Figure 1

(Color online) (a) Scanning electron micrograph of photolithographically prepared $4.5\text{−}\mu \mathrm{m}$-diam, $0.8\text{−}\mu \mathrm{m}$-thick PMMA disks. (b) Schematic of the experimental setup. Particles settle to a monolayer on the bottom of the sample chamber and stand on edge in the applied field. Dashed line indicates the horizontal plane.Reuse & Permissions

Figure 2

(Color online) (a) Optical micrographs of the sample at 2D packing fractions of 0.50, 0.53, and 0.56 from left to right. Inset on right is Fourier transform of 0.56 image. (b) Orientational correlation functions associated with the micrographs. Square, $g_2$; circle $g_4$; up triangle, $g_6$; down triangle, $g_8$. Inset of right image shows the ${\left[g_2\left(r\right)\right]}^4$ and $g_4\left(r\right)$ overlay, indicating both arise from nematic order.Reuse & Permissions

Figure 3

(a) Nematic and tetratic order parameter and (b) nematic and tetratic decay exponent as a function of 2D packing fraction. For the nematic phase, the Franck constant has a relation with ${\eta }_2$: ${\eta }_2=2k_{B}T∕\pi \mathrm{K}$.Reuse & Permissions

Figure 4

(Color online) (a) Disclinations and domain walls for the almost tetratic sample. Star, plus, and square denote $\pi$, $\pi ∕2$, and $-\pi ∕2$ dislcinations, respectively. Lines (blue in color) indicate the location of walls. Bound tetratic disclinations are connected by white segments (white-red in color). The free disclinations are shown in isolated symbols. (b) Schematic of correlation functions arising from topological defects. The dotted line represents the nematic correlation function $g_2\left(r\right)$ expected from the distance between $\pi$ disclinations. The solid line represents the tetratic correlation function $g_4\left(r\right)$ expected from $g_2\left(r\right)$ + the distance between $\pi ∕2$ disclinations. The dashed line represents $g_2\left(r\right)$ expected from the separation length of grain boundaries + the distance between $\pi$ disclinations. The $\pi ∕2$ grain boundaries, of separation $L_{\mathit{\text{wall}}}$, destroy nematic but not tetratic correlations. Thus in our observation window $d\sim 73\mu \mathrm{m}$, $g_4$ dominates over $g_2$.Reuse & Permissions