Pinning–depinning transition of droplets on inclined substrates with a three-dimensional topographical defect

Mhatre, Ninad V.; Kumar, Satish

doi:10.1039/d4sm00081a

Soft Matter

2024

DOI: 10.1039/d4sm00081a

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Pinning–depinning transition of droplets on inclined substrates with a three-dimensional topographical defect

Ninad V. Mhatre,

Satish Kumar

Abstract: Droplets on inclined substrates can depin and slide freely above a critical substrate inclination angle. Pinning can be caused by topographical defects on the substrate, and understanding the influence of...

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2024

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Cited by 3 publications

References 55 publications

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Shear-induced depinning of thin droplets on rough substrates

Mhatre,

Kumar

2024

J. Fluid Mech.

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Depinning of liquid droplets on substrates by flow of a surrounding immiscible fluid is central to applications such as cross-flow microemulsification, oil recovery and waste cleanup. Surface roughness, either natural or engineered, can cause droplet pinning, so it is of both fundamental and practical interest to determine the flow strength of the surrounding fluid required for droplet depinning on rough substrates. Here, we develop a lubrication-theory-based model for droplet depinning on a substrate with topographical defects by flow of a surrounding immiscible fluid. The droplet and surrounding fluid are in a rectangular channel, a pressure gradient is imposed to drive flow and the defects are modelled as Gaussian-shaped bumps. Using a precursor-film/disjoining-pressure approach to capture contact-line motion, a nonlinear evolution equation is derived describing the droplet thickness as a function of distance along the channel and time. Numerical solutions of the evolution equation are used to investigate how the critical pressure gradient for droplet depinning depends on the viscosity ratio, surface wettability and droplet volume. Simple analytical models are able to account for many of the features observed in the numerical simulations. The influence of defect height is also investigated, and it is found that, when the maximum defect slope is larger than the receding contact angle of the droplet, smaller residual droplets are left behind at the defect after the original droplet depins and slides away. The model presented here yields considerably more information than commonly used models based on simple force balances, and provides a framework that can readily be extended to study more complicated situations involving chemical heterogeneity and three-dimensional effects.

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Shear-induced depinning of thin droplets on rough substrates

Mhatre,

Kumar

2024

J. Fluid Mech.

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Uphill directional passive transport of water droplets on axisymmetric surfaces

Mo,

Huang,

Wang

et al. 2024

Physics of Fluids

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Spontaneous directional transport of droplets by a surface curvature gradient, adopted by many biological species such as cactus and sand moss, is particularly suitable for applications including anti-icing, self-cleaning, and water harvesting, which eliminates the need for external energy input. However, this directional droplet transport is limited to short transport distance and no maneuverability, i.e., droplets can only migrate toward a flatter region and gradually stop. Fixed structures that can regulate droplet movement, if they could be created, would significantly advance their applications in a variety of areas. In this work, we propose a method to regulate the spontaneous motion of droplets on solid surfaces using surface curvature gradients. Molecular dynamics simulations show that droplets on general bowl-shaped axisymmetric surfaces can travel in the uphill direction (from the base to the apex) and move continuously to the apex with almost a constant speed. The mechanisms governing opposite directional transport of droplets are explained, and the conditions required to guarantee the transport in the reversed direction are discussed.

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To spill or not: Short-time pouring dynamics of a toppled liquid bottle

Roy,

Kumar,

Ray

et al. 2024

Physics of Fluids

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A typical culinary setting involves liquid condiments with different constitutive behaviors stored in jars, bottles, pitchers, or spouts. In the dynamic kitchen environment, handling these condiments might require pouring, drizzling, squeezing, or tapping, demonstrating the interplay of the container geometry, the fluid properties, and the culinary expertise. There is, of course, the occasional accidental toppling. We investigate the combined effects of surface properties, fluid properties, and confinement dimensions on the short-time spilling or pouring dynamics of a toppled cuvette. While attesting to the fact that smaller cuvettes (which can be termed as capillaries as well) do not spontaneously spill, larger cuvettes exhibit spilling dynamics that are dependent on the surface property, fluid viscosity, and flow rheology. For Newtonian liquids, it is observed that the spilling dynamics are determined largely by the coupling of viscous and gravity forces with surface properties, inducing non-intuitive behavior at higher conduit dimensions. The inclusion of rheology for non-Newtonian liquids in the soup makes the spilling dynamics not only an interplay surface and fluid properties but also a function of meniscus retraction demarcating a “splatter” of three regimes “not spilling,” “on the verge of spilling,” and “spontaneous spilling.” We not only delineate the interactions leading to meniscus motion but also provide a mapping on whether or not a container would spill if it is momentarily toppled and then immediately returned to upright position. This study aids in understanding the fascinating physics of fluid pouring dynamics and could lead to new kitchen, biomedical, and industrial technologies.

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Pinning–depinning transition of droplets on inclined substrates with a three-dimensional topographical defect

Abstract: Droplets on inclined substrates can depin and slide freely above a critical substrate inclination angle. Pinning can be caused by topographical defects on the substrate, and understanding the influence of...

Cited by 3 publications

References 55 publications

Shear-induced depinning of thin droplets on rough substrates

Shear-induced depinning of thin droplets on rough substrates

Uphill directional passive transport of water droplets on axisymmetric surfaces

To spill or not: Short-time pouring dynamics of a toppled liquid bottle

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