Impact of wettability on interface deformation and droplet breakup in microcapillaries

Droplet dynamics post-impingement on a solid surface is of immense technological importance due to its applicability in self-cleaning, anti-frosting, anti-biofouling, anti-icing, thermal management, and inkjet printing. While sustained droplet oscillation is desirable in some applications, a fast seizure of oscillations after the surface touchdown of the droplets is necessary for others. The post-impingement dynamics is a complex non-linear interaction of the fluid and solid properties and a common framework to characterize the short-time response of a droplet is provided here. We describe these post-impact droplet dynamics on a patterned surface in terms of lumped simple harmonic oscillator parameters. We delineate the effect of the substrate and fluid properties and attribute the same to one or more terms in the dynamical model of the oscillator, describing an analogy between the terms. Our analogy gives a universal mapping between the fluid–solid parameters and droplet dynamics, making it feasible to predict with accuracy the post-impact behavior. Here for the first time, we developed a simplified universal approach to predict the droplet oscillation dynamics impacting any solid surface for a broad range of fluid and solid properties and impingement conditions.

Touchdown dynamics of a liquid droplet on a patterned substrate: A generalized lumped parameter-based model

Mondal,

Biswas,

Mukherjee

et al. 2023

A numerical analysis of particle encapsulation in a flow-focusing droplet generation device

Fatehifar,

Revell,

Jabbari

et al. 2023

In this paper, the process of encapsulating solid particle(s) into liquid droplets in a high-throughput flow-focusing microchannel is investigated numerically. Open source software is used, which computes fluid flow in an Eulerian framework and particle dynamics with a Lagrangian approach. Previous studies have demonstrated that if no action is taken, particles suspended in a liquid passing through a flow-focusing microchannel will be encapsulated at random. This is perhaps unsurprising, but in one such study, less than 35% of droplets were found to contain exactly one particle. The two aims of this study are (i) to explore the flow patterns arising in a microfluidic channel and (ii) to elucidate the effect of salient governing parameters on encapsulation efficiency (i.e., the fraction of droplets encapsulating one particle) by focusing on ordering the particles before reaching the droplet generation section. Following validation against experimental reference data, the capillary number is varied across the three droplet generation regimes: squeezing, dripping, and jetting. We demonstrate that under certain conditions, an encapsulation frequency of 100% can be achieved with ordered particles, but in most cases, this is significantly lower. We examine the flow field to help understand how this non-uniform distribution of particles occurs. Notably, we find the dripping to be the best option for particle encapsulation and in this case extend the study to explore the effect of junction angle, finding that an angle of 60° is the most favorable. Improved understanding of the encapsulation process derived from this study can help to improve design of high-throughput droplet generation microfluidic systems.

How adsorbed proteins at solid interfaces impact the droplet propagation through micro capillaries

Giefer,

Heyse,

Drusch

et al. 2024

Proteins are used in food technology as biological emulsifiers which stabilize the liquid/liquid interface by lowering the interfacial tension due to their amphiphilic character. Due to the rising use of sensitive animal- and plant-based proteins in the food industry, homogenization methods with low-shear and low mechanical and thermal stresses like premix-membrane emulsification got more and more attention. However, one drawback remains: the proteins tend to adsorb to the membrane surfaces causing problems like fouling and pore-blocking. Recently, it was also found that the adsorbed proteins alter the wettability of the system which is suspected to affect the droplet breakup [Giefer et al., “Impact of wettability on interface deformation and droplet breakup in microcapillaries,” Phys. Fluids 35, 042110 (2023)], but the mechanism behind is still unknown. The study addresses the question of the extent to which proteins influence droplet propagation in micro-capillaries within membrane emulsification. Protein adsorption at liquid/liquid and liquid/solid interfaces is considered within molecular dynamic simulations and the effect on the interfacial tension is quantified. Considering the Young–Dupré equation [M. E. Schrader, “Young-dupre revisited,” Langmuir 11, 3585–3589 (1995)], interfacial energies are converted into contact angles, which are validated with experimental studies. For this purpose, idealized pore structures are considered under variation of the dynamic contact angle, for which the Kistler equation [S. Kistler, “Hydrodynamics of wetting,” Wettability (CRC Press, 1993), Vol. 6] is used. The results show that the protein adsorption to liquid/solid interfaces alters the wettability. Computational fluid dynamics reveal the dependence of the droplet propagation and interfacial instabilities in dependence on the wettability.