Local Melting Attracts Grain Boundaries in Colloidal Polycrystals

We find that laser-induced local melting attracts and deforms grain boundaries in 2D colloidal crystals. When a melted region in contact with the edge of a crystal grain recrystallizes, it deforms the grain boundary—this attraction is driven by the multiplicity of deformed grain boundary configurations. Furthermore, the attraction provides a method to fabricate artificial colloidal crystal grains of arbitrary shape, enabling new experimental studies of grain boundary dynamics and ultimately hinting at a novel approach for fabricating materials with designer microstructures.

Figure 1

Experimentally fabricated grains. Local melting allows the creation of grains with arbitrary shapes: (a) broken heart, (b) cat, (c) $\mathsf{C}$, (d) smiley face. Particles are colored by the phase of the local orientational order parameter ${\mathrm{\Psi }}_6$, indicating grain orientation.

Figure 2

Optical blasting causes local melting in a 2D colloidal crystal (scale bar: $5\mu \mathrm{m}$). A crystal is shown (a) before blasting, (b) during the blast, (c) melted after the blast, and (d) recrystallized. The melted region is outlined in white. The same region is shown in (e)–(h), with particles colored by modified dynamic Lindemann parameter ${\gamma }_L$, which quantifies local fluctuations in particle separations [color bar in (h) applies to (e)–(h)]. (i) Time series of the average values of the magnitude of the orientational order parameter $|{\mathrm{\Psi }}_6|$ (blue dots) and ${\gamma }_L$ (red triangles), measured within the white outline over the entire experiment (error bars: SEM). The shaded region denotes when the laser is on, and the asterisks indicate the frames shown in (a)–(d).

Figure 3

Optical blasting attracts grain boundaries. (a) An initially flat grain boundary (cyan). (b) An optical blast disrupts the grain boundary (red) from its original position (cyan). (c) When a melted region (white outline) forms after the blast, the grain boundary has been deformed by a distance $y_{\text{imp}}$ relative to the original grain boundary position. The distance $y_{\text{imp}}$ is the average of the displacements on the left and right edges of the melted region. (d) Recrystallization pulls the grain boundary toward the blast location even further, for a total displacement $y$ above the original grain boundary. (e) Finally, the grain boundary relaxes down to a flat configuration.

Figure 4

Lattice model explains attraction to melted region. (a) Schematic diagram of lattice model. A portion of the lower grain (circles) extends into the melted region (dashed square) by a distance $y_{\text{melt}}$. A single grain boundary segment (red) corresponds to a particular sequence of lattice vectors (blue arrows) between adjacent particles at the top of the grain. Each step in the sequence follows one of the three lattice vectors ${\mathbf{v}}_1$, ${\mathbf{v}}_2$, ${\mathbf{v}}_3$. The angle $\theta$ indicates the orientation of the lattice relative to the bottom edge of the melted region, which has side length $L$. (b) Distribution of vertical grain boundary displacements as a fraction of the size of the melted region. The theoretical fractional distance $⟨y_{\text{melt}}⟩/L$ predicted by the lattice model (vertical dashed line) is consistent with the experimental fractional distances $(y-y_{\text{imp}})/L$ measured from 94 optical blasting experiments.

Figure 5

Sculpting an artificial grain. (a)–(c) We continuously move the laser to melt a band of particles directly above a grain boundary, causing the lower crystal grain to protrude into the upper crystal. (d)–(g) By melting the interior of the base of the protrusion, we draw the grain boundary inward to pinch off a new grain. Particle color: magnitude of ${\mathrm{\Psi }}_6$.