Darkfield condenser - oil or water?

[MikeBradley]

I just took delivery of an Olympus darkfield condenser for my BH2, it is their "wet" model (DCW Oil immersion). It works very well when oiled to the slide but it seems to work almost as well when I use water as the medium. Since oil is messy to clean up, what do I lose by using water routinely?

Secondly, what is the best way to clean the condenser after using it with oil?

Thanks
Michael

[75RR]

what is the best way to clean the condenser after using it with oil?

I use isopropanol and cotton wool.
See the booklet "The clean microscope" for specific instructions: viewtopic.php?f=15&t=1428

Here is a list of the refractive index of various liquids.
Note that the refractive index of air is 1, immersion oil is 1.515 and water is 1.33

[Hobbyst46]

The very properties that make oil "nasty" make it appropriate for immersion: will not evaporate, clings to the glass surfaces and, due to its viscosity, remains between the condenser and slide for a long time rather than spreads around and drips down.

I would avoid using solvents, other than isopropanol or light petroleum, to remove oil residues from the condenser (or objectives) and even these two I would use very sparingly.

Alternating between immersion liquids, e.g. water, then oil, then water etc, can be problematic, because they repel each other. Each of them must be well removed from the condenser lens before applying the other.

Water for immersion: I would choose the best distilled water available (distilled, rather than deionized or RO water), to prevent the accumulation of precipitated residues on the condenser glass surface.

[apochronaut]

Using water instead of oil for DF, kind of negates the value of DF, which is to be able to achieve better resolution of difficult to resolve details. The extra N.A. provided by the oil will only be a benefit.

The cleanup of oil is a bit of bugbear to some, myself included but if a lens or condenser is used frequently, and only lightly wiped to remove the free oil on the surface of the exposed lens , any residual oil smear will quickly amalgamate with the oil for the next use. If you are worried about dust adhering to a lightly oil smeared surface, then slip a little bag over the surface between uses.
Obviously, if the optics are not going to be used for a while, the oil will dry out and be a little harder to remove, so attempt a more complete cleanup.
Limiting physical contact with lens surfaces is always a good idea. The more one cleans a surface, the more likely something gets rubbed against it that will damage it. It's kind of a Russian Roulette thing.

Isopropyl isn't that great a solvent for oil , and unless you are doing multiple applications, it will leave an oil film anyway.

[mnmyco]

@75RR I know Zeiss recommends a mix that includes isopropyl alcohol, but it does damage aluminum. Admittedly, I am not sure of the rate of the reaction that occurs.

mnmyco

[75RR]

@75RR I know Zeiss recommends a mix that includes isopropyl alcohol, but it does damage aluminum. Admittedly, I am not sure of the rate of the reaction that occurs.

That made me sit up and wonder if I was slowly dissolving my Zeiss WL!


Searched for info on this on the web and came across a cool video that will have "Mad Scientist" forum members sitting up and looking to buy some more lab equipment.

The product of the reaction is called Aluminum Isopropoxide

The explanation of the procedure is at 5:47

Making Aluminum Isopropoxide: https://www.youtube.com/watch?v=SuoWnueBJaY

Takeaway tip: Do not immerse your microscope in a vat of isopropyl alcohol, mercuric chloride and iodine.

[apochronaut]

@75RR I know Zeiss recommends a mix that includes isopropyl alcohol, but it does damage aluminum. Admittedly, I am not sure of the rate of the reaction that occurs.

mnmyco

Isopropyl makes a decent general cleaner but is only mildly a solvent of oil or grease. That's why Zeiss uses it in a blend. I'm sure one of the other components is a surfactant. I usually mix isopropyl with a mild inscented detergent/water mix, if the oil really needs to be removed. The surfactant gets rid of the oil. I follow up with isopropyl as a rinse agent, several times . For uncoated lenses that have just arrived and are completely choked with residue, I will resort to the same with ethanol, instead of isopropyl.

I wouldn't worry about the aluminum. The small amount of isopropyl evaporates readily.

[Hobbyst46]

...I was slowly dissolving my Zeiss WL!

Fear not, this does not happen.
Aluminum is a quite reactive metal, it reacts with water as well as organic chemicals, alcohol included. BUT - to react, it needs activation, or catalysis, for example - converting the Al to powder form, mixing it with mercury, or heating to (very) high temperatures. Otherwise, Al is not attacked. Furthermore, uncoated aluminum, upon exposure to air, is rapidly oxidized, and the very thin oxide layer protects it from further interaction with chemicals (unless, of course, these chemicals dissolve the oxide - for example, if they are strong acids or strong bases). Moreover, aluminum parts for use in machines and instruments are often anodized, that is, electrolytically oxidized, the coating being 15-30micrometer thick. Finally, these aluminum bars are not pure Al but alloys (e.g. 6061) and I think they are even less prone to chemical attack than chemically pure Al.

So IMO, in microscopy, aluminum is not really attacked by isopropanol

In the "Mad Scientist" and similar events, some catalyst was added. Not relevant to microscope cleaning with isopropanol.

[MikeBradley]

Thanks everyone for the ideas and advice. I think I'll be using oil with the condenser as a routine and cleaning occasionally with isopropanol.
Michael