We discuss gravitational billiards, i.e. the two-dimensional motion of a paint mass inside a hard boundary curve under the influence of a constant (e.g. gravitational) field. A parabolic boundary is shown to be a new example of integrable billiards. The system has a second integral of motion in addition to the energy, which is constructed analytically. Stability and bifurcation properties ofthe central periodic orbits arc discussed. the results also shed new light on the known integrable care of elliptic non-gravitational billiards.

Atmospheric pressure chemical vapor deposition (APCVD) has been used extensively for synthesizing two-dimensional (2D) materials, due to its low cost and promise for high-quality monolayer crystal synthesis. However, the understanding of the reaction mechanism and the key parameters affecting the APCVD processes is still in its embryonic stage. Hence, the scalability of the APCVD method in achieving large scale continuous film remains very poor. Here, we use MoSe 2 as a model system and present a fluid guided growth strategy for understanding and controlling the growth of 2D materials. Through the integration of experiment and computational fluid dynamics (CFD) analysis in the full-reactor scale, we identified three key parameters: precursor mixing, fluid velocity and shear stress, which play a critical role in the APCVD process. By modifying the geometry of the growth setup, to enhance precursor mixing and decrease nearby velocity shear rate and adjusting flow direction, we have successfully obtained inch-scale monolayer MoSe 2 . This unprecedented success of achieving scalable 2D materials through fluidic design lays the foundation for designing new CVD systems to achieve the scalable synthesis of nanomaterials.

In this study, an artificial transparent head surrogate with high-speed photography discovers the formation and collapse of cavitation bubbles near the contrecoup regions as the head is exposed to a sudden translational impact. The cavitation damages the brain surface and produces a shock wave through the brain matter. Based on a novel experimental design, this new finding uncovers the mystery of the motion and deformation of the soft brain matter, which is not visible otherwise. It suggests that current brain injury criteria may underestimate the risk of head collision.

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