by size with 3.3% resolution over a wide turning range. Droplets can be also sorted based on chemical composition because tensiophoresis is inhibited by surface active agents adsorbed on the droplet surface. Studies conducted using Bovine Serum Albumin (BSA) show that the droplet migration velocity scales inversely with protein concentration in the droplet, and migration velocity can be correlated to protein concentration with a 1 femtomole limit of detection. As modern life sciences research becomes increasingly reliant on high throughput workflows, microdroplet technology can meet the growing demand to perform screening at ultra-high throughputs with reduced sample volume. This thesis contributes three important unit operations which expand the capabilities of droplet-based workflows in proteomics, cell biology, and other areas of biomedical research

    Particle sorting and automatic particle identification for advanced medical diagnostics

    Get PDF
    The physical separation of micro-particles is very important in many research field as diverse as chemistry and medicine. The main goal of the current separation techniques is to extract micro-particles such as cells at a high processing rates and purity. Chromatography, for instance, is commonly applied for the detection and enrichment of pathogens, which is useful for the medical diagnostics of parasitic infections. Many separation techniques have been developed over the years, applying physical phenomena of different kinds and/or taking advantage of unique physical properties of the particles themselves. From all of these techniques, one that has remained popular over the years is Dielectrophoresys(DEP). One of the main reasons for its popularity is that it does not require markers of any kind; it takes advantage of differences in the particle’s polarizability, size and shape. Another distinctive characteristic of dielectrophoresis is its selectivity due to its capacity to be controlled using frequency and voltage amplitude and its suitability for small microfluidic systems. In very general terms the work I have done during my PhD studies was oriented towards the development of novel and robust technology for aiding in the micro-particle sorting and bio-particle recognition by using computer tools. The ideas and concepts I will be introducing throughout this document were allowed total freedom to evolve and change to better fulfill the main goals of the project and also to better adapt to the many technical challenges I had to face during my research. As well as developing a new dielectrophoresis method I have also tried to maximize the impact of this work by doing it in a truly accessible way for anyone, regardless if they are interested in basic research, a possible application or just looking to adapt this concepts and tools for a different purpose. The central work in this PhD thesis focuses on two main topics: - Computerized bio-particle tracking and identification using a machine-learning algorithm that incorporates a number of predictors, including colour histogram comparison. - A portable dielectrophoresis(DEP) electronic device able to tailor the potential across a microfluidic channel for particle separation. The first project is about computerized vision system designed to track and identify micro-particles of interest through the use of video microscopy, machine learning and other video processing tools. This system uses a novel particle recognition algorithm to improve specificity and speed during the tracking and identification process. We show the detection and classification of different types of cells in a diluted blood sample using a machine-learning algorithm that makes use of a number of predictors, including shape and color histogram comparison. This software can be considered as a stand alone piece of work. Its open source nature makes it ideal for scientific purposes or as a starting point for a different application. In the context of this PhD thesis, however, it is an invaluable tool for validating and quantifying experimental results obtained from the micro-particle separator experiments presented in Chapter 4. The central piece of work in this PhD thesis is introduced in Chapter 4. This project is about the development of a all-in-one continuous flow DEP based microparticle separator which uses a system of individually addressable electrodes to shape and control the particle’s potential energy profile across the entirety of a microfluidic channel. These tailored potential landscapes are created by averaging the electric field generated by 64 individual electrodes, where the electronic device has complete control over each electrode’s on/off state, frequency, AC voltage amplitude and pulse duration. All the characteristics of the potential landscapes are controlled wirelessly through a mobile phone application. These specially designed potential landscapes allow us to make lateral sorting and/or concentration of a binary mixture of particles at the same time they move through a microfluidic channel; all this without the need for buffer flows or additional external forces. One of the outstanding characteristics of this new sorting technique is that it relays exclusively on negative DEP. Most previous techniques require a combination of positive and negative DEP and possibly and external force of different nature to achieve particle sorting; all of which requires the use of a crossover frequency and hence a careful control of the conductivity of the suspending medium. Here by using only negative DEP we eschew the careful control over the conductivity of the suspending medium and the use of any other external force; all this contributes to make our device small and robust. In addition to this, our electronic device was designed to include all the supporting electronics it needs in a small and robust printed circuit board that can also be operated by batteries. We present simulation results to illustrate the physics behind this new technique along with experimental results demonstrating the separation of polystyrene beads

    Microthermal Devices for Fluidic Actuation by Modulation of Surface Tension.

    Full text link
    Fluid manipulation at the micrometer scale has traditionally involved the use of batch-fabricated chips containing miniature channels, electrodes, pumps, and other integrated structures. This dissertation explores how liquids on non-patterned substrates can be manipulated using the Marangoni effect. By placing miniature heat sources above a liquid film, it is possible to generate micro-scale surface temperature gradients which results in controlled Marangoni flow. A variety of useful flow patterns can be designed by tailoring the geometry of the heat source. As a surface tension-based phenomenon, the Marangoni effect is an efficient actuation mechanism at submillimeter dimensions. With optimized liquid carriers, flow velocities >10 mm/s can be generated with only small perturbations in surface temperature (1700 µm/s flow velocity in mineral oil while consuming <20 mW of power. In water films, the probes can generate surface doublets with linear velocities up to 5 mm/sec and rotational velocities up to 1300 rpm, making them potentially useful for active mixing. The utility of Marangoni flows is demonstrated within the context of digital microfluidic systems. In contrast to conventional microfluidics, where samples are flowed through microchannels, digital microfluidic systems contain liquid samples in micro and nanoliter-sized droplets suspended in an immiscible oil layer. Marangoni flows generated in the oil layer can manipulate droplets without any physical structures, thus avoiding surface contamination. By using point, linear, annular, and tapered heat source geometries, it is possible to engineer Marangoni flows which mimic the functionality of droplet channels, mixers, size-selective filters, and pumps. Arbitrary, two-dimensional actuation of droplets (Ф=400-1000 µm) can also be achieved using an array of heaters suspended above the oil layer. The 128-pixel heater array incorporates addressing logic and a software interface which allows it to programmatically transport and merge multiple droplets through the sequential activation of heaters. The appendices outline other aspects of thermal probes, including i) the structure, fabrication, and operational characteristics of single probes and probe arrays, and ii) scanning thermal lithography, a technique for nanoscale patterning of thin films with heat.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/60871/1/basua_1.pd

    Thermally induced motion, collision and mixing of levitated droplets

    Get PDF
    This dissertation investigates the motion of a levitated droplet experimentally and analytically against the Marangoni flow in an immiscible outer fluid at higher speeds than is possible currently. Based on our earlier experiments, when a droplet is released from a height of 1.5 – 4 times its diameter from the liquid surface, it can overcome the impact and stay levitated at the liquid-air interface due to the existence of an air gap between the droplet and the liquid film. In order to explain this behavior of droplet traveling against the counter-current motion, we propose a simple approach: first, the Marangoni convection inside the thin film is considered without the droplet floating on the surface. By using a level-set method and solving the Navier-Stokes equation, the free surface velocity and deformation are calculated. Then, these quantities are used to solve for droplet velocity and drag coefficient simultaneously using a force balance. In order to compare the simulation results, experiments with levitated water droplets on an immiscible carrier liquid, FC-43, were conducted for various temperature gradients, and droplet velocities were measured at different locations using high-speed imaging. The experimental results are in good agreement with the developed theoretical model. For a Reynolds number range of 2-32, it is shown that the drag coefficients are up to 66% higher than those for the fully immersed sphere at the same Reynolds numbers. A correlation is proposed to calculate the drag coefficient of levitated droplets for various temperature drops across the channel. For the first time, it is shown that it is possible to realize the natural coalescence of droplets through Marangoni effect without any external stimulation, and deliver the coalesced droplet to a certain destination through the use of surface tension gradients. The effects of the various shapes and sizes upon collision are studied. Regions of coalescence and stretching separation of colliding droplets are delineated based on Weber number and impact number. The existence of the transition line between coalescence and stretching separation in this passive mode of transport is similar to what was observed in the literature for forced coalescence at significantly higher Weber numbers. It is also found that a thermocapillary environment improves the mixing process. In order to illustrate and quantify the mixing phenomenon, the dispensed droplets were made of potassium hydroxide and phenolphthalein which is used as a pH indicator. The experiments show the possibility to reach mixing rates as high as 74% within 120 ms. This study offers new insight to thermo-coalescence and demonstrates how natural coalescence could be used to transport, mix and collect biochemical assays more efficiently. The results of this research can be engineered to enhance the performance of self-cleaning surfaces and micro-total analysis systems (µTAS), where sample transport, filtration, chemical reactions, separation and detection are of great interest

    Advances in Optofluidics

    Get PDF
    Optofluidics a niche research field that integrates optics with microfluidics. It started with elegant demonstrations of the passive interaction of light and liquid media such as liquid waveguides and liquid tunable lenses. Recently, the optofluidics continues the advance in liquid-based optical devices/systems. In addition, it has expanded rapidly into many other fields that involve lightwave (or photon) and liquid media. This Special Issue invites review articles (only review articles) that update the latest progress of the optofluidics in various aspects, such as new functional devices, new integrated systems, new fabrication techniques, new applications, etc. It covers, but is not limited to, topics such as micro-optics in liquid media, optofluidic sensors, integrated micro-optical systems, displays, optofluidics-on-fibers, optofluidic manipulation, energy and environmental applciations, and so on

    Development of A Hydrophobicity Controlled Microfluidic Dispenser

    Get PDF
    Ph.DDOCTOR OF PHILOSOPH

    Microdevices and Microsystems for Cell Manipulation

    Get PDF
    Microfabricated devices and systems capable of micromanipulation are well-suited for the manipulation of cells. These technologies are capable of a variety of functions, including cell trapping, cell sorting, cell culturing, and cell surgery, often at single-cell or sub-cellular resolution. These functionalities are achieved through a variety of mechanisms, including mechanical, electrical, magnetic, optical, and thermal forces. The operations that these microdevices and microsystems enable are relevant to many areas of biomedical research, including tissue engineering, cellular therapeutics, drug discovery, and diagnostics. This Special Issue will highlight recent advances in the field of cellular manipulation. Technologies capable of parallel single-cell manipulation are of special interest

    Electrowetting-based Actuation of Model Biological Fluids

    Get PDF
    Electrowetting-based Actuation of Model Biological Fluids Niyusha Samadi, Ph.D. Concordia University, 2016 In the past two decades, microfluidics-based lab-on-a-chip devices have received growing interest, and researchers have been developing chemical and biological analysis systems on very small scales. In Lab-on-a-chip systems, the goal is reducing chemical laboratory procedures and using miniaturized rapid, portable, inexpensive and reliable equipment which can be applied in medical diagnostics, and basic scientific research. The driving force behind the Lab-on-a-chip concept is “microfluidics” where contrary to bulk flows; surface tension is a dominant force for liquid handling and actuation. One method of actuation involves applying an external electric field which changes the surface tension between the solid-liquid interface reducing the meniscus contact angle and inducing motion of a droplet in a microchannel. This phenomenon is called “electrowetting”. In this research, the effect of electrowetting on the behavior of biopolymer solutions such as DNA is experimentally investigated. To better assess the electrowetting phenomenon of such complex solutions, the physics of electrowetting of aqueous biopolymer solutions should be completely understood. Such a fundamental understanding currently does not exist. For this purpose, the effect of fluid composition (i.e. different concentrations of DNA solutions and the type of buffer solution) on the static response of the droplet to electric field variables such as applied voltage is identified. In the transient response, the time and voltage dependency of the parameters such as droplet speed, total displacement, and elongation of the droplets of distilled water, Tris-HCl buffer and the DNA solutions is studied. Among these parameters, the droplet speed is a key factor which controls the rate of microfluidics-based lab-on-a-chip devices. It is found that the negatively charged oxygen ions on the DNA chain will affect the dynamic behaviour of DNA solutions significantly, and the electrophoretic velocity increases with voltage. Besides, it appears that the higher the DNA concentration is, the higher the DNA droplet velocity will be; as the ionic strength and the total effective charge increase with DNA concentration. Therefore, both electrowetting and electrophoretic forces contribute to the movement of the negatively charged droplets of DNA solutions. Overall, the results of this study will help us to better understand, analyze, design and prototype the microfluidic-based systems for DNA solutions
    corecore