Letters to Father Characterization: Q&A

I hope you’ve been a good consultant this year and not been a naughty boy. Thank you for writing to Father Characterization. You may want some new accessories this year from Santa for your rheometer. As is the case for many scientific questions, the answer is “it depends”. It depends on your sample and its inherent flow and viscoelastic properties. May I ask what you’re measuring? With rotational rheology, the sample must keep up with the stress imparted by the rotating disc. If there’s a danger of slippage, a variety of things may be seen depending on the experiment type (constant stress or constant torque; pre-conditioning of the sample) but basically we’re saying that if the disc detaches from the fluid then we create a void or voids in the system. If slippage is gradual, we may see inconsistency in the results, with less contact favoring a lowered viscosity. With sudden slippage, we may even eject the sample from the gap between the plates. For certain samples, we can minimize the chances of slippage by increasing the surface area between the plates and the liquid – for example, with roughened plates, or in the classic example – yogurt – with a vane-type arrangement. We can take measurements in an oscillatory manner to minimize the chances of detachment, too. So maybe a new accessory would be in order depending on your actual application.

Thank you for taking the time to write to Father Characterization. Sadly, Father Characterization does not have a crystal ball, but he does know that every year more and more boys and girls write to him requesting more and more exotic gifts especially of an electronic nature and seem always want them with a substantial discount! It wasn’t so long ago that I was delivering wooden toys (like the Mastersizer Classic in brass and mahogany) in my sleigh. Time moves on, and we have gone from notepad to iPad, and my sleigh is now jet-propelled to get around the world in time. So Father Characterization thinks that there’ll be more of the same sorts of toys, but they’ll be quicker, faster, delivering more information and making intelligent software decisions based on the input sensor and sample information.

Colloids are very interesting and you’re a very lucky boy in being able to play with them.  We have lots of elves and pixies who also play with colloids so there’s lots of help we can offer you. Colloids are small – typically in the nano (< 100 nm) region, and bubbles are often quite large.  Typically, if your bubbles are are smaller than around 20 mm or so they will collapse, as the radius of curvature is too high (as happens with sonication).  We have a webinar on ultrasound and the singing kettle that deals with this subject – you may like to have a watch of this. With small samples in laser diffraction, we need to have sufficient size, concentration or optical contrast to get adequate signal-to-noise.  Say 5 – 20 units of maximum scattering in the intensity-angle profile.  How do you get bubbles?  Well, if you have high concentrations of surfactant.  If you’re using a diffraction instrument as you hint you do with your comments on the residual, then it is possible that you will have a sample with all the material in the colloidal range. In this case, Father Characterization would be thinking of a Zetasizer Nano instrument. You have also hinted at the standard issue with measurements of this type – dilution.  For emulsions and colloids (where there needs to be adequate zeta potential to keep particles apart), dilution in the mother liquor i.e. the same ionic background as the starting dispersant phase, is recommended. In this way, one doesn’t disturb the ionic background to the sample.  In this case therefore there should be no dilution of the necessary surfactant or stabilizer.  If we don’t do this, we risk destabilizing the sample.  In both emulsion and colloid cases, one would seek to minimize the energy input by having the pump stirrer set at a low level, simply to mix rather than shear. The RI and imaginary component should never be unknown.  We can measure these on an Abbe refractometer very easily for the oil and aqueous phases in an emulsion.  The residual is not a particle size parameter, and we can talk about this offline, but it needs care in interpretation.  1% residual or more may be fine for a narrow distribution – see what a standard small latex provides.

Now, stearic acid-covered calcium carbonate is normally a diffraction experiment.  Did you start with a powder?  In diffraction, we mix the sample with a little neat (100%) non-ionic surfactant to wet it thoroughly, and then we measure in DI water.  Other organic solvents would tend to dissolve the hydrophobic coating.  Remember too that calcium carbonate is sparingly soluble in water and often has a positive zeta potential in pH 7 water.  What size do you get when measauring with DLS?  Father Characterization would expect that we would get error messages relating to sedimentation or far-field kicks in DLS.  One of our local Mexican elves such as Edna or David may be able to help here if you can send them the raw data file.

Thank you Colin for having a Nanosight system, and also for writing to Father Characterization. Burning sun particles can hide everything else you want to see, so it might be necessary to filter your samples to remove the big stuff (or allow these particles to settle out before analyzing the sample). You can always extract and export the video and crop afterwards with appropriate software. I use AVS video converter, as I can crop videos and also convert them to less memory-hungry types. Our elves in England would love to make you a cup of cocoa and discuss your requirements in more detail.

Thank you for buying a Malvern Zetasizer ZS and for writing to Father Characterization. Here we have another question, where the basic answer is “it depends”. What types of sample are you measuring? Are you measuring size or zeta potential? Typically for zeta potential measurements, we need conductivity in the dispersant phase (so some liquids such as DI water or hexane may pose problems; typically, a level of around 0.001M NaCl is a good starting point) and for size measurements we’ll need dilution in the mother liquor – the same starting environment. The liquid should be particle-free and filtered to at least the size you’ll want to be measuring.

Hello! This is not an easy question to answer and one of Father Characterization’s favorite elves or pixies may have to help here – Sonja comes to mind. This is really a sample preparation question though, and my first thought is to try the experiment and see what happens. 40 kDa is quite small and probably too small to see the scattering from, especially if the concentration is low – the optical contrast is very low for materials of this type. I doubt you’d see them with flow cytometry either, I’m afraid. Please follow up with us if you are still stuck!

Thank you for buying 2 Malvern instruments – like many of our customers! Consider that we’re measuring different aspects of our particle and that these particle sizes are like fruit. You can’t or wouldn’t want to convert an apple into a pear or an orange. So, you would expect the results from different technologies to give different answers and you need not to even try to convert between them. Neither DLS nor diffraction measures D[3,2] and so if you want a specific surface area, we’d recommend BET surface area, and then you can attempt a back-conversion to D[3,2}. There is assistance with some of the basics in our whitepaper and webinar. The span is calculated in a totally different way to the PDI, and so comparison here is not possible. D[4,3] and the z-average are different types of mean, and the D[0.5] is a median, so again there’s no comparison of like with like. Just report the values you get for the techniques you use – stability in these results is the most important factor. In general terms, the intensity mean must be larger than the volume and surface means but this isn’t always the case.