3.1.2 Siphon Valves
The structure of a siphon valve typically utilizes hydrophilic microchannels to
ensure capillary action dominates at a low spin frequencies. These structures consist
of a reservoir chamber attached to siphon channel that is directed towards the centre
of rotation [ 20 ]. At a crest point, located radially inwards of the reservoir, the
siphon channel turns back away from the centre of rotation to a point radially
outward below the original reservoir.
At rest flow can be restricted from entering the siphon valve by a capillary valve
(described above) when the rotation of the disc is increase to the “Burst Frequency”
the valve will open and fluid will flow up the siphon until it reaches hydrostatic
equilibrium, that is when the force of capillary action is counteracted by the
centrifugal force [ 4 , 20 ]. As this frequency of rotation is lowered the capillary
force will become more dominant until the fluid passes around the siphon, it is at
this point that the siphon is said to be “primed”. Next, when the frequency of
Fig. 5.5Figure5.3showed a metered volume of liquid held back by a “hydrophobic constriction
Valve” (Image (a)), this type of valve stops the flow of liquid due to the repulsive interactions
between the liquid and the surface of the microchannels which have been altered to be hydropho-
bic. Image (b) depicts the concept of “Hydrophilic expansion” whereby the walls of the
microchannel diverge rapidly, this halts the contact between the liquid and the walls of the
microchannel which is drawing the liquid along the channel. Without the new surface to draw
the liquid down the channel the advancing liquid stops at the opening. Both valve types can be
opened by increasing the frequency of rotation to the burst frequency
122 B. Henderson et al.