kg m − 3 and average diameter d = 1 . 034 mm are gently added inside a shear cell filled with an interstitial liquid with the same refractive index as the glass spheres, den- sity ρ$_{l}$ = 1 . 0 × 10 3 kg m − $^{3}$, and viscosity ν = 2 . 2 × 10 − 2 Pa s. Then a flat plate is placed on the top, which is constrained to move only in the vertical and horizontal direction and not allowed to rotate using a rigid set of lin- ear guides. The initial volume fraction of the glass beads is measured using the mass of the particles added and the volume of the cell occupied and found to be φ = 0 . 59. A normal stress σ$_{z}$ = 0 . 4 Pa is applied on the top boundary which is about five times the net gravitational stress due to the weight of the grains alone inside the cell, and is found to eliminate gradients due to gravity in the system. A dye is added to the liquid and a thin slice of the cell is illuminated with a laser and a cylindrical lens [10]. The resulting fluorescent light causes the particles to ap- pear dark against a bright background, and is imaged from an orthogonal direction with a resolution of 20 pix- els to a particle diameter using a 1000 × 1000 pixel 10- bit camera. A stack of images is recorded by linearly translating the plane of illumination. We then examine a 40 d × 5 d × 17 . 5 d central region as in indicated by box in Fig. 1(b,c) to avoid any effect of the boundaries, and locate the absolute position of the spheres to within the slight polydispersity of the particles (see Fig. 1(d)). We impose quasi-static shear strain by varying θ be- tween ± π/ 36 radians with a mean angular speed ω = 8 . 0 × 10 − $^{3}$rad s − $^{1}$, with a wait time of 50 s while the stack of images is acquired every time the system returns to its original position, θ$_{0}$ = 0 rad. The lubrication forces [11] due to liquid draining at contacts can be estimated to be 10 − 5 lower than the confining forces, and the particle Reynolds number is ∼ 10 − $^{1}$. Therefore the particles can be assumed to be in contact during the entire experiment and the interstitial liquid does not have any impact on the observed structure. The mean packing volume fraction of the spheres in- side the entire cell is first obtained by measuring the po- sition of the top plate as a function of the shear cycles. The mean φ is observed to increase logarithmically from the initial value by 5% (Fig. 1e) consistent with previous reports with a similar setup [12]. However, it is note- worthy that this is the total φ inside the cell and can be different than φ in the bulk because of influence of boundaries [10]. Examining the images corresponding to the initial state of the packing, before applying the shear deformation, N = 1 and after shear cycle N = 600 shown in Fig. 1(b,c), we indeed note greater ordering near the top where the boundary shears the spheres and moves to accommodate changes in the total φ . The boundary be- tween ordered and disordered region appears sharp and moves downward as N is increased further, similar to de- velopment of crystalline order inside a Couette shear cell upon extended shearing [7]. Therefore, we focus on the first 600 shear cycles where the particles inside the bulk in the observation window appear uniformly random and obtain φ from the ratio of the particle volume and the Now, the Percus-Yevick equation [9, 14] can be used to analytically find g ( r ) for randomly distributed particles for a given particle size, and volume fraction. Because of the slight polydispersity of the particles used in our ex- periments, we in fact have to compute the Percus-Yevick pair distribution function for polydisperse spherical pack- ings g$_{ij}$ ( r ), where the indices i, j represent the probability of finding a particle with diameter d$_{j}$ at a distance r from a particle with diameter d$_{i}$ . To analyze the structure from the measured particle positions, we first discuss the radial distribution function g ( r ) which represents the probability that the center of a particle is found at a distance r from another particle. g ( r ) obtained from the experimental data is shown in Fig. 2. For the initial volume fraction φ = 0 . 59, g ( r ) shows a tall peak at r ∼ d , and a broad peak at r ∼ 2 d , but for the higher φ obtained after cyclic shear, the second peak splits and a weak secondary peak occurs at r = √$_{3}$$_{d}$$_{, corresponding to the next nearest neighbor in}$ a face-centered-cubic (FCC) lattice. average Voronoi volume in the bulk. The Voronoi vol- ume is defined by points in the volume closest to that particle, and is calculated using algorithms written by Rycroft [13] and measured particle positions. As shown in Fig. 1(e), the evolution of φ in the bulk is observed to be slower compared with φ measured for the entire cell and is used in all subsequent discussion. FIG. 2. The radial distribution function g ( r ) plotted as func- tion of distance r normalized by the mean diameter d for initial packing φ = 0 . 59 and final packing φ = 0 . 605 obtained after shearing, (black), is compared with the theoretical calcu- lation (red/grey) obtained by using the Percus-Yevick equa- tion, and the measured polydispersity of the beads. The φ = 0 . 605 case is offset for clarity. Inset: The calculated pair distribution functions g$_{ij}$ for particles coarsened to three sizes ( φ = 0 . 59). The thick red/grey curve represents the av- erage of the six distinct contributions g$_{ij}$ ( r ) and is used for comparison with the experimental g(r). g$_{P Y}$ ( r ) = 1 n ( n − 1) n ∑ i =1 j ≥ i g$_{ij}$ ( r ) (1) 2 φ  = 0.605 φ  = 0.590