In the laminar regimes typical of microfluidic systems’, mixing is governed by molecular diffusion; however this process is slow in nature. Consequently, passive or active methods are usually sought for effective mixing. In this work, open fluidic channels will be investigated; these channels are bounded on all but one face by an air/fluid interface. Firstly, it will be shown that flow in open channels can merge at a Y-junction in a stable manner; hence two fluids can be brought into contact with each other. Secondly, the mixing of these two fluids will be studied. At high flow rates (>300 μl/min) mixing occurs at the junction without need for additional intervention, this mixing is far swifter than can be expected from molecular diffusion. At lower flow rates, intervention is required. A major motivation for open fluidic channels is the ability to interact with the surrounding air environment; this feature is used to effect the desired mixing. It is shown that by blowing an air jet across the junction, shear stresses at the air/fluid interface causes a flow profile within the fluid inductive to rapid mixing of the fluids

Open fluidic systems have a distinct advantage over enclosed channels in that the fluids exposed nature makes for easy external interaction, this finds uses in introduction of samples by adding liquid droplets or from the surrounding gaseous medium. This work investigates flowing open channels and films, which can potentially make use of the open section of the system as an external interface, before bringing the sample into an enclosed channel. Clearly, in this scenario a key factor is the stability of the flowing open fluid. The open channels investigated include a straight open channel defined by a narrow strip of solid surface, the edges of which allow large contact angle hysteresis, and a wider structure allowing for multiple inputs and outputs. A model is developed for fluid flow, and the findings used to describe the process of failure in both cases.

The ability to control liquid transfer is of fundamental interest in droplet-based microfluidics. Here, we present a method that allows the selective transfer of fluid by means of a gas bubble injected into a liquid droplet. Two mechanisms were studied, one being liquid transfer through the detachment of a liquid bridge, which results in transfer of over 90% of the droplet between substrates in a single step. The other mechanism is liquid transfer via jetting at the moment of bubble rupture; in this case, a smaller daughter droplet is created on the recipient substrate.

The requirement of producing concentration gradients rapidly and controllably is essential in many biochemical applications. The use of microdroplets is advantageous as the droplets act as isolated open reaction vessels that are proficient in containing a variety of reagents. We present a concept of producing concentration gradients over an array of open wells, using a concentrated primary drop which, whilst in motion, merges with successive buffer droplets and, as a result, leaves behind droplets of different concentrations. The primary drop is positioned to slide down a 90° incline over several secondary drops, whereby merging, mixing, and detaching occurs within each well.

Cell lysis is a key sample preparation stage in many biomedical studies as DNA extraction and classification require the use of the nucleic acid and proteins released upon decomposition of a cell membrane. We present an effective method of lysing cells suspended in a microliter droplet placed on a super-hydrophobic surface. When a bubble, injected into the sessile droplet, subsequently ruptures, a rapidly moving fluid jet is formed. In this work, cells that are transported within this fluid jet are captured on a separate hydrophilic substrate and are shown to have been lysed.