bubble formation when glue-bonding with adhesive droplet

Dear reader,
when contacting a solid surface with a sessile liquid drop, a gas bubble
forms depending on viscosity and momentum of contact.
What´s the physicochemical reason?
Any literature?
My problem in detail:
I am trying to join two glass chips (several mm edge length) with a
liquid adhesive. For this I place one chip under a stereomicrosocope,
dispense a drop of adhesive (ca. 30 microL, visc. 4000 mPas, degassed,
UV-cure) onto it, grab the second chip with a tweezer, approach the
drop vertically from atop, make gentle contact and release the second
The drop initially is bubble-free, but in the moment of contact a gas
bubble forms with a diameter of about 0.1mm.
I tried 2K-silicone with the same viscosity: same result.
I tried Polycarbonate and Polystyrene as transparent chip materials
with the same outcome. I placed small droplets on both surfaces: same
When using droplets of higher viscosity, the bubble diameter
I electrostatically discharged all materials.
Only reducing the approach velocity yields a bubble-free joint.
So, it´s not surface tension related (?)
What is it?
Thanx in advance,
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I would guess that either the adhesive is outgassing during cure (despite the fact that the supplier might say otherwise) or that the materials being bonded are reacting to the adhesive and outgassing during cure. It only takes a minute amount of gas to make a big, flat bubble.
The fact that it occurs on many bonded materials makes me lean more towards the former explanation.
What sort of curing conditions are used? There might be a component of the adhesive that's volatile and evaporating (thus forming the bubble) due to the cure conditions.
Reply to
David Harper
Bubble formation occurs instantaneously in the moment of droplet contact way before curing, so curing conditions are not the reason for it.
Outgassing could be the reason, but why is the process momentum dependent?
Is anyone around proficient in microoptics? When glue-bonding (micro)optical components, bubbles should be crucial and avoided, so he might be familiar with this problem and its physicochemical basics.
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