Drifting inwards in protoplanetary discs II: The effect of water on sticking properties at increasing temperatures

TL;DR

This study investigates how water presence affects the sticking properties of dust grains in protoplanetary discs at high temperatures, revealing that dry conditions significantly enhance grain adhesion and may facilitate planetesimal formation.

Contribution

It introduces new preparation methods to measure sticking forces under dry and super-dry conditions, showing temperature-dependent increases in adhesion relevant to planet formation.

Findings

Dry samples show ~10x higher sticking force than wet samples up to 1000 K.

Super-dry samples exhibit exponential increase in sticking force above 900 K, reaching ~100x at 1200 K.

Enhanced sticking at high temperatures suggests a potential zone for efficient planetesimal formation.

Abstract

In previous laboratory experiments, we measured the temperature dependence of sticking forces between micrometer grains of chondritic composition. The data showed a decrease in surface energy by a factor ~5 with increasing temperature. Here, we focus on the effect of surface water on grains. Under ambient conditions in the laboratory, multiple water layers are present. At the low pressure of protoplanetary discs and for moderate temperatures, grains likely only hold a monolayer. As dust drifts inwards, even this monolayer eventually evaporates completely in higher temperature regions. To account for this, we measured the tensile strength for the same chondritic material as was prepared and measured under normal laboratory conditions in our previous work, but now introducing two new preparation methods: drying dust cylinders in air (dry samples), and heating dust pressed into cylinders in vacuum (super-dry samples). For all temperatures up to 1000 K, the data of the dry samples are consistent with a simple increase in the sticking force by a factor of ~10 over wet samples. Up to 900 K super-dry samples behave like dry samples. However, the sticking forces then exponentially increase up to another factor ~100 at about 1200 K. The increase in sticking from wet to dry extends a trend that is known for amorphous silicates to multimineral mixtures. The findings for super-dry dust imply that aggregate growth is boosted in a small spatial high-temperature region around 1200 K, which might be a sweet spot for planetesimal formation.