Geometrical Optics of Dense Aerosols: Forming Dense Plasma Slabs

Assembling a freestanding, sharp-edged slab of homogeneous material that is much denser than gas, but much more rarefied than a solid, is an outstanding technological challenge. The solution may lie in focusing a dense aerosol to assume this geometry. However, whereas the geometrical optics of dilute aerosols is a well-developed field, the dense aerosol limit is mostly unexplored. Yet controlling the geometrical optics of dense aerosols is necessary in preparing such a material slab. Focusing dense aerosols is shown here to be possible, but the finite particle density reduces the effective Stokes number of the flow, a critical result for controlled focusing.

Figure 1

Schematic of two-stage system for compressing and extracting aerosol targets (gray). Stage I forms a jet from the isotropic suspension at the inlet. Stage II extracts this jet by dispersing the carrier gas. ${\rho }_{a,i}$ refers to the mean aerosol density upstream of stage I, ${\rho }_a$ is the density between stages, and ${\rho }_t$ is the final target density downstream of stage II.

Figure 2

Representative particle tracks demonstrating aerodynamic focusing in four combinations of $S$ and upstream ${\rho }_p/\rho ={\rho }_{a,i}/{\rho }_g$. In each frame (a)–(d), the abscissa is the horizontal displacement in multiples of $L_o$, the ordinate is the vertical displacement. The axial extent of the orifice is exaggerated for clarity.

Figure 3

Possible operating points at $n_e\simeq {10}^{19}{\mathrm{cm}}^{-3}$ and ${\rho }_0=2\mathrm{g}/{\mathrm{cm}}^3$ in ($a$, $L_a$) space. Each point is parametric in Re and $M$; the labeled arrows indicate tendencies.