Direct Measurements of Adhesive Forces between Millimetre-Sized Particles: Support of Tribocharging as a Key Mechanism for Overcoming the Bouncing Barrier

The growth of dust aggregates in protoplanetary discs encounters the ‘bouncing barrier’ at millimetre-size, beyond which further growth is no longer possible due to  surface forces alone. To overcome this barrier, larger forces would be required, the origin of which may be electrostatic attraction. This results readily from triboelectric charging caused by collisions in protoplanetary discs. To quantify the adhesive forces between millimetre-sized particles charged by friction, we have developed an experiment in which an ultramicrobalance serves as a force sensor. To validate this method, we measured the forces between two spherical glass beads with a diameter of about 1 mm that were repeatedly brought into contact. The measured adhesive forces extended over a range of more than three orders of magnitude. The lowest values correspond to the typical adhesive forces observed for uncharged particles with typical surface energies. This substantiates the ability of the balance-based setup to measure even down to the expected minimum adhesive force.

A comparison with results from our earlier experiment, which measured the adhesive forces from particle clusters breaking apart due to rotation within an acoustic trap (Schwaak et al. 2024, A&A), shows that previously measured forces lie within the intermediate range of the present measurements.
We interpret these forces as electrostatic interactions between triboelectrically charged particles. Furthermore, the data reveal even stronger forces up to a distinct upper limit. This limit could provide insights into triboelectric charging in general.
Its magnitude is likely to depend on the particle composition, the ambient pressure, and the molecular species deposited on the surface.

The results proove that the presented setup is well suited for further investigations of adhesive forces, providing a wider measurement range as the acoustic trap based experiment and enabling measurements on a wide range of particles including porous aggregates instead of solid spheres.
Overall, the magnitudes of these forces in this simplified setup provide further evidence that triboelectric charging serves as the key to the formation of large particle clusters in protoplanetary discs.