Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session F07: Microscale Flows: Drops |
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Chair: Ian Jacobi, Technion, Israel Institute of Technology Room: Georgia World Congress Center B212 |
Monday, November 19, 2018 8:00AM - 8:13AM |
F07.00001: An investigation of the hydrodynamic resistance of droplets in microchannels using active control Marie Hebert, Carolyn L. Ren, Jan P. Huissoon Droplet microfluidics involves two immiscible phases: typically, water in oil. The passive manipulations of the droplets highly depend on the hydrodynamic resistance of the channel network. Both phases contribute to the overall resistance. However, while the continuous phase (i.e. oil) can be calculated accurately from the literature, the contribution to the resistance from the droplet(s) is a complex relationship that has yet to be fully quantified. Variables contributing to droplet hydrodynamic resistance include: viscosity ratio, droplet speed, surface tension, channel geometry, size and spacing of droplets. The objective of this empirical study is to estimate the resistance of a channel containing a droplet under various conditions. Although this problem has been approached by several researchers, the methods herein proposed is novel as it leverages a visual-feedback assisted active control platform rather than a passive method. This platform allows the generation of a single droplet under various conditions. From the input (pressure) and the output (water-oil interface displacement) with respect to time, the properties of the system can be retrieved using system identification techniques. Hence, an empirical correlation for predicting droplet resistance can be obtained. |
Monday, November 19, 2018 8:13AM - 8:26AM |
F07.00002: Interaction of a Droplet with T-junction of a Microfluidic Bypass Channel Sagar Narhari Agnihotri, Rajneesh Bhardwaj, Adrian Neild, Mohammad Reza Raveshi The transition from no droplet breakup to droplet breakup in a bypass channel of a parallel microfluidic system is studied using experiments and 3D numerical simulations. In experimental work, PDMS microchannels are fabricated using standard photolithography, and the high-speed camera is used to record droplet interaction with the bypass channel. 3D numerical simulations are carried out for conditions causing both droplet breakup and non-breakup. The droplet is generated upstream, using standard T Junction geometry. The generated droplet is then made to interact with the bypass channel. The first aspect of our study is a detailed experimental investigation of the transition process by varying width of the bypass channel (wb) and the capillary number (Ca). There exists a critical value of Ca at a constant value of wb and a critical value of wb at constant Ca, at which droplet breakup occurs. Combining numerical and experimental results, we found out that droplet breakup at the entrance to the bypass channel can exhibit squeezing and dripping regimes which are equivalent to those occurring in droplet generation at a T-junction. |
Monday, November 19, 2018 8:26AM - 8:39AM |
F07.00003: Dual-mode droplet merging in a straight microchannel by using thermocapillary effects June Won, Simon Song Thermocapillary effects induced by interfacial tension gradient established by the temperature difference has been paid attention due to its strong driving force exerted on a droplet. Nevertheless, previous studies utilizing the thermocapillary effects have limitations in that they use a passive structure to assist droplet merging. We aim to develop a dual-mode droplet merging technique using thermocapillary effects in a simple straight channel without passive structure. In order to merge droplets by thermocapillary effects, a laser beam and a mixture of photothermal materials and oleic acid were used. We found that a minimum power of heat source to merge droplet is linearly dependent on both of droplet diameter and droplet velocity. As a result, we were able to perform the active, on-demand droplet merging with a success rate of 95 % by controlling the laser power. As an application of droplet merging by thermocapillary effects, we synthesized nanoparticles on a passive mode of continuous merging and controlled the size of nanoparticles by varying power of heat source. |
Monday, November 19, 2018 8:39AM - 8:52AM |
F07.00004: Microfluidic investigation of dynamic surfactant rearrangement at water-oil interfaces during single droplet formation Ioannis Kiratzis, Daniele Vigolo, Mark Simmons Dynamic surfactant behaviour is of paramount importance in industrial and research application. Microfluidic droplet formation is a dynamic process that involves the use of surfactants for the stabilisation of the produced droplets. It has been shown that during droplet formation surfactant molecules rearrange on the interface creating concentration gradients that lead to surface tension gradients that affect droplet formation. We use a microfluidic setting to study the surfactant rearrangement during droplet formation. We have shown that surfactants with smaller molecular weights, therefore larger diffusion coefficients, are more capable in coping with the short time scales of droplet formation present in microfluidics. This has been done by examining the velocities along the interface moments before detachment. Interfaces where surfactant is present display velocities smaller than surfactant free interfaces. Furthermore, the number of satellite droplets produced appears to be increasing with increasing surfactant concentration, suggesting additional effects, even at concentrations above the critical micellar concentration. |
Monday, November 19, 2018 8:52AM - 9:05AM |
F07.00005: Medium exchange and enrichment of microparticles encapsulated in microscale droplets Jinsoo Park, Ghulam Destgeer, Hyung Jin Sung Droplet microfluidic medium exchange and enrichment of microparticles have long been pursued but remained problematic because the simultaneous and precise control over the droplet interface and suspended microparticles is difficult. This work proposes a new method for in-droplet microparticle medium exchange and enrichment using acoustic waves. Under the influence of the acoustic field within a microchannel, the droplets and encapsulated particles experience acoustic radiation force induced by wave scattering at the liquid/liquid and liquid/solid interfaces. Based on the acoustic radiation force, the medium of in-droplet microparticles was exchanged, and the number of the in-droplet microparticles was increased to the desired quantity. The microparticle concentration location within droplets has been also investigated based on the dimensionless parameters: Helmholtz number and acoustic radiation force factor. The proposed acoustic wave-based method is expected to offer new perspectives to microparticle-based assays in droplet-based microfluidic applications. |
Monday, November 19, 2018 9:05AM - 9:18AM |
F07.00006: Simulation of species leakage in material encapsulation using compound droplets Longlong Wang, Zhizhao Che, Tianyou Wang Compound droplets can be used in substance encapsulation and material compartmentalization. The shell of compound droplets can serve as a protective layer of the inner phase by minimizing the mass transfer between the core and the outer phase. This often relies on that the solubility of the species is low in the shell layer. However, it should be noted that the solubility of the species in the shell phase is not ideally zero. We simulate the leakage of material from the core to the continuous phase or from the continuous phase to the core phase. To reduce the spurious current at the interface, we used to ghost fluid method to consider the surface tension effect. To avoid the numerical diffusion for the mass transfer process, we used the particle tracking method. The mass transfer across the interface is simulated using a Monte Carlo concept. The method presented in this study can also be used in other mass transfer analyses across interfaces such as micro-extraction, micro-mixing, and micro-reaction. |
Monday, November 19, 2018 9:18AM - 9:31AM |
F07.00007: Impingement of droplet train with a immiscible liquid jet: Dynamics and outcomes Xiaodong Chen, Bo Wang Droplet-based microfluidics can generate compound microdroplets of various morphologies, and promote developments of functional materials synthesis, biochemical analysis, and drug manufacturing, etc. However, droplet microfluidics usually uses liquid as an environmental fluid with limited flow rates. In-air microfluidics (IAMF, Ref: Visser et al., Sci. Adv. 2018;4: eaao1175) generates compound microdroplets in the air by impinging microdroplets with a immiscible liquid jet. This study carries out direct numerical simulations to reveal physical details during generations of compound microdroplets using IAMF. A three-phase volume-of-fluid method combining with an adaptive-mesh-refinement technology is used to predict impingement dynamics. Processes such as droplet-jet interaction, triple-line motion, and jet pinch-off are investigated to show effects of flow and physical parameters on shapes (spherical, ellipsoidal and band-shaped) and morphologies (Janus and core-shell) of compound droplets. This study provides physic basis and optimization criteria for the development of IAMF. |
Monday, November 19, 2018 9:31AM - 9:44AM |
F07.00008: Pushing the boundaries of microfluidics; an experimental study on high-speed droplet formation in confined microchannels using air as the continuous phase Pooyan Tirandazi, Julian D. Arroyo, Dac Duc Ho, Carlos H Hidrovo Microfluidic systems for generation of liquid emulsions provide versatile tools for many applications in natural sciences. Droplet formation in these systems are performed in confined microchannels and as a result of the flow of two immiscible liquids (e.g. water in oil). Here we characterize droplet generation in an alternative format where continuous oil phase is replaced by gas in the microchannel. Droplet breakup is experimentally studied in a flow-focusing type channel in which a middle aqueous phase is pinched off by a high-speed air flow. The microchannels are fabricated in PDMS and feature a non-planar architecture enabling the liquid to be fully surrounded by the air flow. We investigate the Dripping and Jetting regimes for this system and identify the effect of geometry as well as flow conditions. We report droplet generation in the order of 10 kHz with diameters of 50µm and 150µm in the Dripping regime while for the Jetting regime we obtain droplets of 10µm 50µm at frequencies higher than 100 kHz which is an order of magnitude higher than the state-of-the-art for oil-based systems. The outcomes of this work are useful in many areas, namely pharmaceutical industries where uniform droplets can be generated purely in air. |
Monday, November 19, 2018 9:44AM - 9:57AM |
F07.00009: The motion of long drops in rectangular microchannels at low capillary numbers Sai Sashankh Rao, Harris H Wong We study the motion of a long drop in a rectangular microchannel in the limit the capillary number Ca-> 0 (Ca = μU/σ, where U is the constant drop velocity, μ is the carrier-liquid viscosity, and σ is the interfacial tension). In this limit, the drop has two end caps connected by a long column, which is surrounded by thin films on the microchannel wall and by menisci along the microchannel corners. Integral axial force balances relate the carrier-liquid pressure gradient to the drop-fluid pressure gradient and the contact-line drag, which is the same as that for a long bubble (known) if the viscosity ratio λ « Ca-1/3 and λ « L, where λ = μ*/μ and μ* is the drop viscosity, and L is the dimensionless drop length. The two pressure gradients also drive unidirectional flows in the drop and in the corner channels along the long middle column. These coupled flows are solved by a finite-element method to yield the pressure gradients for λ = 0 to 100 and various microchannel aspect ratios. We find that in the limit LCa1/3 -> 0, the contact-line drag dominates and the carrier liquid bypasses the drop through the corner channels alongside the drop. For LCa1/3 » 1, the contact-line drag is negligible and the corner fluid is stationary. Thus, the drop moves as a leaky piston. |
Monday, November 19, 2018 9:57AM - 10:10AM |
F07.00010: Segmented flows of viscous threads and droplets in microchannels Thomas Cubaud Segmented flows of droplets and highly viscous oils are focused with low-viscosity oils to examine dynamic interactions between lubricating and recirculating flows and enhance mixing between fluids having large viscosity contrasts in microfluidic channels. A variety of complex flow regimes are analyzed based on initial droplet sizes and concentrations as well as ratio between low- and high-viscosity fluids. Focus is on the formation and stability of viscous threads (i.e., viscous core-annular flows) between droplets in square microchannels.
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