Pooyan Tirandazi

Bio: Pooyan Tirandazi is an academic researcher from Northeastern University. The author has contributed to research in topics: Microfluidics & Flow focusing. The author has an hindex of 4, co-authored 10 publications receiving 48 citations.

Liquid-in-gas droplet microfluidics; experimental characterization of droplet morphology, generation frequency, and monodispersity in a flow-focusing microfluidic device

Abstract: Microfluidic techniques for production of uniform droplets usually rely on the use of two immiscible liquids (eg water-in-oil emulsions) It has been shown recently that a continuous gas flow instead of a second liquid carrier can be used as an alternative approach in droplet microfluidics In this work we experimentally investigate the generation of liquid water droplets within air in flow-focusing configurations Over a wide range of flow conditions we identify six distinct flow regimes inside the microchannel: Co-flowing, Threading, Plugging, Dripping, Multi-Satellite Formation, and Jetting Flow regimes and their transitions are plotted and characterized based on the Weber number (We) of the system We further investigate the impact of liquid microchannel size on the flow maps Generation frequency, morphology, and monodispersity of the droplets are characterized in more detail in the Dripping regime Generation frequency can be related to the product of the liquid and gas flow rates However, droplet morphology (length and width) is more dependent on the gas flow rate We demonstrate the production of monodisperse droplets (d < 100 µm and σ/d < 5 %) up to kHz formation rates in liquid-gas microfluidic systems for the first time The results of this work provide practical and useful guidelines for precise, oil-free delivery of ultra-small volumes of fluid which can be integrated in lab-on-a-chip systems for a variety of applications in biochemical research and material synthesis

An integrated gas-liquid droplet microfluidic platform for digital sampling and detection of airborne targets

Abstract: The use of microfluidic droplets has become ubiquitous in many Lab-on-a-Chip (LOC) applications ranging from material synthesis to novel biochemical sensing. In this paper, we introduce a new droplet-based approach that incorporates a gas phase for generating liquid droplet microreactors in a microfluidic flow-focusing format. We demonstrate the subsequent on-chip transition, collection and handling of the droplets in a secondary liquid carrier inside a multilayer PDMS structure. The presented technique has potential applications in capturing and probing airborne particles and gaseous vapors using high surface-to-volume picoliter droplets. The discrete microfluidic gas-liquid interfaces created in this approach, greatly facilitate absorption and up-concentration of a gaseous target analyte into the droplet volume. The chip-based format of the units also allows for different microfluidic modules and analytical techniques to be integrated in this platform for droplet probing, providing highly-sensitive LOC detection systems. Here, we demonstrate the basic principles of sample partitioning with gas-liquid droplets by capturing and detection of vaporized ammonia at different gaseous concentrations using Nessler’s reaction inside the droplets. The results of this work provide a simple and robust quantification approach for determining gaseous ammonia which can be further expanded to other gas-phase analytes in next generation of airborne target detectors for human breath analysis and environmental monitoring.

Study of drag reduction using periodic spanwise grooves on incompressible viscous laminar flows

Abstract: Engineered surface textures can manipulate boundary layers affecting fluid drag We study periodic, infinitely long spanwise grooves on a laminar boundary layer over a plate for 1000 $R\phantom{\rule{0}{0ex}}{e}_{L}$ 25000 Below a certain width-to-depth aspect ratio (AR), a primary vortex inside each groove causes the freestream to ``slip over'', reducing skin friction Increasing AR poses a tradeoff in drag reduction due to pressure drag from groove vertical walls Overall, transverse grooves for laminar flow can reduce total drag up to 10% compared to a flat plate, despite increasing the wetted surface area

Efficient batch-mode mixing and flow patterns in a microfluidic centrifugal platform: a numerical and experimental study

Abstract: During recent years centrifugal-based microfluidic devices known as Lab-on-a-CD have attracted a lot of attentions. Applications of these CD-based platforms are ubiquitous in numerous biological analyses and chemical syntheses. Mixing of different species in microscale is one of the essential operations in biochemical applications where this seemingly simple task remains a major obstruction. Application of centrifugal force, however, may significantly improve the flow agitation and mixing, especially when it is combined with the Coriolis force which acts perpendicular to centrifugal force. In this study, mixing process in minichambers located on a rotating platform under a periodic acceleration and deceleration angular velocity profile is investigated both numerically and experimentally. We have incorporated various arrangements of obstacles and baffles, which are usually used in stationary mixers, within a batch-mode rotating mixing chamber. Subsequently, the effect of these obstacles on flow field and mixing process has been studied, and among these arrangements four cases have been selected for further experimental analysis. Experimental studies have been performed on a multi-layer CD platform fabricated in polycarbonate plates, and subsequently mixing has been investigated in these minichambers. The quantitative mixing data were obtained after a set of image analyses on the captured images of mixing chamber during the process and the results were compared with the simulation. The results indicate a good resemblance between the two studies both qualitatively and quantitatively. Furthermore, it has been shown that the application of obstacles and baffles together in chamber results in reducing the mixing time more than 50 % as compared to a chamber without any obstacle and/or baffle configuration. Obtaining mixing times less than 10 s in both studies, makes these CD-based platforms an appropriate device for many applications in which a cost-effective device as well as low mixing time is required.

Spheroids-on-a-chip: Recent advances and design considerations in microfluidic platforms for spheroid formation and culture

Abstract: A cell spheroid is a three-dimensional (3D) aggregation of cells. Synthetic, in-vitro spheroids provide similar metabolism, proliferation, and species concentration gradients to those found in-vivo. For instance, cancer cell spheroids have been demonstrated to mimic in-vivo tumor microenvironments, and are thus suitable for in-vitro drug screening. The first part of this paper discusses the latest microfluidic designs for spheroid formation and culture, comparing their strategies and efficacy. The most recent microfluidic techniques for spheroid formation utilize emulsion, microwells, U-shaped microstructures, or digital microfluidics. The engineering aspects underpinning spheroid formation in these microfluidic devices are therefore considered. In the second part of this paper, design considerations for microfluidic spheroid formation chips and microfluidic spheroid culture chips (μSFCs and μSCCs) are evaluated with regard to key parameters affecting spheroid formation, including shear stress, spheroid diameter, culture medium delivery and flow rate. This review is intended to benefit the microfluidics community by contributing to improved design and engineering of microfluidic chips capable of forming and/or culturing three-dimensional cell spheroids.

Microfluidics: Innovations in Materials and Their Fabrication and Functionalization.

Abstract: Microfluidics is a growing field of study because these devices offer novel and versatile approaches for addressing a range of scientific problems. Microfluidics have been used in separations, cell analysis, and microreactors, to illustrate a few applications. Microfluidics have been defined in terms of microliter volumes or micrometer dimensions of channels. For this review, we utilize the latter definition, where microfluidic channels range from 1-1000 µm in width or height. Microfluidic devices are fabricated in a host of ways, with a large variety of materials. Indeed, device material can affect flow, absorptivity, biocompatibility, and function of microfluidic components.

Microfluidic droplet platform for ultrahigh-throughput single-cell screening of biodiversity

Abstract: Significance Biocompatible microfluidic double water-in-oil-in-water emulsion (MDE) enables in-droplet cultivation of different living species. The combination of droplet-generating machinery with FACS followed by next-generation sequencing and liquid chromatography-mass spectrometry analysis of the secretomes of encapsulated organisms yielded detailed genotype/phenotype descriptions. The MDE–FACS platform we developed enabled highly sensitive single-cell selection of predesigned activity and exploration of pairwise interactions between target and effector cells without interference from other microbiota species. Ultrahigh-throughput screening (uHTS) techniques can identify unique functionality from millions of variants. To mimic the natural selection mechanisms that occur by compartmentalization in vivo, we developed a technique based on single-cell encapsulation in droplets of a monodisperse microfluidic double water-in-oil-in-water emulsion (MDE). Biocompatible MDE enables in-droplet cultivation of different living species. The combination of droplet-generating machinery with FACS followed by next-generation sequencing and liquid chromatography-mass spectrometry analysis of the secretomes of encapsulated organisms yielded detailed genotype/phenotype descriptions. This platform was probed with uHTS for biocatalysts anchored to yeast with enrichment close to the theoretically calculated limit and cell-to-cell interactions. MDE–FACS allowed the identification of human butyrylcholinesterase mutants that undergo self-reactivation after inhibition by the organophosphorus agent paraoxon. The versatility of the platform allowed the identification of bacteria, including slow-growing oral microbiota species that suppress the growth of a common pathogen, Staphylococcus aureus, and predicted which genera were associated with inhibitory activity.

Dripping, jetting and tip streaming

Abstract: Dripping, jetting and tip streaming have been studied up to a certain point separately by both fluid mechanics and microfluidics communities, the former focusing on fundamental aspects while the latter on applications. Here, we intend to review this field from a global perspective by considering and linking the two sides of the problem. First, we present the theoretical model used to study interfacial flows arising in droplet-based microfluidics, paying attention to three elements commonly present in applications: viscoelasticity, electric fields and surfactants. We review both classical and current results of the stability of jets affected by these elements. Mechanisms leading to the breakup of jets to produce drops are reviewed as well, including some recent advances in this field. We also consider the relatively scarce theoretical studies on the emergence and stability of tip streaming in open systems. Second, we focus on axisymmetric microfluidic configurations which can operate on the dripping and jetting modes either in a direct (standard) way or via tip streaming. We present the dimensionless parameters characterizing these configurations, the scaling laws which allow predicting the size of the resulting droplets and bubbles, as well as those delimiting the parameter windows where tip streaming can be found. Special attention is paid to electrospray and flow focusing, two of the techniques more frequently used in continuous drop production microfluidics. We aim to connect experimental observations described in this section of topics with fundamental and general aspects described in the first part of the review. This work closes with some prospects at both fundamental and practical levels.