For the typical orifice shape the stream coming out of the orifice is slightly smaller than the orifice (see lindmo in flow cytometry and sorting) but the droplets get bigger as you introduce spaces in the column when you generate droplets. The lower the vibration frequency the bigger the droplet will be as you can see when you have a screen display of your droplets (more space between the droplets). The volume you deliver through an orifice is at least dependent on sheath pressure difference, sample pressure difference, surface tension, viscosity, temperature and orifice geometry. The measurement is normally the easiest and probably the most accurate way of volume determination per droplet. Regards Gerhard -----Original Message----- From: Alice.L.Givan@Dartmouth.EDU [SMTP:Alice.L.Givan@Dartmouth.EDU] Sent: Friday, March 08, 2002 4:44 PM To: Cytometry Mailing List Subject: Re: Bacteria sorting? In my comment on calculating the volume of a droplet, I "guessed" that the diameter of the drop might be approximately the diameter of the nozzle orifice. I knew this was a rough approximation --- as the stream can contract a bit as it leaves the orifice and then the droplet that forms can have a diameter greater than the diameter of the stream. Joe Trotter (whom I always trust) says that the droplet diameter is approximately 1.89 times the nozzle diameter. In fact, I just went and measured the drop and the stream on my monitor screen and I got about 1.6 (using a bad ruler and a roughed up piece of paper). So my approximation that the drop diameter was equal to the nozzle orifice diameter was, obviously, grossly wrong either way. Perhaps the best way to calculate the volume of a drop is by Dirk Van Bockstaele's method (calculating the volume of the "column" of liquid coming out of the nozzle in one second and then dividing that volume into however many drops are being generated in one second). However, in order to do this from first principles you need to know the diameter of the stream and also the velocity of the stream.....possible, but awkward. I suppose you could also do it simply by measuring the volume flowing from the stream in 10 minutes and then calculating the volume for one second (I might be missing something here -- Howard, where are you (I know, he is working on the fourth edition just when I need him). Alice Alice L. Givan Englert Cell Analysis Laboratory of the Norris Cotton Cancer Center Dartmouth Medical School Lebanon, New Hampshire NH 03756 tel 603-650-7661 fax 603-650-6130 givan@dartmouth.edu
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