Further adventures in membrane transfer

This post is a followup to my previous post, which can be found here.

When last I posted, I shared a handful of videos showing membranes transferred onto glass with water vapor being wetted in shear, and noted how the wet-dry interface tended to rend them apart and/or drag them along them surface. I provided a hypothesis for the mechanism that the interface might pull the membrane by, but since then I think we’ve come to a new understanding of the forces at play here.

This video demonstrates nicely what we’re dealing with:

By comparison to the dimensions of the membrane, we can get a good picture of the shape of the water-air interface, which we see here as a dark band: it is a right triangle with an interior angle at its base of approximately 45 degrees, though there may be some bowing. The base of this triangle is acting like a “razor blade” as Greg put it, spreading out under the membrane in a sheet and lifting it onto the interface (notice the slight increase in the thickness of the band when it reaches the membrane.) The space occupied by the water under the membrane can be visualized by the wrinkle it leaves in its wake, which can be seen progressing from right to left and then disappearing before it reaches the opposite edge of the membrane, indicating a region on that edge which is for some reason or another stuck down more firmly than the bulk of the membrane was. When this edge has been reached, the interface is halted and more peripheral regions of the interface begin to flow instead, infiltrating further under the stuck-down area of the membrane from the edges, until the seal is broken and the entire membrane is dragged along with the interface.

The water is then withdrawn back from over the membrane and then set to flow over it again. When the interface reaches the membrane, it joins with the water remaining underneath it, which looks like a sudden jump forward. This process is repeatable, and the water can also be seen hesitating to withdraw from the membrane area in future iterations, suggesting incomplete withdrawal of water from underneath it..

The takeaway is that the water is sheeting under the membrane, pulling it up away from its substrate. A new video also demonstrates a similar process occurs when water is simply condensed onto the surface, without a wet-dry interface flowing over it:

Here, we can clearly see a continuous body of water underneath the membrane and adjacent to it, in addition to obvious signs of curvature on the membrane surface. As the bead evaporates, the water under and adjacent to the membrane do so together. Interestingly, the left-most edge of the membrane does not pop up from the substrate, and prevents the bead under the membrane from meeting with and joining the bead to its left. This is likely working by the same mechanism which accounts for the similar behavior in the first video above, but it remains a mystery.

A third experiment in which a transferred membrane was submerged inverted in water and left overnight also demonstrated similar results: the membrane delaminated from the surface and was visibly floating up over it, but did not fall completely off and sink to the bottom of the flask due to adherent connections at one edge.

One more video can be used to provide some extra hints about the mechanism of water infiltration under the surface:

These videos show an extremely fast wetting which does not appear to appreciably disturb the membrane, followed my withdrawal of the water, drying, and then a slow wetting which disturbs it much more, nucleating defects at wrinkles in the membrane. Notice how regions far from wrinkles tend to apparently stay down better. In addition to providing even more evidence that water infiltrates under the membrane at the interface, this behavior suggests infiltration by capillarity.

Thus, we conclude that the membrane is not stuck down in any robust, repeatable manner that resists water infiltration from the edges. Obviously, this isn’t ideal for a prefilter application, but may still be better than nothing since probabilistically, most diffusive transport from a bulk solution to the surface “protected” by the tent will still be through the pores in the tent, rather than around it.

The advantage of this method is that it doesn’t require and type of destructive surface treatments such as rapid heading or high voltage. This vapor application allows for a gentle laying down of the membrane over the substrate. We may be able to optimize the method to minimize wrinkling and thus keep the volume of water infiltrating under the tent to a minimum.

As far as a more permanent bond goes, it looks like UV/Ozone bonding doesn’t provide a particularly effective seal between the two membranes due to the lack of an organic group to sacrifice in the formation of the Si-O-Si bonds created in that sort of process. Nevertheless, we want to try nuking the surfaces with oxide from an RTP followed by a YES treatment, which may prove sufficiently powerful to form the bonds anyway. In the event that that fails, we may want to pursue alternatives such as physical clamping or anodic bonding.

 

Edit by Greg June 10th 2015

Both of these videos were captured by Tucker.

This video shows repeated wetting and ‘drying’ of the tented NPN nanomembrane over the oxide structure (nitride substrate). Note that even as the meniscus passes over the membrane multiple times, it is very stable. Somehow, we have to find a repeatable way to get the membrane in this stable state.

 

Here is the same tent under continuous fluid flow (high shear, ~100 uL/min). There are some acrylic beads to visualize the flow, but they didn’t show up in the video until near the end (1:34,  bottom edge of channel).