Flow restrictor and system for delivering a flow of liquid in a microcapillary
The present invention relates to apparatus for delivering a controlled or restricted flow of liquid and limiting bubble fragmentation at the inlet. This is achieved in flow restrictor with a flow channel having over most of its length a substantially constant, minimum hydraulic diameter D=4A/W wherein A is the minimum local cross-sectional area of the channel and W is the minimum local wetting perimeter of the channel, by smoothly widening the channel at its inlet such that: at distances z from the inlet face with 0<z<z 1 , the channel has a hydraulic diameter D z ≧k*D wherein k≧1.3; at distances z from the inlet face with z 1 <z<z 2 , the channel has a hydraulic diameter D z with k*D≧D z ≧D; and at distances z from the inlet face with z 2 <z, the channel has a hydraulic diameter D z with D z ≦1.02D, except possibly for a similar widening of the channel at the outlet. It has been found that widening the flow channel at the inlet, such that its diameter increases smoothly and gradually, and preferably by a factor of at least 1.3 over a length of at least 3 channel diameters, significantly reduces the tendency towards bubble fragmentation in microcapillary flow restrictors, and also substantially increases the yield of devices which operate well.
1. A flow restrictor for restricting a flow of liquid, the restrictor comprising a body with an inlet face, an outlet face and a flow channel extending therebetween from an inlet to an outlet, the channel having over most of its length a diameter D, wherein the flow channel has been widened at least at the inlet in such a manner that at distances z from the inlet face:
at z=D the channel has an internal diameter D(z)=3.5D, and
at z≧10.5D the channel has an internal diameter D(z)=D,
and in such a manner that internal diameter, D(z), is narrowed smoothly and gradually as a function of z.
2. The flow restrictor according to claim 1 , wherein the geometry of the flow channel is designed to minimize the change in curvature in the widened part of the flow channel.
3. An apparatus for delivering a restricted flow of liquid to a recipient, the apparatus comprising a reservoir of liquid at higher pressure than the recipient, a flow restrictor as claimed in any preceding claim, a transfer conduit in fluid communication with the reservoir and the inlet of the restrictor, and a delivery conduit in fluid communication with the outlet of the restrictor and the recipient.