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The classic theory of fracture mechanics is found to be valid at the scale of single-walled carbon nanotubes.

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severe respiratory diseases, pneumonia, and systemic inflammatory response syndrome, leading to a worldwide sustained pandemic. Both SARS-CoV-2 and the original severe acute respiratory syndrome coronavirus (SARS-CoV) enter human cells by protein-protein docking to human angiotensin-converting enzyme 2 (ACE2) on the host cell membrane via CoV spike (S) glycoproteins. A recent experimental study found that the binding affinity between ACE2 and the receptorbinding domain (RBD) of the S protein of SARS-CoV-2 is more than tenfold higher than that of SARS-CoV, which may contribute to the higher infectivity and transmissibility of SARS-CoV-2 compared to SARS-CoV. [1-3] Molecular structures of the S protein of SARS-CoV-2 have been observed at high resolution by using cryo-electron microscopy (cryo-EM). [1,4,5] The complex structures of ACE2 bound to the SARS-CoV-2 S have also been experimentally determined. [6-9] Surprisingly, all these experiments showed that the backbone structures of the RBD of SARS-CoV-2 S are almost same as that of SARS-CoV S (see Figure 1a). [6,10] A molecular dynamic (MD) study has shown that the binding energy of SARS-CoV-2 S to ACE2 is almost same as that of A recent experimental study found that the binding affinity between the cellular receptor human angiotensin-converting enzyme 2 (ACE2) and receptor-binding domain (RBD) in the spike (S) protein of novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is more than tenfold higher than that of the original severe acute respiratory syndrome coronavirus (SARS-CoV). However, main chain structures of the SARS-CoV-2 RBD are almost the same with that of the SARS-CoV RBD. Understanding the physical mechanism responsible for the outstanding affinity between the SARS-CoV-2 S and ACE2 is an "urgent challenge" for developing blockers, vaccines, and therapeutic antibodies against the coronavirus disease 2019 (COVID-19) pandemic. Taking into account the mechanisms of hydrophobic interaction, hydration shell, surface tension, and the shielding effect of water molecules, this study reveals a hydrophobic-interaction-based mechanism by means of which SARS-CoV-2 S and ACE2 bind together in an aqueous environment. The hydrophobic interaction between the SARS-CoV-2 S and ACE2 protein is found to be significantly greater than that between SARS-CoV S and ACE2. At the docking site, the hydrophobic portions of the hydrophilic side chains of SARS-CoV-2 S are found to be involved in the hydrophobic interaction between SARS-CoV-2 S and ACE2.