Camille Durand

You ask an interesting question to which there is no real answer. The first question to you is: what is the reason for sonicating your samples? How are you sonicating - bath or probe? What is the nominal energy input?

If you're trying to replicate the amount of energy input into a system then you'll not be able to do this very precisely - the FDA, for example, gave up trying to find out how much energy a baby receives in the womb from an ultrasound scan.

This sentence has major technical problems: 'My sonication system has been specifically optimised for water based materials (1200 rpm, 1 hr)' for a number of reasons:

• 1 hour of ultrasound is way too long - you'll be carrying out ultrasonic milling on your material, disintegrating the ultrasound probe (if used), significantly heating the sample etc.
• An ultrasonic transducer loses around 10% of its energy per year. So optimization today won't be true in the future...
• If using a bath then there are points of high and zero energy input. This can be shown by putting an aluminum foil sheet in the bath - it'll be fractured in the points of high energy input
• Not sure what you mean by 1200 rpm - if an external (to the probe) stirrer then this will simply transfer localized heat away (a desired effect). However, energy transfer is dependent on the temperature of the fluid (hence major differences in cavitation of ships' propellers in tropical and cooler/arctic waters

The are are a number of factors that influence the transfer of energy. Water is probably the best liquid to accomplish this. Liquids of poorer thermal properties provide poorer heat (and energy) transfer. In particular liquids of higher volatility are extremely poor converters of input energy - the energy is converted into volatilizing the liquid rather than separating agglomerates (the standard function of ultrasound) and also into light and sound (sonoluminescence). The heat in a collapsing ultrasound bubble is around 5500 K and the collapse provides an energy input ~ 1011g where g is the acceleration due to gravity.

The compressibility of the fluid is key as Lord Rayleigh pointed out (see attached slide). I dealt with many of these topics and others in a 2013 webinar (registration required):

Ultrasound, cavitation, and the singing kettle

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W130619UltrasoundCavitationSingingKettle.html

Please view this and let me know if there are further questions.

Dear Camille Durand , I agree Alan F Rawle , your question about the effect of viscosity in sonication is indeed interesting, however, your description of the experimental conditions are misleading and it would be useful to clarify them to get the right aid from the community.

For example:

My sonication system has been specifically optimised for water based materials (1200 rpm, 1 hr).

As Alan explained, these conditions are not describing the sonic energy delivered to the sample, but to some process where the sample is stirred for instance with a magnetic bar at 1200 revolutions per minute, for an hour as Alan pointed, but it could also be the settings of a centrifugue, specially from your description of point 2.

Centrifugue´s head need to be equilibrated to avoid damages of the engine and spills of liquid from the tubes (bottles). This equilibration is made by weight, so for simplicity it is better to use the same solvent volume (or two fractions of the same sample). Of course, it is possible to use water too, at least if your solvent and water have similar densities, so that for equal weight the volume in each tube (bottle) would be also similar (it is not a good idea to have a tube fully filled and the other with just a half).

Depending on the energy applied to your sample through sonication, the exfoliation process could yield particles of different size and different layer number (think about graphene), in these cases you could be interested just on some fraction of these exfoliated particles (say just one of few layers graphene), and to separate them you could rely on centrifugation (among other techniques), and there the viscosity of the fluid could help to improve this separation (you can think about this situation as follows:

Imagine you have sand, powder and feathers mixed, and now you pour them in air from a high position. Sand goes swiftly down and some powder too, but fine powder remain in air together with feathers, that would be the case of low viscosity fluid (water in your case)

Now imagine you pour your mixture in water. Again, sand sink relatively fast, dust remain in suspension and feathers float in the surface. It would be the viscous fluid in your case.

I hope this helps. Please, check your method and let us know any relevant information to better help you.

The lineic adsorption coefficient of ultrasound-waves is proportional to the viscosity. So, in my opinion it is likely that the efficiency of ultrasounds for stirring a solute for instance is affected in the same way.

I have done a bit of a blunder it seems here though. I was supposed to ask the question for centrifugation and have instead asked for sonication. My bad. Sorry for all the inconveniences caused thereof. I have restarted another question with centrifugation. Thanks a lot to everyone who read it or pitched in.

Camille Durand

Thank you for your confession! It amused me. I think you need some music to help your day. My suggestion is:

https://www.youtube.com/watch?v=Ln31raI2ezY

To answer your 'new' question, then Stokes' Law links 'g' with viscosity and settling times. Be aware with centrifugation that it's 'g' that's important and not rpm: the length of the rotor arm plays a major role. You need to convert rpm to g (the acceleration due to gravity).

Camille Durand

In terms of 'g', rpm, and rotor arm take a look at:

https://www.researchgate.net/post/How_to_centrifuge_AuCu2o_core-shell_nanoparticles

Alan F Rawle

The song suggestion did help. :D

Thanks a lot. This makes it clear. I will look for the calculation of 'g' and get back incase of any confusion.

Hello Camille,

this paper may help you:

3S (Sonication, surfactant, stability) impact on the viscosity of hybrid nanofluid with different base fluids: An experimental study.

Hayder Ibrahim Mohammed

Thank you for posting the paper. If you read above Camille Durand was referring to centrifugation and not sonication... She has opened a new question:

https://www.researchgate.net/post/How_is_centrifugation_dependent_on_viscosity