Comparative study of the surface potential of magnetic and non-magnetic spherical objects in a magnetized radio-frequency discharge

We report measurements of the time-averaged surface floating potential of magnetic and non-magnetic spherical probes (or large dust particles) immersed in a magnetized capacitively coupled discharge. In this study, the size of the spherical probes is taken greater than the Debye length. The surface potential of a spherical probe first increases, i.e. becomes more negative at low magnetic field ( $B < 0.05\ \textrm {T}$ ), attains a maximum value and decreases with further increase of the magnetic field strength ( $B > 0.05\ \textrm {T}$ ). The rate of change of the surface potential in the presence of a $B$ -field mainly depends on the background plasma and types of material of the objects. The results show that the surface potential of the magnetic sphere is higher (more negative) compared with the non-magnetic spherical probe. Hence, the smaller magnetic sphere collects more negative charges on its surface than a bigger non-magnetic sphere in a magnetized plasma. The different sized spherical probes have nearly the same surface potential above a threshold magnetic field ( $B > 0.03\ \textrm {T}$ ), implying a smaller role of size dependence on the surface potential of spherical objects. The variation of the surface potential of the spherical probes is understood on the basis of a modification of the collection currents to their surface due to charge confinement and cross-field diffusion in the presence of an external magnetic field.

SIMULATOR FOR ROUGHNESS MEASUREMENT WITH SPHERICAL PROBE

A measurement of surface roughness is commonly carried out with the aid of profilometers, but when measuring the roughness of a curved surface arise difficulties. A technical problem consists in a probe feed along the normal to a nominal surface and filtration followed to exclude a low-frequency constituent of a profile. That is why it is purposeful to use coordinate measuring machines which have a probe with a spherical tip. For the first time there is presented a simulator of a spherical probe flat contact and surface roughness. A contact model developed is based on the regulations of analytical geometry. With the use of a numerical algorithm one finds a contact point and the center of a probe circle. On the basis of the simulator there is carried out an investigation of the impact of a probe radius upon roughness parameters Ra, Rmax, Rq. As initial data were used measurement results of the profilometer and profiles modeled with the aid of Monte-Carlo statistical method. It is defined that with the increase of the radius of the spherical probe its penetrating capacity decreases. That is why height parameters of roughness are distorted. The application of the spherical probe with the radius of 5-50 mkm ensures satisfactory results.

Poroelastic properties of hydrogel microparticles

The universal force relaxation of a poroelastic hydrogel particle undergoing constant compression by a spherical probe is determined, allowing analysis of experimental measurements of hydrogel particle material properties for the first time.