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Original Articles

Crossover from Ballistic to Epstein Diffusion in the Free-Molecular Regime

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Pages 738-746 | Received 30 Jan 2014, Accepted 22 Apr 2014, Published online: 25 Jun 2014

Figures & data

FIG. 1. (a) Illustration of a ballistic collision where the light shaded cluster does not make one full persistence length before colliding with its neighbor. (b) Illustration of a diffusive collision where the light shaded cluster makes several persistence lengths before colliding.

FIG. 1. (a) Illustration of a ballistic collision where the light shaded cluster does not make one full persistence length before colliding with its neighbor. (b) Illustration of a diffusive collision where the light shaded cluster makes several persistence lengths before colliding.

FIG. 2. Inverse cluster count versus t + t0. t0 was found from EquationEquation (28). The lower dashed guide line has a kinetic exponent z = 0.80 − 0.88 and the upper guide line has z = 2.2. The points where Knn = 1 are marked and noted in the legend. The points where Knd = 1 are noted as stars in their respective runs. (a) Monte Carlo simulation at fv = 10−4; (b) Brownian Dynamics simulation at fv = 10−4; (c) Monte Carlo simulation at fv = 10−3; (d) Brownian Dynamics simulation at fv = 10−3; (e) Monte Carlo simulation at fv = 10−2; (f) Brownian Dynamics simulation at fv = 10−2. Notice that in (e) and (f) the Epstein diffusion exponent is z = 0.88 as expected when in the cluster dense regime. Also of note in (e) and (f) the trend has upward curvature do to the onset of gelation.

FIG. 2. Inverse cluster count versus t + t0. t0 was found from EquationEquation (28)[28] . The lower dashed guide line has a kinetic exponent z = 0.80 − 0.88 and the upper guide line has z = 2.2. The points where Knn = 1 are marked and noted in the legend. The points where Knd = 1 are noted as stars in their respective runs. (a) Monte Carlo simulation at fv = 10−4; (b) Brownian Dynamics simulation at fv = 10−4; (c) Monte Carlo simulation at fv = 10−3; (d) Brownian Dynamics simulation at fv = 10−3; (e) Monte Carlo simulation at fv = 10−2; (f) Brownian Dynamics simulation at fv = 10−2. Notice that in (e) and (f) the Epstein diffusion exponent is z = 0.88 as expected when in the cluster dense regime. Also of note in (e) and (f) the trend has upward curvature do to the onset of gelation.

FIG. 3. (a) Aggregation kernel homogeneity λ versus nearest neighbor Knudsen number Knn for all Monte Carlo and Brownian Dynamic simulations. (b) The diffusive Knudsen number KnD versus homogeneity λ for all Monte Carlo and Brownian Dynamic simulations. Both measurements show similar behavior and provide a means of detecting the crossover.

FIG. 3. (a) Aggregation kernel homogeneity λ versus nearest neighbor Knudsen number Knn for all Monte Carlo and Brownian Dynamic simulations. (b) The diffusive Knudsen number KnD versus homogeneity λ for all Monte Carlo and Brownian Dynamic simulations. Both measurements show similar behavior and provide a means of detecting the crossover.

FIG. 4. Aggregate size distribution from Brownian Dynamics with l0 = 50 and fv = 10−3. Lines represent fits from EquationEquation (33). The system starts off with ballistic λ then enters an intermediate regime during the crossover and finally at late time and small Knn reaches a diffusive λ. (a) Size distribution from Monte Carlo simulations; (b) size distribution from Brownian Dynamics simulations; (c) rescaled data from (a), here the homogeneity λ transition from ballistic to diffusive can be clearly seen. The system starts with ballistic homogeneity of λ = 0.55 ± 0.1 then changes to λ = −0.3 ± 0.2, consistent to other values of λ reported for Epstein diffusion. (d) Rescaled data from (b), the system starts with ballistic homogeneity of λ = 0.55 ± 0.1 then changes to λ = −0.40 ± 0.2, consistent with Epstein diffusion.

FIG. 4. Aggregate size distribution from Brownian Dynamics with l0 = 50 and fv = 10−3. Lines represent fits from EquationEquation (33)[33] . The system starts off with ballistic λ then enters an intermediate regime during the crossover and finally at late time and small Knn reaches a diffusive λ. (a) Size distribution from Monte Carlo simulations; (b) size distribution from Brownian Dynamics simulations; (c) rescaled data from (a), here the homogeneity λ transition from ballistic to diffusive can be clearly seen. The system starts with ballistic homogeneity of λ = 0.55 ± 0.1 then changes to λ = −0.3 ± 0.2, consistent to other values of λ reported for Epstein diffusion. (d) Rescaled data from (b), the system starts with ballistic homogeneity of λ = 0.55 ± 0.1 then changes to λ = −0.40 ± 0.2, consistent with Epstein diffusion.

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