Droplet-based microfluidic device for multiple-droplet clustering† Jing Xu, Byungwook Ahn, Hun Lee, Linfeng Xu, Kangsun Lee, Rajagopal Panchapakesan and Kwang W. Oh * Received 14th September 2011, Accepted 22nd November 2011 DOI: 10.1039/c2lc20883k We present a multiple-droplet clustering device that can perform sequential droplet trapping and storing. Shape-dependent droplet manipulation in forward and backward flows has been incorporated to achieve high trapping and storing efficiency in a 10  12 array of clustering structures (e.g., storing well, storing chamber, trapping well, and guiding track). In the forward flow, flattened droplets are trapped in each trapping well. In the backward flow, the trapped droplets are released from the trapping well and follow the guiding tracks to their corresponding storing wells. The guided droplets float up out of the confining channel to the super stratum of the storing chamber due to interfacial energy and buoyancy effects. This forward/backward flow-based trapping/storing process can be repeated several times to cluster droplets with different contents and samples in the storing chambers. We expect that the proposed platform will be a valuable tool to study complex droplet-based reactions in clustered droplets. 1. Introduction Droplet-based microfluidics has proven to be a useful tool to study many biological, chemical, and pharmaceutical reactions in a high-throughput manner. 1,2 The pairing of two droplets containing different reagents and samples is one of the essential unit operations in the droplet-based microfluidics platform. The droplet pairing has been demonstrated in continuous-flow channels 3,4 and static arrays. 5,6 Usually, the flow-through approach is suitable for high-throughput end-point detection, and the static array approach is appropriate for large-scale real- time monitoring. The key requirement for the droplet pairing in the continuous-flow channels is the perfect synchronization of two droplets over long-term periods because uncontrolled synchronization results in unreliable droplet pairing. However, the droplet pairing has been challenged largely due to hydrody- namic resistive coupling effects between droplets in the chan- nels. 7 The droplet pairing efficiency in the continuous-flow channels could be improved by well-controlled synchronized droplet generation 3 or pairwise droplet formation. 4 Another approach was to use trapping structures to trap and pair droplets in an in-parallel static array 5 or an in-serial channel array 6 by means of manipulation of flow directions (e.g., trap- ping in the forward flow and pairing in the backward flow 8–10 ). However, their pairing efficiency was poor (e.g., 73% for the in-parallel array 5 and 40% for the in-serial array 6 ) mostly due to the unstable streamline controls or inhomogeneous hydrody- namic nature of the two-phase flow. In addition to the droplet pairing, a more desirable but more challenging unit operation is clustering of multiple droplets containing different reagents and samples with high pairing or clustering efficiency for complex assays. Here, we introduce a droplet clustering array device per- forming trapping and storing by forward and backward flows, which can be repeated several times to cluster droplets with different contents and samples. Fig. 1 shows the schematic illustration of the proposed device consisting of two PDMS layers. The lower layer, including a flow-focusing junction and traps, is used for generating and trapping droplets. The upper layer, including guiding tracks and storing chambers, is used for droplet guiding, storing, and clustering. A large-scale droplet manipulation has been maintained in a 10  12 array by the forward and backward flows. With the proposed trapping structures and guiding tracks, the trapping and storing efficien- cies have been significantly improved (>95%). Also, we have successfully demonstrated multiple-droplet clustering by the repeated trapping and storing with high clustering efficiency (95%). If inhomogeneous droplets are used with different reagents and samples, much more complex droplet reaction assays can be performed. We expect that the method presented in this study will facilitate the wide applications in many biological, chemical, and pharmaceutical reactions. 2. Methods and materials 2.1 Working mechanism and design Fig. 2a shows the schematic view of the proposed multiple- droplet clustering array. A unit cell of the array, consisting of SMALL (Sensors and MicroActuators Learning Lab), Department of Electrical Engineering, University at Buffalo, The State University of New York (SUNY at Buffalo), Buffalo, New York, 14260, USA. E-mail: kwangoh@buffalo.edu † Electronic supplementary information (ESI) available: Movie S1 for the single-droplet storing (the first droplet trapping and storing), Movie S2 for the double-droplet pairing (the second droplet trapping and storing), and Movie S3 for the triple-droplet clustering (the third droplet trapping and storing). See DOI: 10.1039/c2lc20883k This journal is ª The Royal Society of Chemistry 2012 Lab Chip, 2012, 12, 725–730 | 725 Dynamic Article Links C < Lab on a Chip Cite this: Lab Chip, 2012, 12, 725 www.rsc.org/loc PAPER Published on 13 December 2011. Downloaded on 10/11/2014 15:55:20. View Article Online / Journal Homepage / Table of Contents for this issue