The material from which the microfluidic devices are built is a flexible elastomer called poly(dimethylsiloxane) (PDMS). The flexibility of this material is the basis for simple valves. A valve is merely a component that can close or open a channel. With a flexible channel, this can be achieved simply by squeezing on the channel, like a pinch-valve does with tubing. One way to do this with microfluidic channels is to use a single-layer membrane valve, which is a dead-end channel placed in close proximity to the flow channel. Between these two channels is a thin wall of flexible PDMS that can be pushed inward or outward depending on the pressures in the two channels. For example, if the pressure in the control channel is larger than in the flow channel, then the flexible wall is pushed outward, constricting the flow channel and closing it off, as shown in this movie:
The purpose of this device is to characterize the effectiveness of the valve. This is accomplished by measuring the pressure at the exit of the constricted channel as a function the pressure applied to the valve. To measure the exit pressure, a differential manometer is used, which is a device consisting of two parallel channels: the "test" channel on which the valve acts, and a "comparator" that is maintained at constant pressure. When the pressure at the exit of the test channel is reduced by actuating the valve, the liquid from the lower channel moves upward to accommodate more of the outlet channel. To visualize this, the lower liquid is dyed. After a simple calibration of the position of the interface between the two liquids, the pressure at the outlet of the upper channel can be measured.
Membrane valves are very common in single-phase microfluidic devices, in which all the liquids pumped through the device are miscible. This is because in single phase microfluidic they are needed to seal off compartments, so that reactions can be performed in isolated volumes. In droplet-based microfluidcs, valves are much rarer because the drops already perform the function of isolating the reagents. As a result of this seemingly minor difference, single-phase and droplet-based microfluidics are quite different from one another, in terms of the philosophies with which they are build, the principles on which they operate, and the complexity of the channels and control systems. Where as single-phase devices tend to look like a spaghetti-network of channels and control valves resembling the motherboard of your computer, droplet-based microfluidic devices tend to consist of only a few channels in which the flow is always forward and constant. This difference offers distinctive capabilities to the two kinds of devices: Whereas single-phase devices are more programmable, droplet-based microfluidics are much faster and require far less reagent.