Shear-induced ordering in a liquid complex plasma

Kurzfassung

A complex, or dusty plasma is a suspension of small solid particles in an ionized gas. Particles get charged by collecting electrons and ions from the ambient plasma and interact with each other via a screened Coulomb (Yukawa) pair potential. Complex plasmas are often strongly coupled and can exist in liquid and even crystallized forms. In our experiment [1], polymer microspheres with a diameter of 9.19 μm were suspended in the plasma sheath above the lower electrode in a radio-frequency discharge in argon. The particle suspension self-organized in a highly ordered single-layer triangular lattice. Shear stress was applied to this plasma crystal by pushing a stripe of particles by the radiation pressure force of a rastered laser beam. We applied increasing levels of shear stress to the crystal. At low stress, the crystal deformed elastically. At higher stress, the following stages were observed: defect generation while in a solid state, onset of plastic flow, and fully developed shear flow. We observed shear-induced reordering in a flowing liquid complex plasma. We made two related observations. First, a flowing liquid complex plasma is not completely isotropic. Rather, on top of an isotropic particle distribution appears a weak triangular order with Θ=0° (where Θ is the angle between the laser beam and the closely packed rows of particles) even in the cases when the original triangular lattice was oriented at 30°. Similar anisotropy in a liquid complex plasma was observed in [2], where the initial triangular order with Θ=0° was not completely destroyed in a shear melted state. Second, after the laser was switched off, the liquid complex plasma re-crystallized in a triangular lattice with Θ=0°, even though the lattice was oriented at Θ=30° before melting. A flowing liquid complex plasmas, therefore, favors particle ordering where "closely packed lines" are oriented along the flow. The observed particle ordering is similar to the formation of particle strings aligned in the flow direction in sheared 2D colloidal dispersions [3] and simulated liquid of soft disks [4]. Here, we observed it for the first time in a sheared complex plasma. [1] V. Nosenko, A. V. Ivlev, and G. E. Morfill, Phys. Rev. E 87, 043115 (2013). [2] V. Nosenko, A. V. Ivlev, and G. E. Morfill, Phys. Rev. Lett. 108, 135005 (2012). [3] E. J. Stancik, A. L. Hawkinson, J. Vermant, G. G. Fuller, J. Rheol. 48, 159 (2004). [4] S. Butler and P. Harrowell, Phys. Rev. E 54, 457 (1996).