Wednesday 1:30 p.m. Room 1315 May 23, 2018 Chemistry

Dr. Paddy Royall Faculty of Science

University of Bristol  Our   understanding   of   the   mechanism   by   which   the   viscosity   of   supercooled  liquids  increase  by  many  decades  is  hampered  by  the  difficulty  in  discriminating  apparently   incompatible   theoretical   approaches.   The   challenge   lies   in  

equilibrating  samples  at  sufficient  supercooling  that  experimental  or  numerical  techniques  can  probe  suitable  quantities  that  enable  the  theories  to  be  discriminated,  or  by  directly  testing  the  theories.  Recently,  considerable  progress  has  been  made  in  obtaining  data  which  can  help  to  discriminate  such  theories,  in  both  molecular  experiments  and  simulation   [1].  Much  of   these  new  data   tend   to   support   theories  which  imagine  a  thermodynamic  origin  of  the  glass  transition  -­‐  in  contrast  to  a  predominately  dynamic  origin.  Of  the  thermodynamic  theories,  we  begin  by  probing  the  predictions  of  the  geometric  frustration  theory  of  the  glass  transition  [2].  Nevertheless,   plenty   of   compelling   evidence   in   support   of   dynamic   facilitation   (which   posits   that   the   glass  transition   is   driven   by   a   dynamical   phase   transition)   exists   [3],   which   we   have   shown   can   be   accessed   in  experiments  [4].  Here  we  present  new  results,  which  use  novel  techniques  to  obtain  very  deeply  supercooled  configurations  in  an  atomistic  model  glassformer.  These  naturally  lend  themselves  to  a  determination  of  the  dynamical  phase  transition  of  facilitation.  We  find  that  the  dynamical  phase  transition  has  a  lower  temperature  bound,  which  we  interpret  as  a  critical  point.  Now  our  deeply  supercooled  configurations  also  give  access  to  the  configurational  entropy,  from  which  we  can  locate  the  Kauzmann  temperature.  Remarkably,  within  the  accuracy  of   our   approach,   this   point  where   the   thermodynamic   theories   suggest   a   phase   transition   lies   at   the   same  temperature  as  the  lower  critical  point  of  the  dynamical  transition.  We  suggest  that  our  findings  may  lead  to  a  path  to  reconcile  the  competing  thermodynamic  and  dynamic  interpretations  of  the  glass  transition  [5].      [1]  Royall,  C.  P.;  Turci,  F.;  Tatsumi,  S.;  Russo,  J.  &  Robinson,  J.  “The  race  to  the  bottom:  approaching  the  ideal  glass?”,  ArXiV,  1711.04739  (2017).  [2]  Turci,  F.;  Tarjus,  G.  &  Royall,  C.  P.  “From  Glass  Formation  to  Icosahedral  Ordering  by  Curving  Three-­‐Dimensional  Space”,  Phys.  Rev.  Lett.,  118,  215501  (2017).  [3]  Speck,  T.;  Malins,  A.  &  Royall,  C.  P.  “First-­‐Order  Phase  Transition  in  a  Model  Glass  Former:  Coupling  of  Local  Structure  and  Dynamics”  Phys.  Rev.  Lett.  109,  195703  (2012).  [4]  Pinchaipat,  R.;  Campo,  M.;  Turci,  F.;  Hallet,  J.;  Speck,  T.  &  Royall,  C.  P.  “Experimental  Evidence  for  a  Structural-­‐Dynamical  Transition  in  Trajectory  Space”  Phys.  Rev.  Lett.,  119,  028004  (2017).  [5]  Turci,  F.;  Royall,  C.  P.  &  Speck,  T.  Non-­‐Equilibrium  Phase  Transition  in  an  Atomistic  Glassformer:  the  Connection  to  Thermodynamics  Phys.  Rev.  X,  7  031028  (2017).  

Getting to the Bottom of the Energy Landscape to Tackle the Glass Transition: Experiments on

Colloids and New Computer Simulation Techniques