  We address the problem of designing micro-fluidic chips for sample
preparation, which is a crucial step in many experimental processes in chemical
and biological sciences. One of the objectives of sample preparation is to
dilute the sample fluid, called reactant, using another fluid called buffer, to
produce desired volumes of fluid with prespecified reactant concentrations. In
the model we adopt, these fluids are manipulated in discrete volumes called
droplets. The dilution process is represented by a mixing graph whose nodes
represent 1-1 micro-mixers and edges represent channels for transporting
fluids. In this work we focus on designing such mixing graphs when the given
sample (also referred to as the target) consists of a single-droplet, and the
objective is to minimize total fluid waste. Our main contribution is an
efficient algorithm called RPRIS that guarantees a better provable worst-case
bound on waste and significantly outperforms state-of-the-art algorithms in
experimental comparison.
