Formation of Droplets of Alternating Composition in Microfluidic Channels and Applications to Indexing of Concentrations in Droplet-Based Assays

Abstract

For screening the conditions for a reaction by using droplets (or plugs) as microreactors, the composition of the droplets must be indexed. Indexing here refers to measuring the concentration of a solute by addition of a marker, either internal or external. Indexing may be performed by forming droplet pairs, where in each pair the first droplet is used to conduct the reaction, and the second droplet is used to index the composition of the first droplet. This paper characterizes a method for creating droplet pairs by generating alternating droplets, of two sets of aqueous solutions in a flow of immiscible carrier fluid within PDMS and glass microfluidic channels. The paper also demonstrates that the technique can be used to index the composition of the droplets, and this application is illustrated by screening conditions of protein crystallization. The fluid properties required to form the steady flow of the alternating droplets in a microchannel were characterized as a function of the capillary number Ca and water fraction. Four regimes were observed. At the lowest values of Ca, the droplets of the two streams coalesced; at intermediate values of Ca the alternating droplets formed reliably. At even higher values of Ca, shear forces dominated and caused formation of droplets that were smaller than the cross-sectional dimension of the channel; at the highest values of Ca, coflowing laminar streams of the two immiscible fluids formed. In addition to screening of protein crystallization conditions, understanding of the fluid flow in this system may extend this indexing approach to other chemical and biological assays performed on a microfluidic chip. This paper characterizes a method of forming stable alternating droplets of two sets of aqueous solutions in microchannels. In addition to characterization, we describe an application of this method to indexing concentrations of the solutes in the droplets used for assays. Protein crystallization under microbatch conditions was used as an example. Indexing here refers to measuring the concentration of a solute by addition of a marker, either internal or external.Droplets in microfluidic channels allow transport in microfluidic channels with no dispersion 1 and allow rapid mixing using chaotic advection. 1-3 In this paper, we use the word "droplet" to describe a plug-an aqueous droplet that is surrounded by an immiscible fluid and is in apparent contact with all four walls of a hydrophobic microchannel. 4 Such droplets in microfluidic channels are being used as microreactors 5-8 in chemical kinetics, 9 chemical amplification, 10 chemical and biological analysis, 11,12 and protein crystallization. 13,14 Tens to hundreds of crystallization trials or assays can be performed in the same channel inside droplets. We have recently used alternating aqueous droplets formed in a water-permeable carrier fluid to perform protein crystallizations under vapor diffusion conditions and to concentrate nanoliter volumes of solutions. 14 In all of these experiments, each droplet co...

A numerical study on the dynamics of droplet formation in a microfluidic double T-junction

“…It can be seen from Fig. 4(b) that the grid independence is achieved when the droplets form in ADF regime, observed experimentally by Zheng et al 13 in the same flow condition. The droplet diameter obtained in present study lies within the range between the lower and upper bounds reported, 13 showing quantitative agreements with experiments.…”

“…The four regimes, namely, merging (MDF), alternating (ADF), alternating-jetting, and jetting, experimentally observed by Zheng et al 13 are reproduced for the first time by numerical simulations in the present study, which in turn allow comprehensive parametric study not available in previous reports. Here, extensive numerical study is performed to investigate the effect of capillary number (10 À3 Ca 0.5), Reynolds number (7.4 Â 10 À4 Re 0.37), and channel width ratio (k ¼ 0.5, k ¼ 1.0, and k ¼ 1.5).…”

“…It has been reported that 3D effects become less important as the channel aspect ratio (h/W c ) decreases below 0.05. 21 Usefulness of 2D simulation for studying droplet formation in T-junction has also been discussed by Li et al, 33 Liu and Zhang, 34 Leshansky and Pismen, 35 Amaya-Bower and Lee, 36 and Arias et al 37 In order to further validate the present numerical model, our numerical results are compared with experimental observation by Zheng et al 13 Figure 3 illustrates the four regimes of droplet formations in a MFDTD. In this figure, the water fraction wf denotes the ratio between the combined flow rate of aqueous flows (water) and the total flow rate of the carrier fluid (oil) and the aqueous flows.…”

“…Since the droplet pairs with different compositions are possible to be formed, it can be used for indexing the concentrations of solutes in a droplet, protein crystallization 13 or used as an initial process in droplet-merging techniques. [14][15][16] The stable formation of droplets is expected in such a way that the miniature volume of each dispersed flow is controllable as accurately as possible.…”

“…A number of previous studies on MFDTD and associated mechanism of the droplet formation exist. Zheng et al 13 identified four different regimes of the fluid flow in a double T-junction that are characterized as a function of capillary number and water fraction. In particular, for low water fraction and low Reynolds number with capillary number ranging from 0.001 to 0.05 droplets are formed from each of the two inlets of the dispersive flow in an alternating fashion.…”