Droplets sitting on thin elastic sheets: A study with the boundary element method
Pith reviewed 2026-06-28 20:23 UTC · model grok-4.3
The pith
The equilibrium shape of a droplet lens on a clamped elastic sheet depends on sheet thickness, and isotropic stretching tunes its focal length via applied tension.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The boundary element simulations show that for a clamped elastic sheet the lens shape of the droplet crucially depends on the sheet thickness; isotropic stretching supplies an extra control parameter that changes the droplet shape and the tension distribution in the sheet, allowing the focal length of the liquid lens to be adjusted by varying the applied tension; uniaxial stretching elongates the droplet while generating folds and dimples.
What carries the argument
The boundary element method extension that solves the coupled elasto-capillary problem for three distinct boundary-condition protocols on the elastic sheet.
Load-bearing premise
The extended boundary element method together with the three boundary-condition protocols correctly reproduces the equilibrium shapes of the coupled system.
What would settle it
Direct comparison of simulated lens morphology against experiments on clamped sheets of systematically varied thickness would falsify the thickness dependence if the measured shapes show no systematic change with thickness.
read the original abstract
Elasto-capillarity of a droplet wetting an elastic sheet provides an interesting system, both for fundamental and applied research. The droplet sinks into the sheet and assumes the shape of a lens. To determine the equilibrium shape in simulations, we formulate a boundary element method (BEM) extending our earlier approaches, and apply the BEM to three specific protocols for the boundary conditions of the sheet. For a clamped elastic sheet, we use various morphological metrics to demonstrate that the lens shape crucially depends on the sheet thickness. Stretching the sheet isotropically, allows for an additional control parameter to influence the droplet shape and the tension in the sheet, which we quantify by radial profiles of the azimuthal and radial elastic stresses. We further demonstrate how the focal length of a liquid lens can be tuned by varying the applied tension. Finally, stretching the sheet along one direction, elongates the droplet, and the sheet shows folds and dimples.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript formulates a boundary element method (BEM) extending prior approaches to compute equilibrium shapes of droplets on thin elastic sheets under three boundary-condition protocols. For clamped sheets it reports that lens morphology depends on sheet thickness via morphological metrics and radial stress profiles; isotropic in-plane stretching is shown to tune droplet shape and focal length through applied tension; uniaxial stretching produces elongated droplets with folds and dimples.
Significance. If the numerical solutions faithfully represent the coupled elasto-capillary equilibrium, the results establish thickness and isotropic tension as practical control parameters for liquid-lens geometry on elastic supports, with direct relevance to tunable optics and soft-matter patterning. The BEM framework itself constitutes a reusable computational tool for this class of problems.
major comments (1)
- [Abstract / Methods] Abstract and § (Methods/BEM formulation): the central claims on thickness dependence and tension-tuned focal length rest on the fidelity of the BEM extension for fluid-elastic coupling, yet no explicit validation is presented against analytical limits such as the rigid-substrate spherical-cap solution or the small-deformation plate-theory limit for thin sheets. Without such checks, discretization or contact-line artifacts cannot be ruled out as the source of the reported morphological trends.
Simulated Author's Rebuttal
We thank the referee for their detailed review and for highlighting the importance of validating the BEM extension. We address the single major comment below and will incorporate the requested checks in a revised manuscript.
read point-by-point responses
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Referee: [Abstract / Methods] Abstract and § (Methods/BEM formulation): the central claims on thickness dependence and tension-tuned focal length rest on the fidelity of the BEM extension for fluid-elastic coupling, yet no explicit validation is presented against analytical limits such as the rigid-substrate spherical-cap solution or the small-deformation plate-theory limit for thin sheets. Without such checks, discretization or contact-line artifacts cannot be ruled out as the source of the reported morphological trends.
Authors: We agree that explicit validation against analytical limits strengthens the manuscript. Our BEM formulation extends prior work on fluid-structure problems, but the current text does not include direct comparisons for the coupled droplet-sheet case. In the revised version we will add a dedicated validation subsection (likely in Methods) that (i) recovers the spherical-cap solution on a rigid substrate in the limit of large sheet thickness and (ii) reproduces the small-deformation plate-theory predictions for thin sheets under isotropic tension. These checks will be performed with the same discretization and contact-line treatment used for the reported results, thereby confirming that the observed thickness and tension trends are not numerical artifacts. revision: yes
Circularity Check
Minor self-citation for BEM extension; central morphological results are independent numerical outputs with no reduction to fitted inputs or self-citation chains
full rationale
The paper formulates and applies a boundary element method (BEM) to solve the coupled elasto-capillary problem for three boundary-condition protocols. Central claims (thickness dependence of lens shape for clamped sheets; tension-tuned focal length under isotropic stretch) are obtained directly as outputs of these numerical solutions. The abstract notes an 'extension of our earlier approaches,' constituting a self-citation, but this pertains only to the solver formulation and is not invoked to establish uniqueness theorems, forbid alternatives, or force any reported quantity by construction. No parameters are fitted to data subsets and then presented as predictions, no ansatzes are smuggled via citation, and no renaming of known results occurs. The derivation chain is a standard numerical discretization of the physical model, rendering the results independent of the inputs. This is the expected non-circular outcome for a simulation study.
Axiom & Free-Parameter Ledger
Reference graph
Works this paper leans on
-
[1]
In: Fluid-Structure Interac- tions in Low-Reynolds-Number Flows, pp
Duprat, C., Stone, H.A.: Chapter 6: Elas- tocapillarity. In: Fluid-Structure Interac- tions in Low-Reynolds-Number Flows, pp. 193–246. The Royal Society of Chemistry, Cambridge (2016). https://doi.org/10.1039/ 9781782628491-00193
2016
-
[2]
Bico, J., Reyssat, ´E., Roman, B.: Elasto- capillarity: When surface tension deforms elastic solids. Annu. Rev. Fluid Mech. 50(1), 629–659 (2018) https://doi.org/10. 1146/annurev-fluid-122316-050130 14
2018
-
[3]
Andreotti, B., Snoeijer, J.H.: Statics and dynamics of soft wetting. Annu. Rev. Fluid Mech.52(1), 285–308 (2020) https://doi.org/ 10.1146/annurev-fluid-010719-060147
-
[4]
Shuttleworth, R.: The surface tension of solids. Proc. Phys. Soc. Sect. A63(5), 444–457 (1950) https://doi.org/10.1088/ 0370-1298/63/5/302
1950
-
[5]
M¨ uller, P., Sa´ ul, A.: Elastic effects on sur- face physics. Surf. Sci. Rep.54(5), 157– 258 (2004) https://doi.org/10.1016/j.surfrep. 2004.05.001
-
[6]
EPL113, 66001 (2016) https://doi.org/ 10.1209/0295-5075/113/66001
Andreotti, B., Snoeijer, J.H.: Soft wetting and the Shuttleworth effect, at the cross- roads between thermodynamics and mechan- ics. EPL113, 66001 (2016) https://doi.org/ 10.1209/0295-5075/113/66001
-
[7]
Chen, L., Bonaccurso, E., Gambaryan- Roisman, T., Starov, V., Koursari, N., Zhao, Y.: Static and dynamic wetting of soft sub- strates. Curr. Opin. Colloid Interface Sci.36, 46–57 (2018) https://doi.org/10.1016/j.cocis. 2017.12.001
-
[8]
Park, S.J., Weon, B.M., Lee, J.S., Lee, J., Kim, J., Je, J.H.: Visualization of asymmet- ric wetting ridges on soft solids with x-ray microscopy. Nat. Comm.5(1), 4369 (2014) https://doi.org/10.1038/ncomms5369
-
[9]
Xu, Q., Jensen, K.E., Boltyanskiy, R., Sarfati, R., Style, R.W., Dufresne, E.R.: Direct mea- surement of strain-dependent solid surface stress. Nat. Comm.8(1), 555 (2017) https: //doi.org/10.1038/s41467-017-00636-y
-
[10]
Gorcum, M., Andreotti, B., Snoeijer, J.H., Karpitschka, S.: Dynamic solid surface ten- sion causes droplet pinning and depinning. Phys. Rev. Lett.121, 208003 (2018) https: //doi.org/10.1103/PhysRevLett.121.208003
-
[11]
Style, R.W., Che, Y., Park, S.J., Weon, B.M., Je, J.H., Hyland, C., German, G.K., Power, M.P., Wilen, L.A., Wettlaufer, J.S., Dufresne, E.R.: Patterning droplets with durotaxis. Proc. Natl. Acad. Sci. U.S.A. 110(31), 12541–12544 (2013) https://doi. org/10.1073/pnas.1307122110
-
[12]
Langmuir35(23), 7571–7577 (2019) https://doi.org/10.1021/ acs.langmuir.8b02037
Alert, R., Casademunt, J.: Role of substrate stiffness in tissue spreading: Wetting transi- tion and tissue durotaxis. Langmuir35(23), 7571–7577 (2019) https://doi.org/10.1021/ acs.langmuir.8b02037
2019
-
[13]
Lee, I.-N., Dobre, O., Richards, D., Ballestrem, C., Curran, J.M., Hunt, J.A., Richardson, S.M., Swift, J., Wong, L.S.: Photoresponsive hydrogels with photoswitchable mechanical properties allow time-resolved analysis of cellular responses to matrix stiffening. ACS Appl. Mater. Interfaces10(9), 7765–7776 (2018) https://doi.org/10.1021/acsami.7b18302
-
[14]
Aland, S., Mokbel, D.: A unified numeri- cal model for wetting of soft substrates. Int. J. Numer. Methods Eng.122(4), 903–918 (2021) https://doi.org/10.1002/nme.6567
-
[15]
Gomez, H., Velay-Lizancos, M.: Thin-film model of droplet durotaxis. Eur. Phys. J. Special Top.229(2), 265–273 (2020) https: //doi.org/10.1140/epjst/e2019-900127-x
-
[16]
Science354(6317), 1240–1241 (2016) https://doi.org/10.1126/science
Huang, J., Juszkiewicz, M., Jeu, W.H., Cerda, E., Emrick, T., Menon, N., Rus- sell, T.P.: Capillary wrinkling of floating thin polymer films. Science317(5838), 650– 653 (2007) https://doi.org/10.1126/science. 1144616
-
[17]
Soft Matter9, 8289–8296 (2013) https://doi.org/10.1039/ C3SM50736J
Toga, K.B., Huang, J., Cunningham, K., Russell, T.P., Menon, N.: A drop on a float- ing sheet: boundary conditions, topography and formation of wrinkles. Soft Matter9, 8289–8296 (2013) https://doi.org/10.1039/ C3SM50736J
2013
-
[18]
Schroll, R.D., Adda-Bedia, M., Cerda, E., Huang, J., Menon, N., Russell, T.P., Toga, K.B., Vella, D., Davidovitch, B.: Capillary deformations of bendable films. Phys. Rev. Lett.111, 014301 (2013) https://doi.org/10. 1103/PhysRevLett.111.014301
2013
-
[19]
Schulman, R.D., Dalnoki-Veress, K.: Liquid droplets on a highly deformable membrane. Phys. Rev. Lett.115, 206101 (2015) https: 15 //doi.org/10.1103/PhysRevLett.115.206101
-
[20]
Schulman, R.D., Ledesma-Alonso, R., Salez, T., Rapha¨ el, E., Dalnoki-Veress, K.: Liquid droplets act as “compass needles” for the stresses in a deformable membrane. Phys. Rev. Lett.118, 198002 (2017) https://doi. org/10.1103/PhysRevLett.118.198002
-
[21]
Smith-Mannschott, K., Xu, Q., Heyden, S., Bain, N., Snoeijer, J.H., Dufresne, E.R., Style, R.W.: Droplets sit and slide anisotropi- cally on soft, stretched substrates. Phys. Rev. Lett.126, 158004 (2021) https://doi.org/10. 1103/PhysRevLett.126.158004
2021
-
[22]
Schulman, R.D., Dalnoki-Veress, K.: Droplets capped with an elastic film can be round, elliptical, or nearly square. Phys. Rev. Lett. 121, 248004 (2018) https://doi.org/10.1103/ PhysRevLett.121.248004
2018
-
[23]
Science359(6377), 775–778 (2018) https:// doi.org/10.1126/science.aao1290
Kumar, D., Paulsen, J.D., Russell, T.P., Menon, N.: Wrapping with a splash: High- speed encapsulation with ultrathin sheets. Science359(6377), 775–778 (2018) https:// doi.org/10.1126/science.aao1290
-
[24]
Soft Mat- ter14, 4913–4934 (2018) https://doi.org/10
Davidovitch, B., Vella, D.: Partial wetting of thin solid sheets under tension. Soft Mat- ter14, 4913–4934 (2018) https://doi.org/10. 1039/C8SM00323H
2018
-
[25]
Shanahan, M.E.R.: Contact angle equilib- rium on thin elastic solids. J. Adhesion 18(4), 247–267 (1985) https://doi.org/10. 1080/00218468508080461
1985
-
[26]
Shanahan, M.E.R.: Equilibrium of liq- uid drops on thin plates; plate rigidity and stability considerations. J. Adhesion 20(4), 261–274 (1987) https://doi.org/10. 1080/00218468708074946
1987
-
[27]
Kozyreff, G., Davidovitch, B., Prasath, S.G., Palumbo, G., Brau, F.: Effect of external ten- sion on the wetting of an elastic sheet. Phys. Rev. E107, 035101 (2023) https://doi.org/ 10.1103/PhysRevE.107.035101
-
[29]
Li, Z., Ren, W.: The motion of a thin drop on an elastic sheet. J. Fluid Mech. 1022, 4 (2025) https://doi.org/10.1017/jfm. 2025.10779
work page doi:10.1017/jfm 2025
-
[30]
Brinker, M., Dittrich, G., Richert, C., Lakner, P., Krekeler, T., Keller, T.F., Huber, N., Huber, P.: Giant electrochemical actuation in a nanoporous silicon-polypyrrole hybrid material. Sci. Adv.6(40), 1483 (2020) https: //doi.org/10.1126/sciadv.aba1483
-
[31]
Brinker, M., Huber, P.: Wafer-scale electroactive nanoporous silicon: Large and fully reversible electrochemo- mechanical actuation in aqueous electrolytes. Adv. Mater., 2105923 (2021) https://doi.org/10.1002/adma.202105923
-
[32]
Berge, B., Peseux, J.: Variable focal lens controlled by an external voltage: An appli- cation of electrowetting. Eur. Phys. J. E 3(2), 159–163 (2000) https://doi.org/epje/ v3/p159(e9028)
2000
-
[33]
Biomicrofluidics4(3), 031501 (2010) https://doi.org/10.1063/1.3460392
Nguyen, N.-T.: Micro-optofluidic lenses: A review. Biomicrofluidics4(3), 031501 (2010) https://doi.org/10.1063/1.3460392
-
[34]
Frontiers in Robotics and AI8, 166 (2021) https://doi.org/10.3389/frobt
Chen, L., Ghilardi, M., Busfield, J.J.C., Carpi, F.: Electrically tunable lenses: A review. Frontiers in Robotics and AI8, 166 (2021) https://doi.org/10.3389/frobt. 2021.678046
-
[35]
In: 2020 IEEE Aerospace Conference, pp
Fogle, F., Cierny, O., Vale Pereira, P., Kam- merer, W., Cahoy, K.: Miniature optical steerable antenna for intersatellite communi- cations liquid lens characterization. In: 2020 IEEE Aerospace Conference, pp. 1–13 (2020). https://doi.org/10.1109/AERO47225.2020. 9172448
-
[36]
Nature 591(7848), 142–146 (2021) https://doi.org/ 10.1038/s41586-020-2992-3
Agudo-Canalejo, J., Schultz, S.W., Chino, H., Migliano, S.M., Saito, C., Koyama- Honda, I., Stenmark, H., Brech, A., May, A.I., Mizushima, N., Knorr, R.L.: Wet- ting regulates autophagy of phase-separated 16 compartments and the cytosol. Nature 591(7848), 142–146 (2021) https://doi.org/ 10.1038/s41586-020-2992-3
-
[37]
Journal of Cell Biology220(10), 202103175 (2021) https://doi.org/10.1083/jcb.202103175
Kusumaatmaja, H., May, A.I., Knorr, R.L.: Intracellular wetting mediates con- tacts between liquid compartments and membrane-bound organelles. Journal of Cell Biology220(10), 202103175 (2021) https://doi.org/10.1083/jcb.202103175
-
[38]
Nature634(8036), 1204–1210 (2024) https: //doi.org/10.1038/s41586-024-07990-0
Wang, Y., Li, S., Mokbel, M., May, A.I., Liang, Z., Zeng, Y., Wang, W., Zhang, H., Yu, F., Sporbeck, K., Jiang, L., Aland, S., Agudo-Canalejo, J., Knorr, R.L., Fang, X.: Biomolecular condensates mediate bend- ing and scission of endosome membranes. Nature634(8036), 1204–1210 (2024) https: //doi.org/10.1038/s41586-024-07990-0
-
[39]
Mokbel, M., Mokbel, D., Liese, S., Weber, C., Aland, S.: A simulation method for the wetting dynamics of liquid droplets on deformable membranes. SIAM J. Sc. Com- put46(6), 806–829 (2024) https://doi.org/ 10.1137/24M1641142
-
[40]
Soft Matter17, 2454 (2021) https://doi.org/ 10.1039/d0sm02082f
Grawitter, J., Stark, H.: Steering droplets on substrates using moving steps in wettability. Soft Matter17, 2454 (2021) https://doi.org/ 10.1039/d0sm02082f
-
[41]
Soft Mat- ter17, 9469 (2021) https://doi.org/10.1039/ d1sm01113h
Grawitter, J., Stark, H.: Droplets on sub- strates with oscillating wettability. Soft Mat- ter17, 9469 (2021) https://doi.org/10.1039/ d1sm01113h
2021
-
[42]
Soft Matter20, 3161 (2024) https://doi.org/10.1039/d4sm00213j
Grawitter, J., Stark, H.: Steering droplets on substrates with plane-wave wettability pat- terns and deformations. Soft Matter20, 3161 (2024) https://doi.org/10.1039/d4sm00213j
-
[43]
Cambridge University Press, Cam- bridge (1992)
Pozrikidis, C.: Boundary Integral and Sin- gularity Methods for Linearized Viscous Flow. Cambridge University Press, Cam- bridge (1992)
1992
-
[44]
Dover Publications, Mineola/NY (2005)
Kim, S., Karrila, S.J.: Microhydrodynamics. Dover Publications, Mineola/NY (2005)
2005
-
[45]
Soft Matter13, 3544–3555 (2017) https://doi
Schaaf, C., Stark, H.: Inertial migration and axial control of deformable capsules. Soft Matter13, 3544–3555 (2017) https://doi. org/10.1039/C7SM00339K
-
[46]
Soft Matter17, 4804–4817 (2021) https://doi.org/10.1039/ D1SM00276G
Patel, K., Stark, H.: A pair of particles in inertial microfluidics: effect of shape, softness, and position. Soft Matter17, 4804–4817 (2021) https://doi.org/10.1039/ D1SM00276G
2021
-
[47]
Skalak, R., Tozeren, A., Zarda, R.P., Chien, S.: Strain energy function of red blood cell membranes. Biophys. J.13, 245 (1973) https: //doi.org/10.1016/S0006-3495(73)85983-1
-
[48]
Springer, ??? (2012)
Kr¨ uger, T.: Computer Simulation Study of Collective Phenomena in Dense Suspen- sions of Red Blood Cells Under Shear. Springer, ??? (2012). https://doi.org/10. 1007/978-3-8348-2376-2
2012
-
[49]
theory and possible experiments
Helfrich, W.: Elastic properties of lipid bilay- ers. theory and possible experiments. Z. Naturforsch. C28, 693–703 (1973) https:// doi.org/10.1515/znc-1973-11-1209
-
[50]
Kantor, Y., Nelson, D.R.: Crumpling transi- tion in polymerized membranes. Phys. Rev. Lett.58(26), 2774–2777 (1987) https://doi. org/10.1103/PhysRevLett.58.2774
-
[51]
Gompper, G., Kroll, D.M.: Random surface discretizations and the renormalization of the bending rigidity. J. Phys. I6(10), 1305–1320 (1996) https://doi.org/10.1051/jp1:1996246
-
[52]
Moffatt, H.K.: Viscous and resistive eddies near a sharp corner. J. Fluid Mech. 18, 1–18 (1964) https://doi.org/10.1017/ S0022112064000015
1964
-
[53]
Alinovi, E., Bottaro, A.: A boundary ele- ment method for stokes flows with interfaces. J. Comp. Phys.356, 261–281 (2018) https: //doi.org/10.1016/j.jcp.2017.12.004
-
[54]
Langmuir13, 7293 (1997) https: //doi.org/10.1021/la970825v 17
Ruijter, M.J., De Coninck, J., Blake, T.D., Clarke, A., Rankin, A.: Contact angle relax- ation during the spreading of partially wet- ting drops. Langmuir13, 7293 (1997) https: //doi.org/10.1021/la970825v 17
-
[55]
Lee, Y.-J., Lim, S.-M., Yi, S.-M., Lee, J.- H., Kang, S.-g., Choi, G.-M., Han, H.N., Sun, J.-Y., Choi, I.-S., Joo, Y.-C.: Aux- etic elastomers: Mechanically programmable meta-elastomers with an unusual poisson’s ratio overcome the gauge limit of a capacitive type strain sensor. Extreme Mech. Lett.31, 100516 (2019) https://doi.org/10.1016/j.eml. 2019.100516
-
[56]
Course of Theoreti- cal Physics, Vol
Landau, L.D., Lifshitz, E.M.: Theory of Elasticity, 3rd edn. Course of Theoreti- cal Physics, Vol. 7. Elsevier Butterworth- Heinemann, Oxford (1986). https://doi.org/ 10.1016/B978-0-08-057069-3.50003-6
-
[57]
Fortes, M.A.: Microscopic and macroscopic contact angles. J. Chem. Soc., Faraday Trans. I78, 101–107 (1982) https://doi.org/10. 1039/F19827800101
1982
-
[58]
Measurement Science and Technology8(6), 601–605 (1997) https: //doi.org/10.1088/0957-0233/8/6/003
Rheims, J., K¨ oser, J., Wriedt, T.: Refractive- index measurements in the near-ir using an abbe refractometer. Measurement Science and Technology8(6), 601–605 (1997) https: //doi.org/10.1088/0957-0233/8/6/003
-
[59]
Vella, D., Mahadevan, L.: The “Cheerios effect”. Am. J. Phys.73, 817–825 (2005) https://doi.org/10.1119/1.1898523 18
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