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arxiv: 2604.01185 · v2 · submitted 2026-04-01 · ❄️ cond-mat.soft · physics.flu-dyn

Polyelectrolyte adsorption at the solid-liquid interface favors receding contact line instability

L\'ea Delance (1) , Diego D\'iaz (2) , Arivazhagan G. Balasubramanian (2) , Outi Tammisola (2) , Kaloian Koynov (1) , Hans-J\"urgen Butt (1) ((1) Max Planck Institute for Polymer Research , (2) KTH Royal Institute of Technology) This is my paper

Pith reviewed 2026-05-13 21:43 UTC · model grok-4.3

classification ❄️ cond-mat.soft physics.flu-dyn
keywords polyelectrolyteadsorptioncontact lineviscoelasticityfilament formationsliding dropswetting
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0 comments X

The pith

Adsorption of polyelectrolytes at the solid-liquid interface destabilizes the receding contact line of sliding viscoelastic drops, causing charge-dependent filament formation.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper investigates the motion of viscoelastic drops made from polyelectrolyte solutions sliding on hydrophobic surfaces. Using a custom high-speed reflection microscope, the authors visualize the contact line dynamics directly. They find that the receding contact line becomes unstable and forms filaments for cationic and non-ionic polymers, but not for anionic ones. This instability is attributed to the adsorption of the polymers at the solid-liquid interface, which affects the wetting properties differently based on charge. The work provides a microscopic explanation for the observed macroscopic behaviors like reduced sliding velocities and drop elongation in such systems.

Core claim

Viscoelasticity from polyelectrolytes destabilizes the receding contact line in sliding drops, triggering filament formation for the first time reported in this geometry. Cationic and non-ionic polymers promote this while anionic do not, due to their distinct wetting properties from interface adsorption.

What carries the argument

Charge-dependent polyelectrolyte adsorption at the solid-liquid interface that modifies wetting and destabilizes the receding contact line.

If this is right

  • Viscoelastic drops containing charged polymers exhibit slower sliding and elongation due to contact line filaments.
  • Polymer charge selection can control whether instability occurs in drop-based applications.
  • The mechanism connects bulk rheology to surface adsorption effects in fluid dynamics.
  • Filament formation may generalize to other viscoelastic flows on surfaces.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Direct measurements of adsorption layers could quantify how much charge affects the interface properties.
  • Similar instabilities might influence the behavior of charged biological fluids on surfaces.
  • Surface modifications to modulate adsorption could be used to stabilize or destabilize contact lines predictably.

Load-bearing premise

That the observed charge-dependent differences stem from distinct wetting properties induced by adsorption, without detailed contact angle or adsorption quantification.

What would settle it

Direct observation of no filament formation in cationic polymer drops or equal instability across all charges would contradict the claim.

Figures

Figures reproduced from arXiv: 2604.01185 by (2) KTH Royal Institute of Technology), Arivazhagan G. Balasubramanian (2), Diego D\'iaz (2), Hans-J\"urgen Butt (1) ((1) Max Planck Institute for Polymer Research, Kaloian Koynov (1), L\'ea Delance (1), Outi Tammisola (2).

Figure 1
Figure 1. Figure 1: FIG. 1. a) Schematics of the setup used for reflection microscopy of sliding drops. b-c) Typical [PITH_FULL_IMAGE:figures/full_fig_p005_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Side-view images of the drops and reflection microscope images of the receding contact line [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. (a) Effective capillary number as a function of the tilting angle and (b) contact angle [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. (a) Length [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Schematic of the rear side of a sliding drop representing polymer adsorption at the solid [PITH_FULL_IMAGE:figures/full_fig_p010_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Side (a-b) and bottom (c-d) view of the rear side of a moving drop at pH 5 and 11. [PITH_FULL_IMAGE:figures/full_fig_p011_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. Cationic (a) and non-ionic (b) polymer deposition observed by SEM after one drop sliding [PITH_FULL_IMAGE:figures/full_fig_p012_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. Receding contact line on PDMS for a) anionic and b) non-ionic polymer drops imaged by [PITH_FULL_IMAGE:figures/full_fig_p015_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9. Extensional rate (a) and (b) extensional viscosity for the anionic polymer. [PITH_FULL_IMAGE:figures/full_fig_p016_9.png] view at source ↗
read the original abstract

Controlling the motion of non-Newtonian drops on surfaces is crucial for applications ranging from inkjet printing to biomedical devices and food processing. While the macroscopic behavior of viscoelastic drops sliding on tilted hydrophobic surfaces has been characterized, showing reduced velocities and elongation compared to Newtonian fluids, the underlying microscopic mechanisms remain poorly understood. To address this gap, we developed a high-speed, high-resolution reflection microscope that enables direct visualization of the contact line of sliding drops. We used water/soluble polyelectrolyte solutions based on polyacrylamide and let drops sliding on hydrophobic substrates composed of Teflon AF- and PDMS-coated glass slides. The substrate tilting angle was varied between 20{\deg} and 45{\deg}. We reveal how viscoelasticity influences the dynamics of the receding contact line and drop motion. Our experiments demonstrate that viscoelasticity can destabilize the receding contact line, triggering filament formation. This instability previously observed in the coating of thin viscoelastic films, is reported here for the first time in sliding drops. We further highlight the critical role of polymer charge in this process: while cationic and non-ionic polymers promote filament formation, anionic polymers do not, a difference we attribute to the distinct wetting properties of the solutions. In conclusion, we clarify the interplay between rheology, surface interactions, and drop dynamics.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports high-speed, high-resolution reflection microscopy experiments on sliding drops of polyacrylamide-based polyelectrolyte solutions (cationic, non-ionic, and anionic) on Teflon AF- and PDMS-coated hydrophobic substrates at tilt angles of 20–45°. It claims that viscoelasticity destabilizes the receding contact line, producing filaments for the first time in sliding drops (previously seen only in thin-film coating), and that this instability is promoted by cationic and non-ionic polymers but suppressed by anionic ones due to charge-dependent differences in wetting arising from polyelectrolyte adsorption at the solid-liquid interface.

Significance. If the mechanistic attribution to adsorption-driven wetting differences is substantiated, the work would extend known viscoelastic contact-line instabilities from coating flows to sliding drops and clarify the role of polymer charge and surface interactions in non-Newtonian drop motion, with relevance to inkjet printing, biomedical devices, and food processing.

major comments (2)
  1. [Abstract] Abstract and title: the central claim that polyelectrolyte adsorption at the solid-liquid interface favors the receding contact-line instability rests on the observed charge selectivity (filaments for cationic/non-ionic but not anionic polymers) being ascribed to distinct wetting properties, yet the manuscript provides no direct adsorption measurements (surface excess, layer thickness, or adsorbed mass) nor systematic contact-angle data to establish this causal link; comparisons rest solely on polymer charge without controls for viscosity, surface tension, or ionic strength.
  2. [Results] Results section: the abstract states that filament formation is triggered by viscoelasticity but supplies no quantitative thresholds, error bars, sample sizes, or statistical criteria for identifying filaments versus stable contact lines, leaving the robustness of the charge-dependent selectivity difficult to evaluate.
minor comments (2)
  1. [Methods] The description of the high-speed reflection microscope setup would benefit from explicit values for frame rate, spatial resolution, and illumination wavelength to allow reproducibility.
  2. [Methods] Polymer molecular weights, concentrations, and exact charge densities should be stated explicitly rather than generically as “based on polyacrylamide.”

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for the constructive feedback on our manuscript. We address the major comments point by point below and outline planned revisions to strengthen the evidence and quantitative rigor.

read point-by-point responses

  1. Referee: [Abstract] Abstract and title: the central claim that polyelectrolyte adsorption at the solid-liquid interface favors the receding contact-line instability rests on the observed charge selectivity (filaments for cationic/non-ionic but not anionic polymers) being ascribed to distinct wetting properties, yet the manuscript provides no direct adsorption measurements (surface excess, layer thickness, or adsorbed mass) nor systematic contact-angle data to establish this causal link; comparisons rest solely on polymer charge without controls for viscosity, surface tension, or ionic strength.

    Authors: We agree that the manuscript lacks direct adsorption measurements (e.g., surface excess or layer thickness via ellipsometry or QCM-D) and systematic contact-angle data, which weakens the causal attribution to adsorption-driven wetting differences. Our interpretation draws from the observed charge selectivity combined with established literature on polyelectrolyte adsorption to hydrophobic surfaces. In revision, we will add systematic advancing/receding contact-angle measurements for all three polymer types on both Teflon AF and PDMS substrates. We will also include controls by matching viscosity and surface tension across samples (via concentration adjustment or salt addition) and report ionic strength explicitly. The abstract and title will be revised to state that cationic and non-ionic polymers promote filament formation while anionic ones suppress it, with the difference attributed to charge-dependent wetting likely arising from adsorption, while noting the indirect nature of the supporting evidence. revision: partial

  2. Referee: [Results] Results section: the abstract states that filament formation is triggered by viscoelasticity but supplies no quantitative thresholds, error bars, sample sizes, or statistical criteria for identifying filaments versus stable contact lines, leaving the robustness of the charge-dependent selectivity difficult to evaluate.

    Authors: We acknowledge the absence of quantitative thresholds, error bars, sample sizes, and statistical criteria in the current results presentation. In the revised manuscript, we will define explicit criteria for filament identification (e.g., receding contact-line protrusions with length exceeding 0.5 mm or aspect ratio >3), include error bars on all plots (standard deviation from n=5–8 independent drops per condition), report sample sizes clearly, and add a statistical summary (e.g., percentage of drops exhibiting instability for each polymer type). This will allow direct evaluation of the robustness of the charge-dependent selectivity. revision: yes

standing simulated objections not resolved
  • Direct quantitative adsorption measurements (surface excess, adsorbed mass, or layer thickness) are not available from the original experiments and cannot be added without a separate experimental campaign using techniques such as ellipsometry or quartz-crystal microbalance.

Circularity Check

0 steps flagged

No circularity: purely observational experimental study with no derivations or fitted predictions

full rationale

The manuscript reports high-speed microscopy observations of filament formation at the receding contact line of sliding polyelectrolyte drops. No equations, models, or first-principles derivations are presented. The central observation (filaments appear for cationic and non-ionic polymers but not anionic) is directly compared to polymer charge type; the attribution to wetting properties is stated as an interpretation without any fitted parameters, self-citations forming a load-bearing chain, or renaming of prior results. No step reduces by construction to its own inputs. The work is self-contained against external benchmarks as an empirical report.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Experimental study relying on established fluid-dynamics and wetting principles with no new theoretical entities or fitted parameters central to the claim.

axioms (1)
  • standard math Standard assumptions of contact-line dynamics and viscoelastic fluid behavior on hydrophobic surfaces
    The interpretation of filament formation and charge effects presupposes conventional models of wetting and rheology without new derivations.

pith-pipeline@v0.9.0 · 5599 in / 1244 out tokens · 39096 ms · 2026-05-13T21:43:47.503331+00:00 · methodology

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Reference graph

Works this paper leans on

2 extracted references · 2 canonical work pages

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    work page 1997

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    Surface charge at teflon/aqueous solution of potassium chloride inter- faces,

    the origins of charge,” Electrophoresis29, 1092–1101 (2008), https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/pdf/10.1002/elps.200700734. 11T. Preoˇ canin, A. Selmani, P. Lindqvist-Reis, F. Heberling, N. Kallay, and J. L¨ utzenkirchen, “Surface charge at teflon/aqueous solution of potassium chloride inter- faces,” Colloids and Surfaces A: Phy...

    work page doi:10.1002/elps.200700734 2008