Design of a model-based feedback controller for active sorting and synchronization of droplets in a microfluidic loop

The transport of confined droplets in fluidic networks can lead to complex spatiotemporal dynamics, precluding full control of the position of droplets in the network. Here, we report the design of a model-based feedback controller that can actively regulate droplet positions in a network. We specifically consider droplet dynamics in a microfluidic loop where a main channel splits into two and recombines. Consistent with previous studies, we find that without active control, the dynamics of droplets in the loop can range from periodic to chaotic behaviors. However, by implementing the model-based feedback controller, we show that the droplets can be made to sort alternately into the branches of the loop as well as to synchronize the times at which pairs of droplets exit the loop. In particular, our computations demonstrate that the controller is capable of executing remarkable droplet sort-synchronization tasks in the otherwise chaotic dynamics in the loop. The design of our controller incorporates a hydrodynamic network model, that is, capable of predicting droplet positions and subsequently delivering an actuation to the branches in the loop through elastomeric valves. Efficacy of the controller is discussed in terms of actuation characteristics and constraints imposed by elastomeric valves. The model-based feedback controller framework presented in this study is likely to promote the development of lab-on-chip technologies in which droplet manipulation tasks are executed with active control. (C) 2011 American Institute of Chemical Engineers AIChE J, 2012