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Fluid Mechanics Seminar: Pinch‑off Dynamics, Dripping-onto-Substrate (DoS) Rheometry and Printability of Complex Fluids

Event Type
fluid mechanics
Mechanical Science and Engineering
2005 Mechanical Engineering Laboratory (Deere Pavilion)
May 5, 2017   12:00 - 1:00 pm  
Professor Vivek Sharma, Department of Chemical Engineering, University of Illinois at Chicago
Originating Calendar
MechSE Seminars


Liquid  transfer  and  drop  formation/deposition  processes  involve  complex  free-surface  flows including   the   formation   of   columnar   necks   that   undergo   spontaneous   capillary-driven instability,  thinning  and  pinch-off. For  simple  (Newtonian  and  inelastic)  fluids,  a  complex interplay   of   capillary,   inertial   and   viscous   stresses   determines   the   nonlinear   dynamics underlying   finite-time   singularity   as   well   as   self-similar   capillary   thinning   and   pinch-off dynamics. In rheologically complex fluids, extra elastic stresses as well as non-Newtonian shear and extensional  viscosities  dramatically  alter  the  nonlinear  dynamics.  Stream-wise  velocity gradients  that  arise  within  the  thinning  columnar  neck  create  an  extensional  flow  field,  and many  complex  fluids  exhibit  a  much  larger  resistance  to  elongational  flows  than  Newtonian fluids with  similar shear viscosity. Characterization  of pinch-off dynamics and  the  response  to both shear and extensional flows that influence drop formation/ deposition in microfluidic and printing  applications  requires  bespoke  instrumentation  not  available,  or  easily  replicated,  in most  laboratories.  Here  we  show  that  dripping-onto-substrate  (DoS)  rheometry  protocols  that involve  visualization  and  analysis  of  capillary-driven  thinning  and  pinch-off  dynamics  of  a columnar  neck  formed  between  a  nozzle  and  a  sessile  drop  can  be  used  for  measuring  shear viscosity,   power   law   index,   extensional   viscosity,   relaxation   time   and   the   most   relevant processing   timescale   for   printing.   We   showcase   the   versatility   of   DoS   rheometry   by characterizing  and  contrasting  the  pinch-off  dynamics  of  a  wide  spectrum  of  simple  and complex  fluids:  water,  printing  inks,  semi-dilute  polymer  solutions,  yield  stress  fluids,  food materials and cosmetics. We show that DoS rheometry enables characterization of low viscosity printing  inks  and  polymer  solutions  that  are  beyond  the  measurable  range  of  commercially-available  capillary  break-up  extensional  rheometer  (CaBER).  We  show  that  for  high  viscosity fluids, DoS rheometry can be implemented relatively inexpensively using an off-the-shelf digital camera, and for many complex fluids, similar power law scaling exponent describes both neck thinning  dynamics  and  the  shear  thinning  response.  Using  a  particular  example  of  aqueous polymer  solutions,  we  show  the  measurement  of  both  the  extensional  relaxation  time  and extensional viscosity of weakly elastic, polymeric complex fluids with low shear viscosity η < 20 mPa· s and relatively short relaxation time, λ  < 1 ms.  Lastly, we utilize DoS rheometry to probe and elucidate how polymer composition, flexibility, concentration, charge and molecular weight determine the kinetics of capillary-driven thinning and pinch-off in our experiments.

About the Speaker

Dr. Vivek Sharma is an Assistant Professor of Chemical Engineering at the University of Illinois, Chicago. Before joining UIC in November 2012, he worked as a post-doctoral research associate in Mechanical Engineering at Massachusetts Institute of Technology. He received his Ph. D. (Polymers/MSE, 2008) and M. S. (Chemical Engineering, 2006) from Georgia Institute of Technology, an M. S. (Polymer Science, 2003) from the University of Akron, and a bachelor's degree from IIT Delhi. Dr. Sharma's research interests broadly lie in optics, dynamics, elasticity, and self-assembly (ODES) of complex fluids and soft materials. At UIC, Dr. Sharma's Soft Matter ODES-lab combines experiments and theory to pursue the understanding of, and control over, interfacial and nonlinear flows of complex fluids. ODES-lab focuses on the interplay of (a) viscoelasticity and capillarity for printing applications and extensional rheometry, and (b) interfacial thermodynamics and hydrodynamics in fizzics (the science of bubbles, drops, thin films, emulsions and foams).

Host:  Professor Randy Ewoldt

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