Interaction between Nearly Hard Colloidal Spheres at an Oil-Water Interface

Iain Muntz, Franceska Waggett, Michael Hunter, Andrew B. Schofield, Paul Bartlett, Davide Marenduzzo, Job H. J. Thijssen

Research output: Contribution to journalArticlepeer-review


We show that the interaction potential between sterically stabilised, nearly hard-sphere (PMMA-
PLMA) colloids at a water-oil interface has a negligible unscreened-dipole contribution, suggesting
that models previously developed for charged particles at liquid interfaces are not necessarily ap-
plicable to sterically stabilised particles. Interparticle potentials, U(r), are extracted from radial
distribution functions (g(r), measured by fluorescence microscopy) via Ornstein-Zernike (OZ) in-
version and via a reverse Monte Carlo scheme. The results are then validated by particle tracking
in a blinking optical trap. Using a Bayesian model comparison, we find that our PMMA-PLMA
data is better described by screened monopole only rather than a functional form having a screened
monopole plus an unscreened dipole term. We postulate that the long range repulsion we observe
arises mainly through interactions between neutral holes on a charged interface i.e. the charge of
the liquid interface cannot, in general, be ignored. In agreement with this interpretation, we find
that the interaction can be tuned by varying salt concentration in the aqueous phase. Inspired by
recent theoretical work on point charges at dielectric interfaces, which we explain is relevant here,
we show that a screened 1r 2 term can also be used to fit our data. Finally, we present measure-
ments for PMMA-PHSA particles at a water-oil interface. These suggest that, for PMMA-PHSA
particles, there is an additional contribution to the interaction potential. This is in line with our
optical-tweezer measurements for PMMA-PHSA colloids in bulk oil, which indicate that they are
slightly charged.
Original languageEnglish
Article number023388
JournalPhysical Review Research
Issue number2
Publication statusPublished - 23 Jun 2020


  • cond-mat.soft


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