Capillary thinning of elastic and viscoelastic threads: From elastocapillarity to phase separation

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Capillary thinning of elastic and viscoelastic threads: From elastocapillarity to phase separation

H. V. M. Kibbelaar, A. Deblais, F. Burla, G. H. Koenderink, K. P. Velikov, and D. Bonn

Phys. Rev. Fluids 5, 092001(R) – Published 22 September 2020

Abstract

The formation and destabilization of viscoelastic filaments are of importance in many industrial and biological processes. Filament instabilities have been observed for viscoelastic fluids but recently also for soft elastic solids. In this work, we address the central question of how to connect the dynamical behavior of viscoelastic liquids to that of soft elastic solids. We take advantage of a biopolymer material whose viscoelastic properties can be tuned over a very large range by its pH, and study the destabilization and ensuing instabilities in uniaxial extensional deformation. In agreement with very recent theory, we find that the interface shapes dictated by the instabilities converge to an identical similarity solution for low-viscosity viscoelastic fluids and highly elastic gels. We thereby bridge the gap between very fluid and strongly elastic materials. In addition, we provide direct evidence that at late times an additional filament instability occurs due to a dynamical phase separation.

Received 15 April 2020
Accepted 20 August 2020

DOI:https://doi.org/10.1103/PhysRevFluids.5.092001

Physics Subject Headings (PhySH)

Dynamics of phase separation Instability control Instability of free-surface flows

Fluid DynamicsPolymers & Soft Matter

Authors & Affiliations

H. V. M. Kibbelaar ¹, A. Deblais^1,2, F. Burla³, G. H. Koenderink^3,4, K. P. Velikov ^1,2,5, and D. Bonn¹

¹Van der Waals–Zeeman Institute, Institute of Physics, University of Amsterdam, 1098 XH Amsterdam, The Netherlands
²Unilever Innovation Centre Wageningen, Bronland 14, 6708 WH Wageningen, The Netherlands.
³AMOLF, Department of Living Matter, 1098 XG Amsterdam, The Netherlands
⁴Department of Bionanoscience, Kavli Institute of Nanoscience, Delft University of Technology, 2629 HZ Delft, The Netherlands
⁵Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University, 3584 CC Utrecht, The Netherlands

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Issue

Vol. 5, Iss. 9 — September 2020

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Images

Figure 1
Time-lapse photographs of blue-colored droplets of HA (3 mL volume), on a glass substrate, prepared at pH 2.5 representing the elastic state (a, upper panel) and at pH 7 representing the fluid state (b, lower panel). The fluid solution spreads like a liquid, while the elastic sample only flows on a timescale of minutes. The scale represents 15 mm.
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Figure 2
(a) Photographs of the neck breakup dynamics of HA in the fluid state (upper panel, pH 7) and in the elastic state (lower panel, pH 2.5). The scale represents $75 μ m$ in the upper panel and $400 μ m$ in the lower panel. (b) Minimum filament diameter normalized by the initial bridge size as a function of time for three different solutions at different elastocapillary numbers $γ / G^{'} D_{0}$ given in the upper colored bar: blue for pH 1.6 and 7, violet for 2.2 and 2.8, and purple for 2.5. The numbers correspond to the photographs in panel (a). (c) The viscoelastic moduli from oscillatory shear measurements. (d) Relaxation times obtained from panel (b).
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Figure 3
(a) BOAS instabilities are observed when the growth rate of the Plateau-Rayleigh instability is smaller than the inverse characteristic time of the polymer ( $λ_{0}^{- 1} > ω_{R}$ ); for pH 2.5, no bead is formed (the color gradient is the same as in Fig. 2). (b) Viscoelastic moduli at pH 2.8 as function of frequency. The crossover point at very low frequencies (marked with an asterisk) indicates the timescale at which the BOAS instability appears, corresponding to the transition between regimes II and III in Fig. 2.
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Figure 4
Time evolution of the filament shapes in (a) the elastic (pH 2.5, $γ / G^{'} D_{0} = 1.8$ ), 12.5 s between each profile, and (b) the fluid states (pH 7, $γ / G^{'} D_{0} = 40.2$ ), 10 ms between each profile. (Right) The same profiles rescaled by the minimum neck radius, $R_{\min}$ , and with the location for which the experimental profiles are collapsing, $z_{0}$ . The universal self-similar solution is the black line.
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Figure 5
(a) Critical neck diameter measured at the occurrence of the blistering pattern as a function of pH. Blistering is observed for a larger neck diameter when the pH approaches 2.5. Photographs of the breakup dynamics of two samples of 5 mm length at pH 1.9 extended at $100 μ m$ /s (b) and $1000 μ m$ /s (c). In panel (c), a pearl instability is observed (see Supplemental Video 1). The apparent roughness of the thread is consistent with polymer material, while the smoothness of the pearl indicates it contains the solvent. Scale bar is 2.5 mm.
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