System and method for dispensing fluid droplets of known volume and generating very low fluid flow rates

A low volume fluid withdrawal and dispensing system, comprising:

• a bi-directional pump which provides positive or negative pressure at a pump outlet in response to a control signal,
• tubing connected to the outlet of said pump, said tubing at least partially filled with a first fluid,
• an aperture in said tubing downstream of said pump, said first fluid pressurized against said aperture by said pump and passing through said aperture at a flow rate which varies with the magnitude of the pressure applied by said pump on said first fluid,
• a pressure sensor which provides an output signal representative of the pressure of said first fluid against said aperture,
• a controller connected to receive a setpoint signal representative of a desired dispensed volume, said pressure sensor output signal, and a signal representative of dispensed fluid volume as inputs, and to produce said control signal to said pump as an output,
• a withdrawal/dispensing tip on said tubing downstream of said aperture, and
• a droplet volume measuring means arranged to measure the volume of a droplet of a second fluid that forms at the outlet of said withdrawal/dispensing tip, said droplet volume measuring means providing said signal representative of dispensed fluid volume to said controller, said pump, pressure sensor, and controller forming an inner control loop closed around the pressure against said aperture and said pump, droplet volume measuring means, and controller forming an outer control loop closed around droplet volume,
• said controller arranged to, with said withdrawal/dispensing tip immersed in a second fluid, command a negative pressure at said pump outlet to withdraw a quantity of said second fluid into said tip,
• said controller further arranged to vary the pressure against said aperture to produce a droplet of said second fluid at said withdrawal/dispensing tip outlet having the desired dispensed volume.

A method of dispensing a droplet of fluid having a desired volume, comprising the steps of:

• providing a value representing the desired volume of a fluid droplet to be dispensed,
• pressurizing a fluid against an aperture of a known size within a length of tubing,
• measuring the pressure of said fluid against said aperture,
• conveying the fluid which passes through said first aperture in response to said pressure to a dispensing tip,
• measuring the volume of a droplet of said fluid as it forms at the outlet of said tip in response to said pressure,
• computing a pressure value based on said volume measurement and said desired fluid volume value that, when used to pressurize said fluid against said aperture, results in said droplet quickly obtaining said desired volume,
• adjusting the pressure of said fluid against said aperture to correspond to said computed pressure value, and
• reducing said pressure on said fluid to zero when said droplet has obtained said desired volume.

A method of withdrawing fluid from a container and of dispensing a droplet of said fluid having a desired volume, comprising the steps of:

• providing a value representing the desired volume of a droplet of a first fluid to be dispensed,
• immersing the outlet of a dispensing tip in said first fluid,
• establishing a negative pressure within a length of tubing connected to said dispensing tip to withdraw a quantity of said first fluid into said tip,
• removing said tip from said first fluid,
• pressurizing a second fluid against an aperture within said tubing,
• measuring the pressure of said second fluid against said aperture,
• conveying the fluid which passes through said first aperture in response to said pressure toward said dispensing tip via said tubing, thereby applying pressure to said quantity of said first fluid withdrawn into said tip,
• measuring the volume of a droplet of said first fluid as it forms at the outlet of said tip in response to said pressure,
• computing a pressure value based on said volume measurement and said desired fluid droplet volume value that, when used to pressurize said second fluid against said aperture, results in said droplet quickly obtaining said desired volume,
• adjusting the pressure of said second fluid against said aperture to correspond to said computed pressure value, and
• reducing said pressure on said second fluid to zero when said droplet has obtained said desired volume.

A fluid droplet dispensing tip and volume measuring system, comprising:

• a channel having first and second ends, said channel having a predetermined spring rate along its length between said first and second ends,
• an aperture formed at said second end of said channel, said channel arranged such that a fluid introduced at said first end is conveyed by said channel to said aperture, said aperture arranged such that fluid conveyed to said aperture by said channel forms into a droplet around said aperture, said channel fixed at said first end and extending freely and approximately horizontally from said fixed end, said predetermined spring rate such that said aperture end progressively droops with respect to said first end as the weight of said droplet increases, said droplet having a predetermined density,
• a reflective area near said aperture that moves with said aperture as the weight of said droplet increases,
• a laser beam directed onto said reflective area,
• a position sensitive detector (PSD) arranged to receive said reflected beam from said reflective area and to output a signal representative of the position of said beam, and
• a processor arranged to determine the volume of said droplet based on said PSD output, said predetermined spring rate, and said predetermined density, and to output a signal representative of said volume.