surfaces - lammpslammps.sandia.gov/workshops/aug15/pdf/poster_bala.pdfshear induced adsorption of...

SHEAR INDUCED ADSORPTION OF POLYMER ONTO NEUTRALSURFACES

VENKAT BALA & DR. COLIN DENNISTON, THE UNIVERSITY OF WESTERN ONTARIO, CANADA, VBALASU8@UWO.CA

INTRODUCTIONExperiments involving the protein VWF

(von Willebrand factor), present in our bloodhave shown its importance in preventing bloodloss under high shear stress conditions. Ele-vated shear levels found during blood loss ac-tivate this protein’s adhesion potential causingit to stick to walls for blood platelets to adhere.

http://medimoon.com/2013/12/new-drug-tretten-approved-by-fda-to-prevent-bleeding-in-rare-clotting-

disorder/

The unusually large size of this multimeric pro-tein, 2050 amino acids/monomer and lengthsreaching upto ≈ 100µm are attributed as rea-sons as to why this counter-intuitive phenom-ena occurs.

RESULTS• Thermal fluctuations cause small protrusions into the bulk, which get pulled by the shear and

unfold the polymer causing it to stick to the wall. This creates an immobilized grid to whichblood platelets adhere causing the blood to clot

• Characterizing adhesion by counting the number of contact points along the chain, M withinhalf the cut-off distance of the polymer-wall attractive potential, i.e rz < rc = 2.5σ

φ =<M >

N

a=0.7 nm a=0.1 nm

0

0.05

0.1

0.15

0.2

0.25

0 0.005 0.01 0.015 0.02 0.025 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1e-05 2e-05 3e-05 4e-05 5e-05 6e-05 7e-05

γ̇τ γ̇τ

Contact parameter (steady state time-averaged) as a function of shear for various adhesion strengths εw .Here τ = a2/(µ0kBT ), µ0 being the Stokes mobility, is the single monomer diffusion time

• Cumulative probability distribution of the net force acting on the polymer in the z direction

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-50 -40 -30 -20 -10 0

P(f

z)

Net Force,Fz

a=0.1 nm a=0.7 nm

– Results for εw = 0.095, γ̇/τ = 0.5. Higher proba-bility for Fz to be negative for the larger monomercase

– Difference quite small

– Need sufficiently high number of terms in a mul-tipole expansion to properly account for the bead-bead HI

DISCUSSION• Recent works using Brownian dynamics sim-

ulations, unable to observe shear inducedadsorption with HI [2]

• In the bulk, polymer gets stretched out →dramatic loss of conformational entropy, ad-hered state to the wall preferred!

• Particle size effects, Stoke’s flow equations→LINEAR. Green’s function solution satisfy-ing no-slip condition at a plane wall→ Blaketensor, µB

∇p(r)− η∇2v(r) = f(r), ∇ · v = 0

• Hydrodynamic interactions between spheresof radii a located at r, r’, no shear, [3]

µ(r, r’) =(1 +

a2

6∇2

r +a2

6∇2

r’

)µB (r, r’)

• Higher order terms in the multipole expan-sion could possibly yield correct dynamicsnear wall

• Effect of external shear flow on the HI tensormust be taken into account

eps=0.095

REFERENCES

[1] S. Plimpton, J. Comput. Phys., 117 (1995), pp. 1-19[2] M. Radtke, M. Radtke, and R. Netz, Eur. Phys. J. E, vol.

37, no. 3, 2014[3] S. W. Schneider, S. Nuschele, a Wixforth, C. Gorze-

lanny, a Alexander-Katz, R. R. Netz, and M. F. Schnei-der, Proc. Natl. Acad. Sci. U. S. A., vol. 104, no. 19, pp.7899-7903, 2007

[4] F. E. Mackay, S. T. T. Ollila, and C. Denniston, Comput.Phys. Commun., vol. 184, no. 8, pp. 2021-2031, 2013

METHOD• MD simulations with full hydrodynamic interactions

were carried out using LAMMPS/USER-LB packagewith nanoscale units ([1],[4])

• 12-6 lj/cut repulsive potential was used to model ex-cluded volume interactions between monomers withε = 4.14195, σ = 1.5 and rc = 21/6σ

• Polymer was composed of 96 monomers, each en-veloped within a spherical shell consisting of 30 atomsinteracting with the thermal lb/fluid bath

• Chain held together by Finitely Extensible Non-linearElastic (FENE) bonds

• 12-6 lj/cut potential used for the attractive interaction be-tween the polymer and wall atoms, with σ = 1.5 andrc = 2.5σ

• Interaction strength varied,εw = 0.045, 0.055, 0.065, 0.075, 0.085, 0.095