We study the effects of a line of spherical interstitial particles (or intruders) placed between two adjacent uncompressed chains of larger particles in a square packing of spheres, using experiments and numerical simulations. We excite one of the chains of particles adjacent to the intruders with an impact and show how energy is transmitted across the system until equipartition is reached from the excited (or impacted) chain to the absorbing (or adjacent) chain. The coupling of the two chains, although a purely two-dimensional effect, is modeled by a simplified one-and-a-half-dimensional (1.5-D) system in which transverse motions of the particles are neglected.PACS numbers: 46.40.Cd Introduction. Granular crystals are unique nonlinear systems that exhibit interesting properties stemming from the nonlinear contact interactions (Hertzian [1]) between two individual particles. Uniform one-dimensional chains of spheres have been shown to support the propagation of a new type of solitary wave. The width of these waves is independent of its velocity [2][3][4][5]. The degree of nonlinearity can be tuned from highly nonlinear to linear by the addition of a precompression force [3,4,6]. Additionally, it was shown that homogeneous granular media can support families of strongly nonlinear traveling and standing waves [7], whereas heterogeneous media can exhibit resonance [8] and anti-resonance phenomena [9]. Another interesting property of these materials is the reflection of the solitary waves at an interface between two granular crystals [10][11][12], which could be used to develop new impulse trapping granular materials [13][14][15]. Several groups studied the interaction of a solitary wave with defect particles [16][17][18][19]. Twodimensional (2-D) granular crystals have been relatively unexplored, and prior works mainly consisted of numerical studies or experiments visualizing dynamic stress in photo-elastic disks [20][21][22][23]. Solitary waves have been observed in 2-D square packings of spherical particles [24]. Granular crystals have been proposed as new structured materials for the control and redirection of stress waves (see for example [10,13,14,25,28,33]). The experiments reported in this paper provide the first observation of energy equipartition between two adjacent and nonlinearly coupled chains of particles. In particular, we show that when one chain is excited by an impulse while the other is as rest, the energy is redistributed between the two chains within a short spatial distance. A similar equipartition phenomenon was studied numerically in an earlier work [26]. This phenomenon is of interest to create new acoustic wave guides, delay lines and stress mitigating materials. Energy transfer and equipartition phenomena in weakly coupled one-dimensional granular chains were studied [26,27], and in [28] through a macroscopic realization of the Landau-Zener tunneling quantum effect. The energy equipartition principle is well known for elastic waves. Seismic waves for example have well known regimes wh...