Droplet encapsulation of particles in different regimes and sorting of particle-encapsulating-droplets from empty droplets
TL;DR: The study reveals that the size of the positive (particle-encapsulating) droplet is larger or smaller compared to that of the negative (empty) droplets in the dripping and jetting regimes but no size contrast is observed in the squeezing regime.
Abstract: Encapsulation of microparticles in droplets has profound applications in biochemical assays. We investigate encapsulation of rigid particles (polystyrene beads) and deformable particles (biological cells) inside aqueous droplets in various droplet generation regimes, namely, squeezing, dripping, and jetting. Our study reveals that the size of the positive (particle-encapsulating) droplets is larger or smaller compared to that of the negative (empty) droplets in the dripping and jetting regimes but no size contrast is observed in the squeezing regime. The size contrast of the positive and negative droplets in the different regimes is characterized in terms of capillary number C a and stream width ratio ω (i.e., ratio of stream width at the throat to particle diameter ω = w / d p). While for deformable particles, the positive droplets are always larger compared to the negative droplets, for rigid particles, the positive droplets are larger in the dripping and jetting regimes for 0.50 ≤ ω ≤ 0.80 but smaller in the jetting regime for ω < 0.50. We exploit the size contrast of positive and negative droplets for sorting across the fluid–fluid interface based on noninertial lift force (at R e ≪ 1), which is a strong function of droplet size. We demonstrate sorting of the positive droplets encapsulating polystyrene beads and biological cells from the negative droplets with an efficiency of ∼95% and purity of ∼65%. The proposed study will find relevance in single-cell studies, where positive droplets need to be isolated from the empty droplets prior to downstream processing.
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21 Mar 2008
TL;DR: In this article, a cross-streamline noninertial migration of a vesicle in a bounded Poiseuille flow is investigated experimentally and numerically, where the combined effects of the walls and of the curvature of the velocity profile induce a movement toward the center of the channel.
Abstract: Cross-streamline noninertial migration of a vesicle in a bounded Poiseuille flow is investigated experimentally and numerically. The combined effects of the walls and of the curvature of the velocity profile induce a movement toward the center of the channel. A migration law (as a function of relevant structural and flow parameters) is proposed that is consistent with experimental and numerical results. This similarity law markedly differs from its analog in unbounded geometry. The dependency on the reduced volume ν and viscosity ratio λ is also discussed. In particular, the migration velocity becomes nonmonotonous as a function of ν beyond a certain λ.
147 citations
TL;DR: In this article, the authors studied the mechanism of cross-stream migration and the coalescence of aqueous droplets flowing in an oil-based ferrofluid with a co-flowing aouous stream in the presence of a magnetic field and revealed that the migration phenomenon is governed by the advection and magnetophoretic (τm) time scales.
Abstract: Manipulation of aqueous droplets in microchannels has great significance in various emerging applications such as biological and chemical assays. Magnetic-field based droplet manipulation that offers unique advantages is consequently gaining attention. However, the physics of magnetic field-driven cross-stream migration and the coalescence of aqueous droplets with an aqueous stream are not well understood. Here, we unravel the mechanism of cross-stream migration and the coalescence of aqueous droplets flowing in an oil based ferrofluid with a coflowing aqueous stream in the presence of a magnetic field. Our study reveals that the migration phenomenon is governed by the advection (τa) and magnetophoretic (τm) time scales. Experimental data show that the dimensionless equilibrium cross-stream migration distance δ* and the length Lδ* required to attain equilibrium cross-stream migration depend on the Strouhal number, St = (τa/τm), as δ* = 1.1 St0.33 and Lδ*=5.3 St−0.50, respectively. We find that the droplet-stream coalescence phenomenon is underpinned by the ratio of the sum of magnetophoretic (τm) and film-drainage time scales (τfd) and the advection time scale (τa), expressed in terms of the Strouhal number (St) and the film-drainage Reynolds number (Refd) as ξ = (τm + τfd)/τa = (St−1 + Refd). Irrespective of the flow rates of the coflowing streams, droplet size, and magnetic field, our study shows that droplet-stream coalescence is achieved for ξ ≤ 50 and ferrofluid stream width ratio w* < 0.7. We utilize the phenomenon and demonstrated the extraction of microparticles and HeLa cells from aqueous droplets to an aqueous stream.
20 citations
8 citations
TL;DR: A technique for the detection and selective isolation of target cells encapsulated in microdroplets in single-cell format for downstream analysis of genomics, transcriptomics, proteomics, and metabolomics characterisation at the individual cell level is demonstrated.
Abstract: Single-cell analysis has emerged as a powerful method for genomics, transcriptomics, proteomics, and metabolomics characterisation at the individual cell level. Here, we demonstrate a technique for the detection and selective isolation of target cells encapsulated in microdroplets in single-cell format. A sample containing a mixed population of cells with fluorescently labelled target cells can be focused using a sheath fluid to direct cells in single file toward a droplet junction, wherein the cells are encapsulated inside droplets. The droplets containing the cells migrate toward the centre of the channel owing to non-inertial lift force. The cells present in the droplets are studied and characterised based on forward scatter (FSC), side scatter (SSC), and fluorescence (FL) signals. The FL signals from the target cells can be used to activate a selective isolation module based on electro-coalescence, using suitable electronics and a program to sort droplets containing the target cells in single-cell format from droplets containing background cells. We demonstrated the detection and isolation of target cells (cancer cells: HeLa and DU145) from mixed populations of cells, peripheral blood mononuclear cells (PBMC) + cervical cancer cells (HeLa) and PBMC + human prostate cancer cells (DU145), at a concentration range of 104-106 ml-1 at 300 cells per s. The performance of the device is characterised in terms of sorting efficiency (>97%), enrichment (>1800×), purity (>98%), and recovery (>95%). The sorted target cells were found to be viable (>95% viability) and showed good proliferation when cultured, showing the potential of the proposed sorting technique for downstream analysis.
8 citations
TL;DR: In this article, the role of a shear-thinning induced lift force (FSM) in cross-stream migration of droplets in a confined STVE flow was investigated.
Abstract: Shear-thinning viscoelastic (STVE) flows exhibit intriguing phenomena owing to their complex rheology and the coupling of various forces involved. Here, we present an understanding of the cross-stream migration of droplets in a confined STVE flow and unravel the role of a shear-thinning induced lift force (FSM) in their dynamical behavior. We perform experiments with popular STVE liquids of different molecular weights and concentrations (c) for Reynolds numbers Re < 1 and Weissenberg numbers Wi = 0.01–7.4. Our results reveal larger droplets (of drop-to-channel ratio β ≥ 0.28) that follow their original streamlines, whereas smaller droplets (β ≤ 0.2) exhibit center ward migration and the migration rates depend upon the drop-to-medium viscosity (k) and elasticity (ξ) ratios. The lateral displacement of droplets is tracked using high-speed imaging that is used to estimate the relevant forces using suitable correlations. We find that the migration dynamics of droplets is underpinned by the non-inertial lift (FNIL), viscoelastic lift (FVM, FVD), and shear-thinning induced lift (FSM) forces. We provide experimental evidence of the proposed FSM and, from analytical scaling and empirical modeling, develop an expression for FSM ∼ ΔμΔγD3.7/h1.7 (with R2 = 0.95) for an object at a distance h from the wall and with a drop in viscosity Δμ and strain rate Δγ across its diameter D. Our study sheds light on the underlying dynamics on droplets in an STVE medium and opens up avenues for sorting and focusing of drops in an STVE medium at low Re.
7 citations
References
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TL;DR: A theoretical model based on the Poisson distribution accurately predicted the observed enrichment values using the starting cell density (cells per droplet) and the ratio of active to inactive cells, and all of the recovered cells were the active strain.
Abstract: We describe a highly efficient microfluidic fluorescence-activated droplet sorter (FADS) combining many of the advantages of microtitre-plate screening and traditional fluorescence-activated cell sorting (FACS). Single cells are compartmentalized in emulsion droplets, which can be sorted using dielectrophoresis in a fluorescence-activated manner (as in FACS) at rates up to 2000 droplets s−1. To validate the system, mixtures of E. colicells, expressing either the reporter enzyme β-galactosidase or an inactive variant, were compartmentalized with a fluorogenic substrate and sorted at rates of ∼300 droplets s−1. The false positive error rate of the sorter at this throughput was <1 in 104 droplets. Analysis of the sorted cells revealed that the primary limit to enrichment was the co-encapsulation of E. colicells, not sorting errors: a theoretical model based on the Poisson distribution accurately predicted the observed enrichment values using the starting cell density (cells per droplet) and the ratio of active to inactive cells. When the cells were encapsulated at low density (∼1 cell for every 50 droplets), sorting was very efficient and all of the recovered cells were the active strain. In addition, single active droplets were sorted and cells were successfully recovered.
911 citations
TL;DR: This paper demonstrates in droplet the rapid laser photolysis of the single cell, which essentially "freezes" the state that the cell was in at the moment ofphotolysis and confines the lysate within the small volume of the droplet.
Abstract: This paper describes a method, which combines optical trapping and microfluidic-based droplet generation, for selectively and controllably encapsulating a single target cell or subcellular structure, such as a mitochondrion, into a picoliter- or femtoliter-volume aqueous droplet that is surrounded by an immiscible phase. Once the selected cell or organelle is encased within the droplet, it is stably confined in the droplet and cannot be removed. We demonstrate in droplet the rapid laser photolysis of the single cell, which essentially “freezes” the state that the cell was in at the moment of photolysis and confines the lysate within the small volume of the droplet. Using fluorescein di-β-d-galactopyranoside, which is a fluorogenic substrate for the intracellular enzyme β-galactosidase, we also assayed the activity of this enzyme from a single cell following the laser-induced lysis of the cell. This ability to entrap individual selected cells or subcellular organelles should open new possibilities for carr...
592 citations
TL;DR: Encapsulation of cells within picolitre-size monodisperse drops provides new means to perform quantitative biological studies on a single-cell basis for large cell populations by evenly spacing cells as they travel within a high aspect-ratio microchannel.
Abstract: Encapsulation of cells within picolitre-size monodisperse drops provides new means to perform quantitative biological studies on a single-cell basis for large cell populations. Variability in the number of cells per drop due to stochastic cell loading is a major barrier to these techniques. We overcome this limitation by evenly spacing cells as they travel within a high aspect-ratio microchannel; cells enter the drop generator with the frequency of drop formation.
549 citations
TL;DR: The motion of small particles, drops, and bubbles in a viscous fluid at low Reynolds number is one of the oldest classes of problems in theoretical fluid mechanics, dating at least to Stokes's analysis of the translation of a rigid sphere through an unbounded quiescent fluid at zero Reynolds number as discussed by the authors.
Abstract: The motion of small particles, drops, and bubbles in a viscous fluid at
low Reynolds number is one of the oldest classes of problems in theoretical
fluid mechanics, dating at least to Stokes’s (1851) analysis of the translation
of a rigid sphere through an unbounded quiescent fluid at zero Reynolds
number.
535 citations
TL;DR: Both passive and active methods for droplet production are examined and how these can be used to deterministically and non-deterministically encapsulate cells are explored.
Abstract: There is a recognized and growing need for rapid and efficient cell assays, where the size of microfluidic devices lend themselves to the manipulation of cellular populations down to the single cell level. An exceptional way to analyze cells independently is to encapsulate them within aqueous droplets surrounded by an immiscible fluid, so that reagents and reaction products are contained within a controlled microenvironment. Most cell encapsulation work has focused on the development and use of passive methods, where droplets are produced continuously at high rates by pumping fluids from external pressure-driven reservoirs through defined microfluidic geometries. With limited exceptions, the number of cells encapsulated per droplet in these systems is dictated by Poisson statistics, reducing the proportion of droplets that contain the desired number of cells and thus the effective rate at which single cells can be encapsulated. Nevertheless, a number of recently developed actively-controlled droplet production methods present an alternative route to the production of droplets at similar rates and with the potential to improve the efficiency of single-cell encapsulation. In this critical review, we examine both passive and active methods for droplet production and explore how these can be used to deterministically and non-deterministically encapsulate cells.
382 citations