High Speed camera recording of NPN membranes bursting
Scott Russell from Mech. E. and Steve Gillmer came over to help Sarah and I try to record the burst of membranes at up to 2000 frames per second. Generally we used 100o frames per second because it was the best compromise between our ability to time the 1 s recording time and the amount of light necessary.
The HotShot camera had to be positioned in front of the eyepiece of the stereo microscope. (The “better” microscopes did not have enough working distance to allow the burst pressure rig to fit underneath the lens) We focussed on the surface of the membrane. We experimented with many different light sources. Halogen, fluorescent, and LED bulbs, fiber bundles, side and top down illumination. There is plenty of room for optimization. We had to do a “ready set go!” count down to synchronize when the recording started and when I applied pressure. Because of this we chose to overshoot the burst pressure by 1.5-2 fold to make sure they broke when we applied the pressure.
We started out looking at the light reflected off of the top surface of the membrane. Unfortunately, as the membrane bulged the light was reflected out of the lens and the sides went dark. Following are two examples using some chips with square membranes with side lengths ~1.3-1.5 mm. I cropped the time stamp off of the first video, but the entire clip is about 50 ms.
So we tried mounting the sample on a transparent polycarbonate holder that would let us look at transmission:
We made need to go to even higher frames per second to capture details about the rupture, but we definitely need to get a better optical image of the bulged membrane. We were considering putting nanoparticles or microspheres on the surface. These would scatter light instead of just reflecting it, and as opposed to coating it with metal, the NPs are less likely to effect the mechanics of the membrane.
I’m really happy with the fact that we got any recordings at all after all of the setup that was required and the difficulty in timing the burst and camera trigger. Big thanks to Steve and Scott for their hard work and patience!