45 research outputs found
Stationary shapes of deformable particles moving at low Reynolds numbers
Lecture Notes of the Summer School ``Microswimmers -- From Single Particle
Motion to Collective Behaviour'', organised by the DFG Priority Programme SPP
1726 (Forschungszentrum J{\"{u}}lich, 2015).Comment: Pages C7.1-16 of G. Gompper et al. (ed.), Microswimmers - From Single
Particle Motion to Collective Behaviour, Lecture Notes of the DFG SPP 1726
Summer School 2015, Forschungszentrum J\"ulich GmbH, Schriften des
Forschungszentrums J\"ulich, Reihe Key Technologies, Vol 110, ISBN
978-3-95806-083-
Semipermeable elastic microcapsules for gas capture and sensing
Monodispersed microcapsules for gas capture and sensing were developed consisting of elastic semipermeable polymer shells of tuneable size and thickness and pH-sensitive, gas selective liquid cores. The microcapsules were produced using glass capillary microfluidics and continuous on-the-fly photopolymerisation. The inner fluid was 5-30 wt% K2CO3 solution with m-cresol purple, the middle fluid was a UV-curable liquid silicon rubber containing 0-2 wt% Dow Corning® 749 fluid, and the outer fluid was aqueous solution containing 60-70 wt% glycerol and 0.5-2 wt% stabiliser (polyvinyl alcohol, Tween 20 or Pluronic® F-127). An analytical model was developed and validated for prediction of the morphology of the capsules under osmotic stress based on the shell properties and the osmolarity of the storage and core solutions. The minimum energy density and UV light irradiance needed to achieve complete shell polymerisation were 2 J∙cm-2 and 13.8 mW·cm-2, respectively. After UV exposure, the curing time for capsules containing 0.5 wt% Dow Corning® 749 fluid in the middle phase was
30-40 min. The CO2 capture capacity of 30 wt% K2CO3 capsules was 1.6-2 mmol/g depending on the capsule size and shell thickness. A cavitation bubble was observed in the core when the internal water was abruptly removed by capillary suction, whereas a gradual evaporation of internal water led to buckling of the shell. The shell was characterised using TGA, DSC, and FTIR. The shell degradation temperature was 450-460°C
Adhesion of microcapsules
The adhesion of microcapsules to an attractive contact potential is studied
theoretically. The axisymmetric shape equations are solved numerically. Beyond
a universal threshold strength of the potential, the contact radius increases
like a square root of the strength. Scaling functions for the corresponding
amplitudes are derived as a function of the elastic parameters.Comment: 4 pages, 4 figure
Swinging and tumbling of elastic capsules in shear flow
The deformation of an elastic micro-capsule in an infinite shear flow is
studied numerically using a spectral method. The shape of the capsule and the
hydrodynamic flow field are expanded into smooth basis functions. Analytic
expressions for the derivative of the basis functions permit the evaluation of
elastic and hydrodynamic stresses and bending forces at specified grid points
in the membrane. Compared to methods employing a triangulation scheme, this
method has the advantage that the resulting capsule shapes are automatically
smooth, and few modes are needed to describe the deformation accurately.
Computations are performed for capsules both with spherical and ellipsoidal
unstressed reference shape. Results for small deformations of initially
spherical capsules coincide with analytic predictions. For initially
ellipsoidal capsules, recent approximative theories predict stable oscillations
of the tank-treading inclination angle, and a transition to tumbling at low
shear rate. Both phenomena have also been observed experimentally. Using our
numerical approach we could reproduce both the oscillations and the transition
to tumbling. The full phase diagram for varying shear rate and viscosity ratio
is explored. While the numerically obtained phase diagram qualitatively agrees
with the theory, intermittent behaviour could not be observed within our
simulation time. Our results suggest that initial tumbling motion is only
transient in this region of the phase diagram.Comment: 20 pages, 7 figure
Swinging and tumbling of elastic capsules in shear flow
The deformation of an elastic micro-capsule in an infinite shear flow is
studied numerically using a spectral method. The shape of the capsule and the
hydrodynamic flow field are expanded into smooth basis functions. Analytic
expressions for the derivative of the basis functions permit the evaluation of
elastic and hydrodynamic stresses and bending forces at specified grid points
in the membrane. Compared to methods employing a triangulation scheme, this
method has the advantage that the resulting capsule shapes are automatically
smooth, and few modes are needed to describe the deformation accurately.
Computations are performed for capsules both with spherical and ellipsoidal
unstressed reference shape. Results for small deformations of initially
spherical capsules coincide with analytic predictions. For initially
ellipsoidal capsules, recent approximative theories predict stable oscillations
of the tank-treading inclination angle, and a transition to tumbling at low
shear rate. Both phenomena have also been observed experimentally. Using our
numerical approach we could reproduce both the oscillations and the transition
to tumbling. The full phase diagram for varying shear rate and viscosity ratio
is explored. While the numerically obtained phase diagram qualitatively agrees
with the theory, intermittent behaviour could not be observed within our
simulation time. Our results suggest that initial tumbling motion is only
transient in this region of the phase diagram.Comment: 20 pages, 7 figure
Two-Dimensional Fluctuating Vesicles in Linear Shear Flow
The stochastic motion of a two-dimensional vesicle in linear shear flow is
studied at finite temperature. In the limit of small deformations from a
circle, Langevin-type equations of motion are derived, which are highly
nonlinear due to the constraint of constant perimeter length. These equations
are solved in the low temperature limit and using a mean field approach, in
which the length constraint is satisfied only on average. The constraint
imposes non-trivial correlations between the lowest deformation modes at low
temperature. We also simulate a vesicle in a hydrodynamic solvent by using the
multi-particle collision dynamics technique, both in the quasi-circular regime
and for larger deformations, and compare the stationary deformation correlation
functions and the time autocorrelation functions with theoretical predictions.
Good agreement between theory and simulations is obtained.Comment: 13 pages, 7 figure