The collision of droplets with water-repellent surfaces has been studied for over a century because of its role in nature and technology, from self-cleaning to heat transfer. For Newtonian fluids, a complete rebound without splashing is possible only at moderate speeds; at high impact speeds, the droplet breaks into satellite droplets. Adding polymers makes the fluid viscoelastic, but it was previously thought that this suppresses rebound. This work stands apart: neighboring studies include droplets on glass, rain launching microplastics, and soft particles in turbulence, but no one had connected elasticity with complete rebound on a superhydrophobic surface at high speeds.
Droplets of aqueous polyacrylamide (PAA) solution, 2.5 mm in diameter, fell onto a Glaco superhydrophobic surface made of silica nanoparticles at speeds from 0.23 to 3.4 m/s. Polymer concentration was varied from 0.025 to 1% by mass, giving relaxation times from 0.6 ms to 7 s. The process was recorded with two high-speed cameras (up to 4900 frames/s) from the side and below. To understand the mechanics, axisymmetric numerical simulations were performed in Basilisk C with the L-PTT model for polymer stresses, using measured dynamic contact angles.
At Weber numbers above 136, water splashes, while viscoelastic droplets with polymer concentrations of 0.1% and above form a vertical filament (ligament) that stretches, develops a "balloon" at its end, and fully detaches from the surface. The rebound occurs without a single secondary droplet. The dynamic contact angle at filament detachment drops from 140° to 60°, indicating a transition from the Cassie-Baxter state to the Wenzel state: the liquid penetrates the surface's micro-gaps. Numerical simulation confirmed that the pressure peak at impact is tens of times higher than the pressure during Worthington jet formation, so impregnation occurs precisely at the moment of contact. The maximum filament length grows linearly with the Weber number, and the trajectory of the droplet's center of mass is described by a ballistic model: dissipative forces are an order of magnitude smaller than gravity.
This work shows for the first time that polymer additives do not hinder but promote complete rebound at high impact speeds, provided the surface has a microstructure. This differs from neighboring studies, where elasticity usually suppressed rebound or only Newtonian droplets were considered. Here, the key was not just rebound but the formation and detachment of a filament, governed by surface impregnation. This changes our understanding of how viscoelasticity interacts with superhydrophobicity.
The topic could develop toward a detailed study of the role of surface microstructure: pore size, shape, and distribution. It is important to understand how to control filament length and detachment time through fluid rheology and wettability. Surfaces that repel complex fluids without mass loss under extreme loads may emerge.
The results will be useful in inkjet printing, pesticide application, and cooling of hot surfaces, where avoiding splashes and fluid loss is crucial.
The next step is to systematically measure the threshold for the transition to the balloon regime as a function of surface roughness and wettability, and to test other polymers.
For droplet hydrodynamics, the open question remains predicting the conditions under which a viscoelastic filament does not break before detachment. This work shows that the key lies in the balance between elastic stresses and capillary pressure, but a universal criterion is still lacking.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This study examines the collision of water droplets containing polyacrylamide with a superhydrophobic Glaco surface at high speeds. Experiments and numerical simulations revealed that for Weber numbers above 136, the droplet forms a vertical filament that stretches, develops a 'balloon,' and detaches completely from the surface without producing secondary droplets. The key mechanism turned out to be the impregnation of the surface's micro-gaps at the moment of impact, which enhances adhesion and triggers filament growth; the polymer's elasticity prevents the filament from breaking.
Where its strength is
This work demonstrates for the first time that polymer additives at high impact speeds do not suppress but rather promote complete rebound when the surface is microstructured. Combining viscoelasticity with surface impregnation and the formation of a stable filament provides a new mechanism for controlling the rebound of complex fluids.
Where it could go next
Worth trying
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.56, against a usual 0.64. It looks like few have drilled nearby.
The pressure when a droplet hits a surface is comparable to the water pressure at a depth of several meters, but it acts for only a few microseconds.