Droplet breakup is an important phenomenon in the field of microfluidics to generate daughter droplets. In this work, a novel breakup regime in the widely studied T-junction geometry is reported, where the pinch-off occurs laterally in the two outlet channels, leading to the formation of three daughter droplets, rather than at the center of the junction for conventional T-junctions which leads to two daughter droplets. It is demonstrated that this new mechanism is driven by surface tension, and a design rule for the T-junction geometry is proposed. A model for low values of the capillary number Ca is developed to predict the formation and growth of an underlying carrier fluid pocket that accounts for this lateral breakup mechanism. At higher values of Ca, the conventional regime of central breakup becomes dominant again. The competition between the new and the conventional regime is explored. Altogether, this novel droplet formation method at T-junction provides the functionality of alternating droplet size and composition, which can be important for the design of new microfluidic tools.

The Poisson limit is a major problem for the isolation of single cells in different single-cell technologies and applications. In droplet-based single cell assays, a scheme that is increasingly popular,...

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The generation of single-cell biological assays has become a strategic need for fundamental and clinical studies. Droplet microfluidics being sensitive, high throughput, and low cost, has been widely adopted for single-cell assays. However, precise manipulation of single cells using droplet microfluidics is still limited by the intrinsic randomness during single cell encapsulation, known as the Poisson limitation. It results in ambiguous single-cell assays with considerable cell losses. Here we present a novel cell-triggered splitting (CTS) method for deterministic and passive single cell encapsulation into droplets. We demonstrated the high efficacy (negligible cell loss) and high specificity (high cell loading percentage) of this method. With a simple working principle, this passive and label-free method reaches an unprecedented throughput of more than 3k Hz for droplet sorting, corresponding to 22,000 cell-loaded droplets per minute with less than 3% false events (empty droplets or doublets), a 100-fold improvement compared to existing label-free active droplet sorting methods. This asynchronous method requires no flow tuning and provides a general solution to different application scenarios, from high throughput encapsulation to manipulation of small cell populations. This method also applies to the encapsulation of other deformable particles such as hydrogel beads. The novel versatile tool may impact applications in single-cell sequencing, cell-cell/cell-bead interaction, and rare cell isolation. This passive and deterministic single-cell encapsulation method provides a practical solution to overcome the long-existing difficulty associated with the Poisson limit.

The generation of single-cell biological assays has become a strategic need for fundamental and clinical studies. Droplet microfluidics being sensitive, high throughput, and low cost, has been widely adopted for single-cell assays. However, precise manipulation of single cells using droplet microfluidics is still limited by the intrinsic randomness during single cell encapsulation, known as the Poisson limitation. It results in ambiguous single-cell assays with considerable cell losses. Here we present a novel cell-triggered splitting (CTS) method for deterministic and passive single cell encapsulation into droplets. We demonstrated the high efficacy (negligible cell loss) and high specificity (high cell loading percentage) of this method. With a simple working principle, this passive and label-free method reaches an unprecedented throughput of more than 3k Hz for droplet sorting, corresponding to 22,000 cell-loaded droplets per minute with less than 3% false events (empty droplets or doublets), a 100-fold improvement compared to existing label-free active droplet sorting methods. This asynchronous method requires no flow tuning and provides a general solution to different application scenarios, from high throughput encapsulation to manipulation of small cell populations. This method also applies to the encapsulation of other deformable particles such as hydrogel beads. The novel versatile tool may impact applications in single-cell sequencing, cell-cell/cell-bead interaction, and rare cell isolation. This passive and deterministic single-cell encapsulation method provides a practical solution to overcome the long-existing difficulty associated with the Poisson limit.