Optical Tweezers as a Micromechanical Tool for Studying Defects in 2D   Colloidal Crystals
  This paper reports on some new results from the analyses of the video microscopy data obtained in a prior experiment on two-dimensional (2D) colloidal crystals. It was reported previously that optical tweezers can be used to create mono- and di-vacancies in a 2D colloidal crystal. Here we report the results on the creation of a vacancy-interstitial pair, as well as tri-vacancies. It is found that the vacancy-interstitial pair can be long-lived, but they do annihilate each other. The behavior of tri-vacancies is most intriguing, as it fluctuates between a configuration of bound pairs of dislocations and that of a locally amorphous state. The relevance of this observation to the issue of the nature of 2D melting is discussed. 
Sungcheol Kim, Lichao Yu, Stephanie Huang, Alexandros Pertsinidis, and   Xinsheng Sean Ling
cond-mat.soft