Download Computational aspects The role of the gel

Desorption of soft particles from
a fluid interface
Hadi
1,2
Mehrabian ,
Jacco H. Snoeijer
1,2,
Jens Harting
3,2,1
1. Faculty of Science and Technology, University of Twente, Enschede, The Netherlands
2. Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands
3. Research Centre Jülich GmbH, Helmholtz-Institute Erlangen-Nuremberg (IEK-11), Nuremberg, Germany
Introduction and abstract
Computational aspects
Soft particles are considered to be better stabilizers for emulsions as
compared to rigid particles due to their stronger attachment to a fluid
interface. Preforming constrained molecular dynamics simulations (using
GROMACS 5) and the thermodynamic integration method, the desorption
energy of soft gel-like particles from a fluid interface is calculated. The role of
the particle softness on the desorption energy is studied. Contrary to the
common belief about the desorption energy of soft particles, it is shown that
the higher softness of the particle does not affect the maximum force
required to detach the particle from the interface, and it does not change the
detachment work significantly. For swollen gel particles, it is shown that both
the detachment work and the maximum detachment force increase
considerably as compared to rigid particles.
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Each simulation requires about 750 nanoseconds to be finished and
the average timestep is 5 femtoseconds!
A simulation takes two weeks on 256 cores of the JURECA
supercomputer.
Using GPUs increases the speed of computations considerably.
Caretsisus with 3 gpus and 48 threads
(NVIDIA Tesla K40m and Intel(R) Xeon(R)
CPU E5-2450 v2 @ 2.50GHz)
JURECA with 256 threads ( Intel Xeon E52680 v3 Haswell)
Grouch GPU node with 24 threads and 2
gpus (Titan black gpus and Intel(R) Xeon(R)
CPU E5-2680 v3 @ 2.50GHz)
Desktop pc with 24 threads and 2 gpus (GTX
980 and Intel(R) Xeon(R) CPU X5670 @
2.93GHz)
A soft gel-like particle produced in the molecular dynamics
simulations using cross-linked polymeric chains. The
density of cross-linking controls the rigidity of the particle1.
Desktop pc with 24 threads (Intel(R) Xeon(R)
CPU X5670 @ 2.93GHz)
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speed of computations (nanoseconds/day)
Computational setup and method
A large polymeric particle is placed in a bath of Lennard-Jones particles.
The particle-liquid and liquid-liquid Lennard-Jones interactions determine
the surface tensions. The particle is moved upward in a quasi-static manner
and the force acting on the particle is calculated at each step.
The role of the particle softness
Calibration of the method
The simulations can correctly reproduce the detachment work of a rigid
particle. The continuum force-distance relationship for the desorption of a
solid particle from a fluid interface is compared between theory and
molecular dynamics simulations.
The role of the gel-like structure
The integral of the force-distance equation gives the desorption energy of the
particle. Comparing the desorption energy of a rigid and soft particle shows
that soft particles need a similar amount of energy to get removed from a fluid
interface.
Gels let one of the liquid phases wet their polymeric chains. This makes the
gel swell and the liquid drainage out of it a very slow process and hence
increases the desorption energy considerably.
Conclusion and outlook
Acknowledgment
By means of molecular dynamics simulations, we have shown that using
soft particles instead of rigid particles will not produce more stable particlestabilized (Pickering) emulsions but particles with a gel-like structure can
considerably enhance the stability of such emulsions. As a next step, we will
explore the role of anisotropic wettability on the desorption energy.
We acknowledge the Jülich Supercomputing Centre and SurfSARA for access
to JURECA and CARTESIUS.
References
Mehrabian, H., Snoeijer, J.H., Harting, J., Soft particles at a fluid interface, Soft
Matter, 2016, 12, 1062-1073.