Nanoporous Wallpaper

ByGreg Madejski

In my recent tenting posts, the membrane has shown that it can deform and wrap around edges. I wanted to follow up on this phenomenon and see if we could get a contiguous sheet of membrane around a PDMS edge (not completely sharp). If we can get some membranes around a ~90 sidewall, we could have interesting membrane compartment shapes. Particularly, we could compare to flexible electrospun membranes at much thinner membrane thicknesses.

I aligned and pressed a NPN membrane orthogonally to a 50 um high PDMS microchannel, and then delaminated the membrane with tweezers by poking the freestanding portion in the middle. I hoped that the membrane would unravel from the center towards the edges of the microchannel and stick on the sidewalls. For SEM work, I coated the surfaces with 5-10 nm of platinum (100 sec in Pt sputterer in SEM prep room, 100 mTorr, 15-20 mA sputter current).

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Microchannel Macroscopic view. The Membrane is delaminated at 90 degrees in the central channel.
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Normal view. The edge of the microchannel is apparent in the middle of the image, and the membrane is draped down to the bottom in the top part of the image. Appears mostly intact over a ~ 100 micron linear strip.
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Normal view. Alternate edge of microchannel, where the membrane is draped from the top part of the image to the bottom of the microchannel in the bottom part of the image.
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Normal view. The material that was draped is nanoporous.
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45 degree view. Charging artifacts (streaks) and previous ebeam damage (facets) make it difficult to identify features, but the characteristic edge is apparent in the middle of the image. The membrane appears to wrap around the edge and then delaminate to the bottom of the channel. If we control the length of the delamination carefully (laser), we can probably control the wallpapering effect.
Nanoporous_Wallpaper_15
45 degree view. The membrane conforms to the micrometer-rough edge without breaking, making it around the edge before tenting down to the bottom surface.

 

 

I think the easiest way to get the nanoporous membrane to transfer into devices is actually to control the delamination process. If we can use a better method of rupturing the membrane (laser), I believe we will get larger areas of contiguous membrane. It seems that we will be able to get the wall papering effect around rough, non 90-degree edges in PDMS systems, but it is possible that the membrane will fracture against a smooth, sharp edge.

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