I spent most of last week at the European Union’s Joint Research Centre in Ispra, Italy. I have been collaborating with the scientists in the European Union Reference Laboratory for alternatives to animal testing [EURL ECVAM]. We have been working together on tracking nanoparticles and, more recently, on the validity and credibility of models. Last week I was there to participate in a workshop on Validation and Acceptance of Artificial Intelligence Models in Health. I presented our work on the credibility matrix and on a set of factors that we have developed for establishing trust in a model and its predictions. I left the JRC on Friday evening and slipped back in the UK just before she left the Europe Union. The departure of the UK from Europe reminds me of a novel by José Saramago called ‘The Stone Raft‘ in which the Iberian penisula breaks off from the Europe mainland and drifts around the Atlantic ocean. The bureaucrats in Europe have to run around dealing with the ensuing disruption while five people in Spain and Portugal are drawn together by surreal events on the stone raft adrift in the ocean.
Most of us have a sub-conscious understanding of the forces that control the interaction of objects in the size scale in which we exist, i.e. from millimetres through to metres. In this size scale gravitational and inertial forces dominate the interactions of bodies. However, at the size scale that we cannot see, even when we use an optical microscope, the forces that the dominate the behaviour of objects interacting with one another are different. There was a hint of this change in behaviour observed in our studies of the diffusion of nanoparticles [see ‘Slow moving nanoparticles‘ on December 13th, 2017], when we found that the movement of nanoparticles less than 100 nanometres in diameter was independent of their size. Last month we published another article in one of the Nature journals, Scientific Reports, on ‘The influence of inter-particle forces on diffusion at the nanoscale‘, in which we have demonstrated by experiment that Van der Waals forces and electrostatic forces are the dominant forces at the nanoscale. These forces control the diffusion of nanoparticles as well as surface adhesion, friction and colloid stability. This finding is significant because the ionic strength of the medium in which the particles are moving will influence the strength of these forces and hence the behaviour of the nanopartices. Since biological fluids contain ions, this will be important in understanding and predicting the behaviour of nanoparticles in biological applications where they might be used for drug delivery, or have a toxicological impact, depending on their composition.
Van der Waals forces are weak attractive forces between uncharged molecules that are distance dependent. They are named after a Dutch physicist, Johannes Diderik van der Waals (1837-1923). Electrostatic forces occur between charged particles or molecules and are usually repulsive with the result that van der Waals and electrostatic forces can balance each other, or not depending on the circumstances.
Giorgi F, Coglitore D, Curran JM, Gilliland D, Macko P, Whelan M, Worth A & Patterson EA, The influence of inter-particle forces on diffusion at the nanoscale, Scientific Reports, 9:12689, 2019.
Coglitore D, Edwardson SP, Macko P, Patterson EA, Whelan MP, Transition from fractional to classical Stokes-Einstein behaviour in simple fluids, Royal Society Open Science, 4:170507, 2017. doi: .
Patterson EA & Whelan MP, Tracking nanoparticles in an optical microscope using caustics. Nanotechnology, 19 (10): 105502, 2009.
Image: from Giorgi et al 2019, figure 1 showing a photograph of a caustic (top) generated by a 50 nm gold nanoparticle in water taken with the optical microscope adjusted for Kohler illumination and closing the condenser field aperture to its minimum following method of Patterson and Whelan with its 2d random walk over a period of 3 seconds superimposed and a plot of the same walk (bottom).
Together with colleagues at the JRC Ispra, my research group has shown that the motion of small nanoparticles at low concentrations is independent of their size, density and material , [see ‘Slow moving nanoparticles‘ on December 13th, 2017]. This means that commercially-available instruments for evaluating the size and number of nanoparticles in a solution will give erroneous results under certain conditions. In a proposed PhD project, we are planning to extend our work to develop an instrument with capability to automatically identify and size nanoparticles, in the range from 1 to 150 nanometres, using the three-dimensional optical signature, or caustic, which particles generate in an optical microscope, that can be several orders of magnitude larger than the particle , [see ‘Toxic nanoparticles?‘ on November 13th, 2013]. The motivation for the work is the need to characterise particles present in solution in legacy ponds at Sellafield. Legacy ponds at the Sellafield site have been used to store historic radioactive waste for decades and progress is being made in reducing the risks associated with these facilities . Over time, there has been a deterioration in the condition of the ponds and their contents that has resulted in particles being present in solution in the ponds. It is important to characterise these particles in order to facilitate reductions in the risks associated with the ponds. We plan to use our existing facilities at the University of Liverpool to develop a novel instrument using simple solutions probably with gold nanoparticles and then to progress to non-radioactive simulants of the pond solutions. The long-term goal will be to transition the technology to the Sellafield site perhaps with an intermediate step involving a demonstration of the technology on pond solutions using the facilities of the National Nuclear Laboratory.
The PhD project is fully-funded for UK and EU citizens as part of a Centre for Doctoral Training and will involve a year of specialist training followed by three years of research. For more information following this link.
 Coglitore, D., Edwardson, S.P., Macko, P., Patterson, E.A., & Whelan, M.P., Transition from fractional to classical Stokes-Einstein behaviour in simple fluids, Royal Society Open Science, 4:170507, 2017.
 Patterson, E.A., Whelan, P., 2008, ‘Optical signatures of small nanoparticles in a conventional microscope’ Small, 4(10):1703-1706.
 Comptroller and Auditor General, The Nuclear Decommissioning Authority: progress with reducing risk at Sellafield, National Audit Office, HC 1126, Session 2017-19, 20 June 2018.
If you enjoyed a holiday dinner lit by candles then you might be interested to know that the majority of the light from the candle does not come from the combustion of the candle wax in the flame, but from the unburnt soot glowing in the intense heat of the flame. The combustion process generates the heat and the blue colour in the centre of the flame. However, due to the lack of sufficient oxygen, the combustion of the candle wax is incomplete and this produces particles of unburnt carbon. The unburnt carbon forms soot or graphite, but also more exotic structures of carbon atoms, such as nano-diamonds. The average candle has been estimated to produce about 1.5 million nano-diamonds per seconds, or maybe 10 billion nano-diamonds per Christmas dinner! Unfortunately, they are too small to see otherwise they would add a lot of sparkles to festive occasions.
The picture is an infrared image of a 1cm diameter candle. About 2cm of the candle height extends from the bottom of the picture and the visible flame is about 2cm high.
Helen Czerski, Storm in a Teacup: The Physics of Everyday Life, London: Penguin Random House, 2016.