Capillary Force Valves

Capillary force valves are fluid control structures that use superficial tension at the interface between different fluids to block and/or restore the entrance of fluids in microchannels filled with a second immiscible fluid. For most of the microfluidic applications, the second fluid is air, and the liquid-air interface at a narrow hydrophobic stricture is used to prevent the liquid from entering a capillary. 

Capillary Force Valves, Fig. 1 Superficial tension forces at the interface between a liquid and air. For molecules in the liquid, there is a balance of the cohesive forces (F = 0), while for molecules closer to the interface, the cohesive forces with other molecules in the liquid are larger than the interaction with air molecules (resultant force F toward the bulk of the liquid)... 

Capillary Force Valves, Fig. 3 Capillary forces in hydrophilic capillaries result in increased pressure in trapped air bubbles. Capillary forces in hydrophobic capillaries prevent liquid from entering the capillary and can be overcome by larger pressure... 

Capillary Force Valves, Fig. 4 Schematics of a capillary valve using a hydrophobic patch. The pressure required to move the liquid (Af i) is higher when the liquid-air interface reaches the region of increased hydro-phobicity (Af 2)... 

Capillary Force Valves, Fig. 6 Schematics of capillary valves using changes in the diameter of the capillary. Rapid enlargement of a capillary changes the physical angle between the liquid interface and the solid surface and can form a temporary barrier for the advancing liquid... 

Capillary Force Valves, Fig. 7 Hydrophilic and hydrophobic patches are used to control the flow of liquid streams (here rhodamine in water) in a wall-less configuration, just by relying on the surface tension of the hquid [2]... 

Capillary Force Valves, Fig. 8 Capillary burst valves are used to control the sequential mixing of reagents in a Lab-on-a-CD-type device. The solutions of enzyme, inhibitor, and substrate were loaded in reservoirs that were connected to channels labeled Rl, R2, and R3, respectively. R4 is a reservoir for waste collection. The rotation of the disk at different speeds controls the opening of the valves and the sequence of mixing the enzyme with the inhibitor, followed by mixing with the substrate and detection [3]... 

Capillary Force Valves, Fig. 9 A volume of 25 pL is isolated by the creation of two new liquid-air interfaces, through the use of two thermal actuators. Two air bubbles isolate the separated volume from the rest of the liquid in the main channel [6]... 

Boundary Slip of Liquids Bubble Dynamics in Microchaimels Capillary Filling Capillary Force Valves... 

Capillary Force Valves, Figure 2 Capillary forces at the interaction between air, liquid and solid surfaces. Each of these forces works to minimize the energy of the interface between liquid and solid, air and solid, liquid and air s. Fas, fia. respectively. At equilibrium, the horizontal projection of and fis and Fas cancel each other. The corresponding angle between a and fis the contact angle between liquid and solid ( )... 

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