Capillary Valves operate by superficial tension to block or restrict flow in a channel. These valves have become very important with the integration of paper microfluidics in many low cost applications. Capillary flow requires no outside assistance to move a substance along a fluid channel. Capillary flows occurs from the cohesion forces are larger than that of the other liquid molecules which causes the are pulled toward the liquid which propels the liquid forward.1
Background and Methods
There are two main methods of capillary valves, local changes of contact angle and local changes of surface geometry. These two methods are both driven by the change in pressure needed to push the liquid forward through the valve.
Change in hydrophobicity
The local change in contact angle can be summarized as the change of surface interface which will change the pressure need to push the liquid along. If the surface increases in hydrophobicity there is a larger pressure needed to push the liquid. Passive valving featuring extreme wetting states in a single microchannel 5 PMMA was used to show this type of valve. C4F8 fluorocarbon plasma deposition was used to create hydrophobic parts on the channels which stopped flow through the channels.
Change in Geometry
A local change in geometry can change the contact angle of the liquid as in Fig 2 which will change the pressure needed to push the liquid along. The barrier need to push the liquid across this change in contact angle is called the burst pressure.4 This is usually less effective method for capillary valves so it is combined with the other method of adding a hydrophobic surface.
Capillary soft valves use the idea of the local change in geometry to prevent flow and are activated by simply compressing the valve to change the shape. This causes the liquid to fill the valve and flow is continuous. The flow cannot be stopped once the valve is compressed and liquid flows through.
Capillary stop valves are used to stop flow in a channel using a sudden expansion of the channel cross-section. This method is very effect and used in silicone based wafers. This process is important for biological process than cannot be interfered with by hydrophobic interfaces.
Alternative methods require a change of parameters such as heating or cooling the microfluidic channel. Heating a material over the cannel such as wax can stop the flow or the opposite can be used where the liquid is locally frozen causing flow to stop. These valves require more equipment and can interfere with the material being studied or tested.3
Valves Freezing/thaw valve uses a thin channel that has a freeze plug when axial force is applied will constrict. These valves are used for narrow bore capillaries.