Session J18: Complex Surfaces and Drops

Modeling The Maximum Deposition of a Core-shell Compound Droplet Impacting a Solid Surface

Compound droplets composed of core and shell liquids have earned significant attention in recent years, owing to their potential advantages in various industrial applications including inkjet printing, 3D printing, pharmaceutics and food industry. Predicting and controlling the maximum deposition of such drops after impact on solid surfaces is crucial for these applications. The maximum spreading factor, which is the maximum deposition normalized by the initial droplet diameter, is determined by the energy budget of the falling droplet before impact and at the maximum spreading. A semi-analytical model has been derived using the energy balance of the primary driving forces of the spreading phase namely, kinetic energy, interfacial energy and viscous dissipation. To solve the energy balance equation, simplifications are utilized, particularly concerning the geometry and viscous dissipation terms for the two segments of the compound drop. The model has been validated with the experimental investigations of the low Weber number water-in-oil drop impact events within a wide range of core sizes and viscosity values of the core and the shell layers.  

  

Spreading of drops exhibiting complex rheology

Understanding the drop spreading behaviour of rheologically complex liquids, like dense granular suspensions or concentrated polymer solutions, is of crucial importance for applications such as inkjet printing, spraying paints or coating. In this presentation, we focus in the correlation between complex rheological behavior, like shear thickening or shear thinning, and the on the early dynamics of drop spreading on hard substrates. 

As shear thickening systems, we study the spreading behaviour of suspensions on clean hydrophilic glass surfaces, with increasing particle mass fractions (Φ = 0.3 to 0.65) and varying particle diameter (d = 10 to 20 μm), chemistry and morphology. Depending on the particle mass fraction, being close or not to the jamming transition of the suspension, two outcomes were observed: the spreading curve either exhibited a spreading behaviour qualitatively similar to the one of the Newtonian carrier liquid, with two spreading regimes or a spreading behaviour diverging from the reference, with three spreading regimes and an increase in spreading rate in-between two. The different spreading behavior strongly correlates with an increasing shear thickening of the suspensions. The correlation between rheological behavior and spreading dynamics is general for all investigates particle types. Shear thinning polymer solution also exhibit strong correlation of the spreading dynamics with the rheological properties of the solutions, like the shear rate at the end of the Newtonian plateau or the exponent in an assumed power law model. In both examples the relation of the time scale of the experiment and the intrinsic relaxation time of the sample is relevant and will be discussed. 

In both cases, we relate the observed correlations between rheological behaviour and spreading dynamics of drops to the dissipative mechanisms close to the moving contact line of the drop.

    

Shape Deformation of Liquid Droplets Falling in a Liquid Media

We experimentally investigate the dynamics of aqueous droplets falling through a stationary oleic micellar solution under gravity. Our study focuses on understanding the influence of droplet size, interfacial tension, nanoparticle concentration in droplet, and surfactant concentration in the continuous phase on droplet deformation. Side view images of the falling process reveal that the DI-water drop undergoes a sinusoidal deformation cycle from a sphere shape to oval and ultimately pancake shape. However, the addition of nanoparticles in the aqueous drop leads to a greater droplet deformation but reduced oscillation. Ultimately, at the nanoparticle concentrations above 4 wt.%, the oscillations decay over time where the droplets keep their flattened pancake shape. While the ultra-low interfacial tension contributes in the pancake drop shape, it is hypothesized that the formation of a viscoelastic interfacial layer, made of emulsions, at the drop interface attenuates the drop oscillation. These findings have potential applications in various fields including microfluidics, emulsion formation, and industrial processes involving liquid-liquid interactions.   

Investigating the impact of nanoparticles and surfactants on the surface wettability

Wettability is one of the key parameters that affect the shape of droplets on a surface and thus affects the droplet interactions. Wettability is found to have effects on cell membrane remodeling, water transport in earth soil, protein, and biomolecular adsorption, and nanomaterial synthesis. Various properties have been observed to manipulate the wettability of liquid droplets affecting their behavior and can change the contact angle between the liquid droplets and solid surface, thus changing the wettability. Seminal studies have focused on the effect of surfactants or nanoparticles on wettability and contact angle, but the dynamic and temporal behavior of contact angle variations as a function of time is not well understood. In particular, the mechanism behind the impact of those chemicals on the droplet contact angle is yet to be explored and there is lacking quantitative study of the impact of surfactants and nanoparticles on surface wettability. The objective of this research is to examine the behavior of surfactants and nanoparticles with a specific emphasis on their effectiveness in wettability alteration through experimental measurements via the drop shape analyzer. The results obtained from this study are expected to advance the fundamental understanding of the behavior of liquid droplets in contact with solid surfaces with the addition of various chemical additives.   

Drop Impact of suspensions of fibers on a solid surface

The impact and spreading of drops of particulate suspensions on solid surfaces is relevant in various industrial applications such as in inkjet printing, or spray coating tablets in the pharmaceutical industry. The capillary dynamics of elongated particles, such as fibers, are also relevant for bioprinting cells, or fiber suspensions used for printing fiber composites. However, the role and dynamics of fiber suspensions during capillary flows remain elusive. In particular, fibers can influence the impact of a droplet, from its spreading to its fragmentation. In this study, we experimentally investigate the impact of droplets of fiber suspensions on a hydrophilic surface. We report the evolution of the spreading dynamics, film thickness, and fiber orientation as a function of the radial distance for fiber suspensions of varying mass fraction and aspect ratios. Our experiments show a substantial modification in the spreading dynamics of the drop due to the presence of fibers.   

Binary Mixture Droplet Wetting and Evaporation Phase Change on Hydrophilic Structured Surfaces

There is a promising future for the development of structurally and chemically decorated surfaces in a variety of applications, including everyday practices as well as industrial and biomedical applications. When structured surfaces are used, their intrinsic hydrophobicity/hydrophilicity strongly affects the wettability and evaporation process of microliter droplets. In this work, pure water, pure ethanol, and their binary mixtures are used to examine their wettability and evaporation behaviour on 6 intrinsically hydrophilic micro-structured surfaces having the same height-to-diameter aspect ratio and varying the spacing between pillars. Upon deposition, the wettability of droplets on short-spacing surfaces depends highly on the liquid surface tension and spacing between pillars; however, on large-spacing surfaces, droplets behave similarly as on their smooth counterparts independently of the liquid studied. Thereafter, the full evaporation process for the same liquids is examined on the same hydrophilic structured surfaces, where three different evaporative modes are revealed: the constant contact radius (pinning), stick-slip mode and mixed mode, in the absence of the constant contact angle mode. The duration of each mode has been analysed to clearly show the dependency of the evaporation modes on the different initial wetting regimes and liquid surface tensions used. Accordingly, choosing the proper spacing of the structure combined with the proper binary mixture concentration, the initial wetting regime, the initial pinning time, and the duration of evaporation modes can be optimized according to the application and objective, making these fundamentals useful in a variety of biological, agricultural and medical fields among others.   

Impacting Dynamics of Surfactant-Laden Droplets

Surfactants are often used in liquid systems to modify surface tension and thus interfacial behaviour - for instance, to improve or adjust wetting in poorly wetting surfaces. In inkjet and spray technologies, surfactants are used to influence spreading, and therefore control the ink & paint coverage, and drop formation dynamics. In this work, we study the effect of surfactants on the behavior of droplets impacting flat dry surfaces. We combine high-speed imaging and quantitative image analysis to analyse the spreading dynamics on different surfaces through the dynamic contact angle. In our experiments, we use various surfactant solutions with the dynamic surface tension being characterised by high-speed tensiometry at time scales relevant to the droplet impact dynamics. By comparing the effects of different surfactants, surfaces, and dynamic conditions, we have gained insights into how the surfactant-induced dynamic surface tension influences the overall post-impact dynamics.   

The skating of drops impacting over gas or vapor layers

We report numerical simulations confirming the predictions and scaling relationships deduced in Gordillo and Riboux, JFM 941, A10, 2022. where we elucidated the lubrication mechanism by which a drop of a low viscosity liquid impacting over a smooth solid substrate skates over a thin gas or vapor layer that prevents the contact with the wall. In contrast, our numerical results do not validate the approximate equations given in Mandre and Brenner, JFM 690, 148–172, 2012, who claimed that the minimum thickness of the gas film separating the drop from the substrate is attained when a certain type of self-similar solution, valid right after the impact, can no longer describe the unsteady flow near the wall. Moreover, with the purpose of explaining the so-called lift-off mechanism reported in Kolinski, et al, PRL, 112 (13), 134501, 2014 here we derive expressions for the time-varying thickness of the gas layer at the region where the pressure is maximum, finding that these equations closely follow the numerical results.   

Microbubble entrainment on inclined thin liquid films during drop impact.

Drop impact causes air entrainment between the drop and solid substrate due to a large increase in the pressure underneath the drop, which distorts the bottom interface, creating a thin gas film O(10 nm – 1 μm).We propose that the creation of a central disc of air generates microbubbles while the air escapes. In this study, we report new experimental results on drop impact on an immiscible thin film to show the dynamics of microbubble entrainment. Using high speed imaging, we visualized the formation of the microbubbles across various surface inclination angles and normalized Weber numbers (0 ≤ θ ≤ 75; 5.6 ≤ We_n ≤ 112). We found that with increasing inclination angle, the microbubbles appear even at high We numbers, We_n ~ 112, and that the area of microbubbles nonmonotonically increases then decreases at an optimum We.