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# Preface! â†’ âś¨

Weâ€™re back, reader! Itâ€™s your favorite material scientist, and today, Iâ€™m going to explain the difference between a supercritical fluid and a subcritical fluid. I have talked about its role in the drying of any type of aerogels, but not exactly why it does what it does!

Sure, Carlos! But can you explain it in under 5 minutes? Iâ€™m tired from doing homework, so please make it quick!

Hahaha, got you, reader! Iâ€™ll make sure to do so! In the meantime, Patrick, you can kick off â€śSupercritical vs. Subcritical Fluids!â€ť

# Whatâ€™s A Supercritical Fluid? â†’ âť—

Just to briefly define a supercritical fluid, it means that both the temperature & pressure of either a material/substance reach their critical point. The Critical Point is when two phases of a substance are unrecognizable from one another! In the case of aerogel, these two phases/states are the liquid state and the gas state! Both the liquid and gas states are existing at the same time!

Thanks for the explanation, Carlos, but can you demonstrate how the supercritical state would look like?

Tell you what, reader. Iâ€™ll explain the supercritical fluid & the critical point using Supercritical CO2! We will use Supercritical CO2 (SC-CO2) because it is the fluid that will turn the gel into aerogel (replacing the methanol in the gel)!

Now, this may look like a bunch of gibberish, but watch how we decipher the Supercritical CO2 Diagram!

The Critical Temperature of CO2 is 31Â°C or about 300 Kelvin. That is the temperature required for CO2 to reach the critical temperature.

But Carlos, you forgot the pressure! Remember what you said?

The Critical Pressure of CO2 is 74 bar (Pascals), which is our unit of pressure! We need to reach this specific pressure for CO2 to reach the critical pressure.

But because of the pressure and the temperature, you can pressurize CO2 (74â€“100 bars) while maintaining low temperatures (40â€“60Â°C) to reach CO2â€™s Supercritical Point! This is what is known as Low-Temperature Supercritical Drying (LTSCD)!

Cool, Carlos! But if thatâ€™s supercritical drying, whatâ€™s subcritical drying?

# Whatâ€™s A Subcritical Fluid? â†’ âť—

I will use the CO2 example again to demonstrate what a subcritical fluid actually is, reader.

The Critical Temperature for CO2 is 31Â°C, and The Critical Pressure for CO2 is 74 bar.

To reach the subcritical fluid of CO2, all you have to do is lower the temperature below 31Â°C, and the SC-CO2 will go below The Critical Point, returning to a liquid state.

Wait, what? That was it? đź¤Ż

Yup! Subcritical actually means to be below a certain requirement/threshold, and lowering the temperature puts us below the threshold, therefore, we are in a subcritical state.

See what I did there, reader?

# Closing Thoughts! â†’ đź’­

Congratulations, reader! You now know the differences between the supercritical state and the subcritical state of liquid CO2!

And I explained the difference between the two states in under 5 minutes!

Hahaha, good job, Carlos! Whatâ€™s for the next article? After all, you are 1/4 of the way with â€ś100 Nano-Storiesâ€ť!

# Vocabulary â†’đź““

Bar â†’ The metric unit of pressure; equivalent to 100,000 Pascals.

Pascal â†’ The SI Unit of Pressure, and is equal to one kilogram per meter per second squared.

Critical Point â†’ The pressure-temperature curve designates conditions under which a liquid and its vapor allow for properties of both liquid and gas to coexist.

CO2 â†’ Carbon Dioxide.

Supercritical Drying â†’ Liquid in a gel/substance is being transformed into a gas with the absence of the surface tension and the capillary stress of the gel. Itâ€™s what allows for a silica gel to transform into an aerogel.

Subcritical Drying â†’ The temperature is below the supercritical temperature of a substance.