EQ! (Permeability)
SepCon EQ reaches Equilibrium in 2 1/2 hours!
w310 (no pinholes)
1x PBS (no protein)
2 hours @ 1000rpm (3.3psi) -> 103 uL passage (56 uL/(min-cm2-psi) )
2 1/2 hours @ 1500 rpm (5 psi) -> 204 uL passage (>74 uL/(min-cm2-psi) )
In the second case, the sample visibly reached what looked like equilibrium. It was not quite exact, but as the difference in water heights goes to zero, the driving force also goes to zero. One sample did break at the higher pressure. I checked the 204uL sample by reloading it and testing it’s integrity over another 5 minutes. No fluid visibly passed, confirming that the membrane was intact without major defects.
One caveat about the permeability measurements – these are conservative because the effective pressure is decreasing as the water height decreases. I used the starting height to calculate these values. With that said it’s quite surprising that the high rpm sample had an even higher permeability. If it was linearly dependent on pressure, then the two values should be the same. If you consider the average pressure was much lower (as water height decreased to 0) than what I used in the calculation, the second value should be smaller. Does this mean that the higher pressure somehow opened more of our pores? Both membranes were from high R values without pin holes.
I think it’s impressive that we are passing real-world volumes within ~2 hours with only two slits!
Bad news – w320 is very wrinkled (visible by eye) with an abundance of rather large pin holes. w302 was smooth, but had a number of small pin holes.
It’s good to see the spin time creeping downward. I still contend that these flow rates are 5X to 10X lower than theory predicts, so I’m looking forward to seeing how oxidized material behaves.
Wafer 320 was annealed at an unusually high temperature and like oxidized, as Dave mentioned in his production post for this set. It’s basically junk, but can be played with if anyone wants to try it. Wafer 310 and the others in that series are some of the nicest that we have produced – we’re still trying to understand why, since there have been no changes in our core process. The weather perhaps?? Maybe no dirty processes were run at RIT that week?
To clarify, when loaded with 500 uL of fluid, the EQ format reaches near equilibrium with only 200 uL passing? Or are they being loaded with less fluid? Is there any way to pass more fluid, or do we need to re-design the SepCon? I’m concerned that having a 200 uL fitrate and 300 uL retentate is going to be problematic for separation studies, and a new plastic design will take weeks.
Chris –
Yes, you are understanding the EQ situation correctly. I do believe we can load more than 500uL, probably more like 700-800uL if we fill closer to the top (where the snap top was designed). This will increase the initial pressure due to a higher water height.
Today I will also explore options for putting our tubes in larger or different conicals for a “quick” fix and then evaluate some options for a longer-term solution that is closer to something we could hand off to users outside of our lab (Wedekind/Turner/etc.)
So Jess was likely seeing something close to equilibrium reached in her experiments too. This is definitely a more positive perspective on the flow in these experiments than I had before.
Your point about a conservative permeability calculation is very important. We need to work out a simple theory for the EQ equilibration dynamics like the one I worked up for the WetDry configuration. With the correct description of the flow vs. time dynamics, it should be possible to calculate the permeability from a measurement of the transfered volume at any time before equilibrium.
Chris is right about the mismatch with theory. My earlier post used pore sizes in pixels instead of nanometers when piping through the Tong equation and underestimated the flow for wafer 310. The actual theoretical permeability I calculate for this wafer is 700 ul/(cm2-min-psi). I’m reading through the papers that derived the key parts of the Tong equation to understand the prediction better. I’ll repost when I get it sorted out. In the meantime, a proper model of the EQ format dynamics should reveal that we have a higher hydraulic permeability. I would not be surprised if it is several fold higher because the EQ driving pressure drops faster than the Wet/Dry configuration.
So, we have some work left to do but the news here is quite good: The flow rates are high enough to be useful and our hydraulic permeability is as high or higher than all other ultrafiltration membranes.
I think the current EQ format is working for assaying separations – at least for our current needs. Jess showed that we can concentrate the filtrate to get signals on gels that are higher than the undiluted retentate (see her post on Dec. 28th). So we are able to judge the content of the filtrate, which is what I think we need for our current research purposes.