Rheol Acta (2013) 52:767–783 DOI 10.1007/s00397-013-0709-3 ORIGINAL CONTRIBUTION Elastic response from pressure drop measurements through planar contraction–expansion geometries by molecular dynamics: structural effects in melts and molecular origin of excess pressure drop Jorge Castillo-Tejas · Shirley Carro · Octavio Manero Received: 8 November 2012 / Revised: 15 April 2013 / Accepted: 16 April 2013 / Published online: 25 May 2013 © Springer-Verlag Berlin Heidelberg 2013 Abstract In this work, we use nonequilibrium molecu- lar dynamics to simulate a contraction–expansion flow of various systems, namely melts with molecules of various conformations (linear, branched, and star), linear molecules in solution, and a reference Lennard–Jones fluid. The equa- tions for Poiseuille flow are solved using a multiple time scale algorithm extended to nonequilibrium situations. Sim- ulations are performed at constant temperature using the Nose–Hoover dynamics. The main objective of this analysis is to investigate the molecular origin of pressure drop along planar contraction–expansion geometry, varying the length of the contraction, and the effect that different molecular conformations have on the resulting pressure drop along the geometry. Pressure drop is closely related to mass distribu- tion (in neutral and gradient directions) and branching index of molecules. Also, it is shown that remarkable increases of pressure drops are also possible in planar geometries, pro- vided large extensional viscosities combined with moderate values of the first normal stress difference in shear are con- sidered, in addition to considerable reductions of the flow area at the contraction region. Keywords Nonequilibrium molecular dynamics · Pressure drop · Contraction–expansion flow J. Castillo-Tejas () · S. Carro Facultad de Ciencias B´ asicas, Ingenier´ ıa y Tecnolog´ ıa, Universidad Aut´ onoma de Tlaxcala, Calzada Apizaquito S/N, Apizaco, Tlaxcala 90300, M´ exico e-mail: j castillo tejas@hotmail.com O. Manero Instituto de Investigaciones en Materiales, Universidad Nacional Aut´ onoma de M´ exico, Ciudad Universitaria, M´ exico, Federal District 04510, M´ exico Nomenclature (variables are given in dimensionless or reduced units) Rheological properties η Viscosity σ Stress tensor τ Viscous stress tensor N 1 First normal stress function η 0 Limiting viscosity at zero shear rate η s Viscosity of solvent [η] Intrinsic viscosity λ Relaxation time c* Critical concentration φ Concentration per site σ 11 , σ 22 Normal stress components in the x 1 and x 2 directions Dynamic properties Q Volume flow rate P 0 ,P 1 Pressure evaluated at the beginning and end of the measurement region P Total pressure drop P Entry Excess pressure drop originated by the reduction in flow area P 0 , P 1 Pressure drop under developed flow condi- tion prior and past the contraction P Adim Relationship between the pressure drop experienced by the solution and L-J fluid • ε = v 1 /x 1 Strain rate 〈v〉 0 Mean velocity of fluid at the beginning of the measurement region E V Rate of dissipation U NHi Energy removed by thermostat per particle