Stress-induced microcracking and cooperative motion of cold dusty plasma liquids

We investigate the microresponse of the quasi-two-dimensional dusty plasma liquid around freezing to the shear force from a laser beam through the center of the liquid cluster. It is found that the cold liquid can be viewed as a patchwork of crystalline ordered domains (CODs) which are solidlike but can be cracked and rearranged by weak thermal agitation and external stress, through COD rotations and drifting. Under weak external stress comparable to thermal agitation, the laser zone is not the preferred region mastering cracking initiation. CODs in the laser zone can either break locally, or sustain and propagate the stress to remote regions for cracking, in the form of intermittent bursts. The COD rotation and drifting induced by the persistent torques and momentum from the stress causes the formation of the center shear band with a higher longitudinal speed. Increasing stress can enhance cracking initiation around the shear zone and then spread to other remote regions. It deteriorates the local structural order and causes strong shear banding dominated by longitudinal cooperative hopping.

Figure 1

The plots of the dust trajectories and the bond-angle variations $\delta \theta$ in 5 s intervals for runs I and II [28]. The background triangulated grids show the initial dust configuration of the 5 s interval. The squares and the triangles correspond to the sevenfold and fivefold defects, respectively. The regions $\delta \theta >$ ($<0$) correspond to the patches or bonds in the narrow shear strip with counterclockwise (clockwise) rotations. For run II, static, drifting, and rotating CODs separated by strong shear strips, similar to run I, are the basic cooperative excitations. The arrows are the examples showing their rotations. The rotation of two adjacent CODs causes the formation of strongly sheared strips, with the deep color along the COD interface, in which cracking and reconnection occur. For run II, in the loading zone indicated by large arrows, the stick-slip cooperative motions have a zigzag shape along the lattice lines, indicated by small arrows.

Figure 2

(a) and (b) Typical temporal evolutions and power spectra of ${\langle \delta y\rangle }_c$, ${\langle \delta r\rangle }_c$, and $f_{bc}$, spatially averaged over the laser zone for the three runs, respectively. The stick-slip type cooperative motion causes intermittent type temporal fluctuations and the emergence of shoulders, followed by power law decays in the power spectra with a scaling exponent of about $-1.6$. The arrows in (a) indicate the times for the onset of large $f_{bc}$ bursts for Figs. 3 and 5. (c) The temporal cross correlation functions of ${\langle \delta y\rangle }_c$ and $f_{bc}$, and of ${\langle \delta r\rangle }_c$ and $f_{bc}$ for run II.

Figure 3

(a) The initiation and spreading of the cracking sites in 2.5 s intervals for run II, starting from different times at the onset of large $f_b$ bursts, as indicated by the arrows in the $f_{bc}$ vs $t$ plot (b). The background grids indicate initial particle positions. Cracks are initiated in the form of clusters and propagate to other cracking clusters. For run II, there is no dominant region mastering cracking initiation, similarly to run I (Fig. S2 of Ref. [26]).

Figure 4

The spatial dependence of $\overline{\langle \delta y\rangle }$, $\overline{\langle \delta r\rangle }$, $\overline{f_b}$, and $\overline{\langle \left|{\Psi }_6\right|\rangle }$ for runs I, II, and III.

Figure 5

The plots of 5 s dust trajectories and $\delta \theta$ in 5 s intervals (a) and (b), and the initiation and spreading of cracking sites in 2.5 s intervals (c), starting at the onsets of a burst of $f_{bc}$ [Fig. 2] for run III, and using the same plotting and color coding methods as Figs. 1 and 3. The strong stress induces a forward drifting band extended over a long distance, in which lattice orientations are mainly aligned along the laser direction. Cracking initiation more likely occurs around the loading zone where the averaged COD size is also smaller. The circled region indicates that sometimes the forward drifting can be frustrated and change direction locally, and associate the initiation of clusters of cracking sites.