Capillary Valves - Jeremy Allen
Capillary Valves are passive non-mechanical valves which operate by surface tension to block or restrict flow in a channel. Unlike Pneumatic Valves, capillary valves operate by no moving parts. These valves have become very important with the integration of paper microfluidics in many low cost applications. Some valves like electrowetting valves can be very advanced and costly for a simple microfluidic device like paper microfluidics. Capillary valves take advantage of forces that are created by solid liquid interface created from capillary flow. Capillary flow requires no outside assistance to move a substance along a fluid channel which makes it appealing to the low cost applications. This flows occurs from many different forces all related to surface tension. The basic idea is the attempt to minimize surface energy. Energy that is stored in the surface is a function of surface area which causes fluid to minimize the surface area along a solid. The interface between the liquid and solid stores less energy than the air and solid so the liquid will move towards the less energetic state along the surface.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 surface and liquid interface either by changing the surface angle or the surface properties.2
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 past that point which gives it the properties of a valve (Figures 1,2). A hydrophobic surface will cause the angle of the solid-liquid interface to be greater than the a hydrophilic surface and prevent the liquid to be pushed by the surface forces.2
See more at hydrophobic valves.
Change in Geometry
A local change in geometry can change the contact angle of the liquid as in Figures 3,4, 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. This is usually less effective method for capillary valves so it is combined with the other method of adding a hydrophobic surface.5
Capillary stop valves are used to stop flow in a channel using a sudden expansion of the channel cross-section. The method for starting flow again is either adding more pressure by centrifuging or changing the shape.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.4 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.6
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 valves uses a thin channel that has a freeze plug when axial force is applied will constrict (Figure 5). These valves are used for narrow bore capillaries.This is important for high pressure applications that can avoid valve leaking. The decrease in temperature can affect the substance though so this valve can only be used in certain applications. To stop flow liquid carbon dioxide or liquid nitrogen can be used. To heat and melt the liquid freeing flow, a hot stream of air or electrical current can be applied.7
Another examples of valves that use the above principles are liquid triggered valves. These have 2 channels coming together at a hydrophobic inlet and the pressure required to overcome the barrier is greater than the capillary force(link) of the single liquid. The liquid only starts flowing when another liquid comes to the barrier. These valves are very useful to ensure air bubbles are not trapped in the fluid.8
1- Vowell, S. Microfluidics Effects of Surface Tension. 2009-03-19)[2010-09-21].
2- Li, D. Encyclopedia of microfluidics and nanofluidics; Springer Science & Business Media: 2008;
3- Tsougeni, K.; Papageorgiou, D.; Tserepi, A.; Gogolides, E. “Smart” polymeric microfluidics fabricated by plasma processing: controlled wetting, capillary filling and hydrophobic valving. Lab on a Chip 2010, 10, 462-469.
4- Maria, M. S.; Rakesh, P.; Chandra, T.; Sen, A. Capillary flow-driven microfluidic device with wettability gradient and sedimentation effects for blood plasma separation. Scientific reports 2017, 7, 43457.
5- Gliere, A.; Delattre, C. Modeling and fabrication of capillary stop valves for planar microfluidic systems. Sensors and Actuators A: Physical 2006, 130, 601-608.
6- Hitzbleck, M.; Avrain, L.; Smekens, V.; Lovchik, R. D.; Mertens, P.; Delamarche, E. Capillary soft valves for microfluidics. Lab on a Chip 2012, 12, 1972-1978.
7- Gerhardt, G. C.; Bouvier, E. S.; Dourdeville, T. Fluid flow control freeze/thaw valve for narrow bore capillaries or microfluidic devices 2003. ( I could not find a DOI for this source)
8- Oh, Kwang W., and Chong H. Ahn. "A review of microvalves." Journal of micromechanics and microengineering 16, no. 5 (2006): R13.