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From: "Verlee, Donald J            AP" 
To: mems-mgr 
Subject: RE: Fluid Interface
Date: Wed, 10 Sep 1997 16:38:00 -0500
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Angela,
   I have encountered the problem you have described many times.  It is
easily explained but not easily solved.  For porous media there is a
number known as the Bond number that has been used to determine whether
physical forces have a greater influence on fluid flow behavior or
whether the chemical forces at the interface between fluid and solid
begin to dominate (capillarity).  (For glass (hydrophyllic) and water,
capillarity forces begin to dominate below about 1 mm X 1 mm channels.
Theory suggests that below .9 um dimensions, the capillary force is
greater than 14.7 psi - one can't vacuum water out!)    With the
dimensions you have described your device is acting very much like a
very tiny hydrophobic capillary tube.  Ethanol is very much like water
in that it "wets out" most of the same surfaces that water will.  It is
also a mild surfactant and will lower the surface tension of water.  The
problem is that the channel walls in your device are exhibiting a
hydrophobic surface energy (ethanol would "bead up" on a wide flat
surface of the same molecular constituency of your channel walls).  This
means that your channels have a built in "surface chemical pressure"
barrier to entry.
   One solution might be to change the surface energy of the channel
walls to be more hydrophyllic.  Then the ethanol should readily wick in
- - but then it may also be difficult to get out!  Furthermore any air
bubbles entrained may create "vapor lock".  Surfactants in the liquid
may help to some extent if you can tolerate them.
   I'm willing to discuss this with you further.

Don VerLee
Associate Research Fellow
Abbott Laboratories
D43E Bldg. AP52
Abbott Park, Illinois 60064

(847) 937-2420
E-mail : [email protected]

 ----------
From: mems-mgr
To: MEMS
Subject: Fluid Interface
Date: Thursday, September 04, 1997 4:10PM

Hello,

I am working on a microfluidic device.  I am having problems getting the
liquids to go into the microchannels.  Has anyone worked in this area
and have any advice on getting the liquids into microchannels.  The
channels are very small (~2um depth, 20um width, and ~500um long).  Is
this the problem?  or is it the liquid(ethanol) that is a prob.?  Any
suggestions, experience, or help would be greatly appreciated.
Thanks

 -Angela Rasmussen


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