Technological/Societal Impact (1)SEM images of Silicon films deposited in pulsed laser ablation in vacuum Courtesy of A. Perrone Fluence of 3.0 J/cm^2 Fluence of 4.5 J/cm^2 (2) Retrieved MIR solar cell arrays exposed to the harsh LEO environment

Multiple Length Scales in Supersonic Jet Expansion to Vacuum Unique capability to understand and utilize multiple flow techniques Main challenge is to establish connection between methodologies needed to describe diverse processes of clustering Kn = mean free path/ diameter of nozzle

The DSMC Method 1.New particles from jets or the flow field are introduced according to a distribution function. 2.Particles are advanced within the simulation domain over the time step, ignoring potential collisions. 3.Particle-surface collisions are calculated. 4.Particles are indexed within cells. 5.Particle collisions are calculated using Monte Carlo techniques. 6.Post-collision velocities are determined, observing conservation of energy and momentum. Procedure DSMC follows the motions of many virtual particles on a grid over a series of time steps, tracking their collisions. It may also track reactions and changes in internal energies and velocities of the components.

MD DSMC Method kinetic kinetic Application Solid, Liquid, Gas Gas (Kn > 0.1) Simulated Particle geometry point-size sphere a F N ~ Interactions Potential Collision Time Step Sec ~ Sec System Capabilities Computational Domain ~nm > mm # of Real Particles ~10 6 > Time Scale ~100ps > Sec a F N: the number of real atoms represented by a simulated particle. Comparison of Methodologies

=  The nucleation process (CNT):  The cluster-monomer collision:  The cluster-cluster collision: The Ashrigz-Poo model is used to determine the outcome of cluster-cluster collisions based on We number.  The cluster evaporation process: Microscopic Models used in Classical Nucleation Theory-Coupled to DSMC = = Sticking Non-sticking

DSMC/CNT Simulation of Ar Jet P_o=6400 Pa, T_o = 170 K, D = 1.4 mm, external vacuum Starting surface generated by NS/CFD CFD solution overlapped with DSMC

The DSMC model is used to verify Hagena’s scaling laws by simulating Argon condensation in free-expanding plumes from sonic nozzles. The mean terminal cluster size in expanding supersonic jets into a vacuum remains constant if : a) stagnation pressure,, and sonic nozzle throat diameter,, satisfy: b) or stagnation pressure,, and stagnation temperature,, fall into the range between and where q is a constant Hagena’s scaling laws

T 0 = 167 K 1) The terminal cluster size are almost the same for the five cases with an error less than 5%. The normalized curves along the centerline are agree with each other well. 2) Hagena’s experiments suggest the constant q = 0.8, DSMC model suggests that q is about 0.85 (estimated by linear regression method). DSMC/CNT Validation of Hagena’s scaling laws (1):