Coagulation of dust

Gases and grains in interstellar clouds probably experienced many shock events during the formation of planetesimals and meteorites. These events are as follows 1) coagulation of dust into clumps, which settle to the equatorial plane of the nebula 2) breakup of the gravitationally unstable dust disk into clusters of dust clumps 3) coalescence of the clusters into 1 km planetesimals ... 

Most planetary systems are pervaded by dust due to the planet formation process, where through coagulation of dust and gas accretion in the disks that develop during the collapse and infall of massive protostar envelopes planets are formed. By studying the structure and dynamics of this dust, which is very bright at the Far Infrared wavelengths, one can gain information on how such systems were formed. Once the planets are formed, as their motion influence the distribution of the dust, planetary orbits can be traced. 

The increase in weight (density) of the dust particles facilitates their capture and, therefore, improves the efficiency of separation. Thus, the coagulation of dust in the highly desirable. 

Wet scrubbing uses liquid droplets to remove fine dust in a gas stream. In all types of wet scrubbing, the basic cleaning mechanism involves the attachment of particles to the droplets. The function of the droplets in scrubbers is similar to that of spherical fibers in filtration. Likewise, the primary collection mechanisms in scrubbing are similar to those in filtration, i.e., inertial impaction, interception, and diffusion [e.g., Fan, 1989]. Secondary collection mechanisms include thermophoresis due to temperature gradients, coagulation of particles due to particle electrification, and particle growth due to liquid condensation. 

The focus of the chapter is on the smallest range of particle sizes (< 1 cm) and thus the initial phases of dust coagulation in young gas-rich disks, since these are the ones that can be observed in extrasolar systems and have preserved some very early record in chondritic meteorites and interplanetary dust particles (IDPs). 

Dependencies of grain composition, e.g. icy grains versus silicates or carbon, as well as their charge state, are also of interest, and for destructive collisions, the resulting fragment size distribution is important (Wurm et al. 2005). These are dependent on the microphysics of the adhesive forces between monomers (Chokshi et al. 1993 Blum Schrapler 2004). A review of laboratory simulations of dust coagulation can be found in Blum Wurm (2008). 

Silicate dust in both the diffuse ISM as well as in dense protostellar envelopes has been observed to exhibit a nearly constant band profile dominated by small particles (Bouwman et al. 2001 Kemper et al. 2005). In sharp contrast, silicate bands observed in emission from protoplanetary disks exhibit a wide range of silicate band profiles, indicating a dominant presence of dust particles larger than the Rayleigh limit (Bouwman et al. 2001 van Boekel et al. 2005 Kessler-Silacci et al. 2006). The interpretation of this observation is that the characteristic size of the dust particles has grown either by a move to a shallower dust size distribution, or by the removal of smaller particles (cf. Fig. 7.1). Either way, the inference usually made is that small particles have been removed by a coagulation process not occuring in the ISM. 

Below we review the characteristics of fine-grained Solar System dust as preserved in primitive chondrite matrices and draw inferences from these observations about the mechanisms of coagulation of nebular dust as well as the timing and location of coagulation. 

Remote observations provide information about the processes of dust formation and coagulation in extrasolar protoplanetary disks. In contrast, the suite of fine-grained Solar System materials available for study in the laboratory consists of materials that formed 4.56 Gyr ago during the earliest stages of the formation of the Solar System. 

In order to consider the processes of dust coagulation in the early Solar System, we first review the characteristics of this material. Of considerable importance is the fact that these samples - represented principally by chondritic meteorites, but also by IDPs and by samples from Comet Wild 2 collected by the Stardust mission - all come from parent bodies of different kinds. As a result, even the most primitive of these materials has been processed, both physically and chemically, to different degrees. The processes that affected Solar System dust may have occurred in different environments such as the solar nebula (e.g. evaporation/condensation, annealing) and asteroidal parent bodies (aqueous alteration and/or thermal processing, mild compaction to extensive lithihcation). A major challenge is to understand the effects of this secondary processing. 

A key question in addressing the issue of dust coagulation concerns the grain size of dust. This is closely linked to the formation mechanisms of the dust as discussed above. It is important to note that larger grains reaching sizes of up to 10 pm (e.g. Fig. 7.6a,b) do occur in the matrices of primitive chondrites. However, these grains typically represent <1 vol% of primitive matrix and hence are a very minor component. 

The presence of rims of fine-grained material (e.g. Figure 7.4a) on chondrules in many chondrites provides useful constraints on when coagulation of fine-grained dust into larger aggregates occurred. Several models for the origin of these rims have been presented in the literature. Most models support formation of the rims by accretionary processes in the solar nebula (e.g. Metzler et al. 1992), but there are... 

---