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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
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Temperature Dependent Deformation01:12

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In a nonhomogeneous rod made up of steel and brass, restrained at both ends and subjected to a temperature change, several steps are involved in calculating the stress and compressive load. Due to the problem's static indeterminacy, one end support is disconnected, allowing the rod to experience the temperature change freely. Next, an unknown force is applied at the free end, triggering deformations in the rod's steel and brass portions. These deformations are then calculated and added...
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Metallic solids such as crystals of copper, aluminum, and iron are formed by metal atoms. The structure of metallic crystals is often described as a uniform distribution of atomic nuclei within a “sea” of delocalized electrons. The atoms within such a metallic solid are held together by a unique force known as metallic bonding that gives rise to many useful and varied bulk properties.
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Related Experiment Video

Updated: May 26, 2025

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets
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Entanglement transition in random rod packings.

Yeonsu Jung1, Thomas Plumb-Reyes1, Hao-Yu Greg Lin2

  • 1John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138.

Proceedings of the National Academy of Sciences of the United States of America
|February 21, 2025
PubMed
Summary

Random packings of stiff rods form stable structures. Increasing rod aspect ratio enhances entanglement, leading to a mechanical stability transition, as revealed by X-ray tomography and simulations.

Keywords:
jammingphysical entanglementrod packing

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Area of Science:

  • Physics of disordered materials
  • Materials science
  • Complex systems

Background:

  • Random packings of stiff rods exhibit self-supporting mechanical properties due to contact-induced interactions.
  • Understanding the geometrical and topological complexity is crucial for predicting material behavior.

Purpose of the Study:

  • To investigate the relationship between rod aspect ratio, entanglement, and mechanical stability in random packings.
  • To develop an entanglement phase diagram for filamentous materials.

Main Methods:

  • X-ray computerized tomography to visualize packing structure, density, orientational order, and entanglement.
  • Numerical simulations of contacting elastic rods under static and dynamic loading.
  • Analysis of published experimental data from various filamentous systems.

Main Results:

  • Increasing rod aspect ratio leads to increased entanglement, which percolates through the packing.
  • A sharp transition in mechanical stability correlates with increased entanglement.
  • An entanglement phase diagram was constructed, mapping stable entanglement regimes.

Conclusions:

  • Entanglement is a key factor in the mechanical stability of random rod packings.
  • The findings have implications for diverse systems including reconfigurable architectures, textiles, and biological assemblies.