The ability to actively control the spatial and temporal dynamics of droplets in microfluidic networks can be harnessed for several applications. Achieving such control is nontrivial due to the nonlinear and interactive nature of such systems, where droplets in different branches of the network can affect each other through resistive signalling. In our previous work we have demonstrated the application of a Model Predictive Control (MPC) framework for sort-synchronization in a simple microfluidic loop device, assuming that the final control elements are elastomeric valves. In this paper, we explore the ability of the MPC framework for more intricate control, where the relative drop distances at the exit of a loop are required to conform to desired profiles. We demonstrate that through appropriate MPC objective function choices, a variety of digital signals based on the relative exit distance can be generated. The importance of such control is highlighted. (C) 2012 Elsevier Ltd. All rights reserved.