We model a range of materials including sustainable plastics, polymer thin films and composites, molecular crystals, solid-liquid interfaces and oxide materials. In these systems the interfacial properties are crucial in understanding and controlling the system, for example, heterogeneous crystal nucleation of polymer or small molecule systems.

We enjoy collaborating with experimentalists and industrial partners and we apply multiscale simulations to gain knowlegde on how to improve materials and their properties.

We have a broad interest in materials science and engineering, and have pursued some curiosity-driven science including our work on curdling of soy milk, which we also delivered as an outreach activity. If you are interested in talking to us and potentially collaborating please get in touch.

Sustainable Plastics

Plastic pollution is a major societal problem, as many plastics are derived from petrochemicals, take hundreds of years to degrade and lead to microplastics. A variety of polymers are sustainably-sourced and compostable including polyhydroxyalkanoates (PHAs), and alginates. We are investigating how the addition of filler particles and plasticisers may help to improve the properties and enable replacement of petroplastics with sustainable alternatives in applications such as food packaging.

Majerczak et al. Polymer Int. 71 1398 (2022)

Polymer crystallisation and barrier properties

Many polymers are semi-crystalline and their properties, such as gas barrier, depend on their crystallinity and microstructure. Filler particles are often added to enhance polymer crystallisation but, for example in polyhydroxybutyrate, while fillers clearly act as nucleants there are no clear trends with filler type. We are using a generic polymer model to simulate polymer crystallisation at a filler surface.

Wadkin-Snaith et al. Polymer, 281 126113 (2023)

Glass transition of polymer thin films

The shift in polymer glass transition temperature, Tg, between bulk and thin films has been the topic of much scientific debate. The glass transition can be calculated using MD simulations, however, this provides a number of challenges due to statistical variation. We have explored simulation methodologies for obtaining Tg, and and found that Tg of a thin film of polyethylene between graphene layers is higher than bulk polyethylene.

McKechnie et al. Polymer 195 122433 (2020)

Catching nucleation in action


We are using SERS to investigate the early stages of nucleation of glycine on gold nanoparticles. DFT simulations were used to predict the vibrational spectra of neutral and zwitterion glycine on gold, which will help interpret SERS spectra.

Mohammadpour and Johnston, Comput. Theor. Chem. 1227 114224 (2023)

Effect of interfaces on glycine crystallisation


It is believed that nucleation mainly occurs via heterogeneous mechanisms, with the nucleus forming on a surface or interface, rather than in bulk solution. Oil-water interfaces were unexpectedly found to increase glycine nucleation. Simulation showed the formation of a higher concentration layer of glycine (blue) at the oil layer (green), which likely to play a role in decreasing the nucleation barrier.

McKechnie et al. J. Phys. Chem. C 126 38 (2022)
McKechnie et al. J. Phys. Chem. Lett. 11 2263 (2020)

Glycine polymorph crystallisation


Glycine exists in several polymorphic forms: metastable α and β forms and the stable γ form. We investigated the effect of agitation on polymorphic outcomes in glycine crystallisation from aqueous solutions across a wide range of supersaturations under quiescent and stirring conditions. In the absence of stirring, γ was predominant at higher glycine concentrations but under stirred conditions α was predominant at all concentrations and temperatures investigated.

Vesga et al. Cryst. Eng. Comm. 21 2234 (2019)

Force field optimisation for nucleation studies

To simulate crystal nucleation and growth from solution using classical molecular dynamics, it is necessary to have a force field that describes both the solid and solution phases. Usually force fields are validated for just one phase. We have been optimising force fields to describe a variety of crystallising systems, including urea, and polyhydroxyalkanoates.

Anker et al. in preparation (2023)



Adsorption and interface force field optimisation

Hybrid systems often involve hard/soft matter interfaces. To model these systems using classical MD it is crucial to have a force field that accurately describes the interaction at the interface. We use DFT calculations of adsorbed molecules on a surface as a benchmark to assess force field accuracy and optimise force field interface parameters.

Berg et al. JCTC 13 5610 (2017)
Johnston et al. JPCC 116 19781 (2012)
Johnston et al. Soft Matter 8 6320 (2012)
Johnston et al. JPCC 115 14707 (2011)
Herbers et al. PCCP 13 10577 (2011)

Self-assembled monolayers

Halogenated aromatic molecules on platinum exhibit a bistable adsorption energy profile. The relative stability of these states can be tuned by varying the number or type of halogen atoms. A lattice model was used to investigate the self-assembly of halogenated aromatic monolayers on platinum. In the animation we see molecules move from a physisorbed state (blue/green) to a chemisorbed state (red/black). The molecules may move when physisorbed but stick to the surface when chemisorbed.

Fortuna et al. JPCC122 14459 (2018)
Fortuna et al. JCP 144 134707 (2016)
Johnston et al. Surf. Sci. 644 113-121 (2016)
Peköz et al. JPCC 118 6235-6241 (2014)

"Curdling" of Soymilk in Coffee

Soy milk is known to 'curdle' when added to coffee. We found that adding more soy milk or cooling the mixture can 'reverse' the curdling to give a homogeneous solution. The underlying curdling process (e.g. protein denaturing or aggregation) is still not understood and will be the topic of future work. The image shows the accelerated curdling process (real time of about 5 minutes).

Brown et al. Food Hydrocolloids 98 462 (2019)