Stretching Polymers in Droplet-Pinch-Off Experiments

Abstract

Droplet pinch off, which occurs when a drop of liquid detaches from a capillary, can be strongly modified in the presence of polymers, giving rise to long and slender filaments that thin slowly in time. However, little is known experimentally about the molecular conformations of the polymers in the filament itself. Since the thinning dynamics of these filaments can be used to extract macroscopic quantities of interest such as the elongational viscosity of polymer solutions, which is of importance in a variety of physical processes (electrospinning, spraying, or drag reduction), the link with the molecular scale dynamics would be an important step towards understanding the extensional properties of such solutions. We show here, experimentally, that the polymers are highly extended within this filament region and that the distribution of these extensions is stationary in time. We then determine the elongational viscosity from the full filament dynamics. Such a determination turns out to be consistent with the polymer extensions observed and also consistent with simple models of polymer elongational viscosities. The direct observation of stretched macromolecules within such suspended liquid filaments suggests that these filaments may be useful for preparing polymeric materials with specific anisotropy or for optical screening of biopolymers such as DNA.

Pore‐Scale Flow Characterization of Polymer Solutions in Microfluidic Porous Media

“…Channels with hyperbolic constrictions can also be designed to minimize shear effects; pressure drop measurements across the constriction, corrected for shear contributions, then provide a direct measure of the extensional viscosity ( Figure ). Other microfluidic designs use the breakup of aqueous droplets in an oil sheath fluid; the extensional viscosity of the polymer solution can then be obtained from the filament thinning dynamics . Intriguingly, such microfluidic experiments have revealed that for sufficient extension rates, the extensional viscosity can abruptly increase—similar to transport measurements on bulk porous media—suggesting a potential contribution to the increased macroscopic flow resistance.…”

“…Hence, the extensional viscosity η e only quantifies the elongation contribution to fluid stress when averaged over sufficiently large spatial and temporal scales. Recent work has also demonstrated the strong role of polymer elongation on the breakup dynamics of aqueous droplets, indicating that such transient extension likely plays an important role in flows involving fluid interfaces, as in EOR and groundwater remediation . While these transient effects are typically neglected in models of polymer solution flow through porous media, ongoing work leveraging microfluidics is beginning to shed light on the central influence of unsteady flows in determining macroscopic transport properties.…”

“…Other microfluidic designs use the breakup of aqueous droplets in an oil sheath fluid; the extensional viscosity of the polymer solution can then be obtained from the filament thinning dynamics. [67][68][69] Intriguingly, such microfluidic experiments have revealed that for Small 2020, 16,1903944 DayQuil is a commercially available solution of CMC, a rigid polymer; Chili is a commercially available solution of xanthan, a semirigid polymer. Reproduced with permission.…”

Drop formation in shear-thickening granular suspensions

“…On the other hand, drop formation in non-Newtonian fluids, which is important in areas such as emulsification, inkjet printing, and agricultural spraying, is still ill understood in many cases [1][2][3][4] and continues to attract considerable attention [5][6][7][8][9][10][11][12][13]. For instance, theoretical analysis predicts that the drop breakup in shear thinning fluids proceeds faster than that in the Newtonian case due to the high elongational rates present in the fluid neck during the thinning [14,15].…”

Nonuniversality in the Pinch-Off of Yield Stress Fluids: Role of Nonlocal Rheology

“…We follow a simple and direct method to probe the extensional properties of a variety of yield stress fluids by following the capillary thinning dynamics of a fluid neck until it breaks up [5][6][7]. We find that, contrary to expectations, the thinning dynamics shows a nonuniversal behavior that leads to fluid viscosities that depend on the imposed initial deformation.…”