Ellipsoidal water-based dust grains in a dusty plasma

Dusty plasmas are present in space and in technology. Particles that suspend within the plasma environment, called grains, can be made of various materials and are charged differently depending on their size and shape. Water-based dust grains, for example, have been found in Saturn’s E ring and around the supermassive black hole centering our galaxy. Understanding water-based dust grains is thus significant for exploring the solar system.

Two key assumptions about grain structure and behavior are commonly made: that the grains are spherical and conductive. However, the existence of ellipsoidal ice grains has recently been observed in laboratory experiments. To explain these experimental observations, Kritikos et al. examined the competition between electrostatic stresses and surface tension stresses on water ice grains in a weakly ionized plasma.

“When internal electrostatic forces generated by electron-electron repulsions overcome surface tension, the grain elongates,” said author Kritikos. “The conditions for elongation are typically satisfied when grains are at nanoscale sizes, and the exact conditions for elongation are determined by the plasma parameters and material properties.”

Through quantum mechanics computations, the authors also demonstrated ways in which the behavior of water ice grains challenges the assumptions of the orbital motion limited theory, the standard theory of how plasma species charge dust grains, revealing that water molecule clusters behave as insulators rather than conductors at nano timescales.

Next, the authors plan to study what happens before and after the elongation of nanoscale grains.

“This includes the nucleation process and how nanoparticles grow after they deform,” said Kritikos. “Specifically, it is unclear how the elongated shape of grains affects their interactions with their surroundings. Their collision with electrons, ions, and neutral molecules must be reexamined.”

Source: “Can electrostatic stresses affect charged water structures in weakly ionized plasmas?,” by Efstratios M. Kritikos, William A. Goddard III, and Paul M. Bellan, Physics of Plasmas (2025). The article can be accessed at https://doi.org/10.1063/5.0270908 .