Microscopic view of water droplets interacting with a chemically imperfect surface.

Trapped: How Surface Defects Control Droplet Movement—and Why It Matters

Theo Raines in Tech & Innovation September 2025 • 4 min read.

"Unlocking the secrets of droplet behavior on surfaces with chemical 'flaws' opens doors to revolutionary microfluidics and industrial applications."

In an era defined by miniaturization and precision, controlling the behavior of fluids at the microscale has become increasingly crucial. Droplets, those seemingly simple entities, play a pivotal role in a myriad of applications, from lab-on-a-chip diagnostics to advanced drug delivery systems. The ability to manipulate these droplets with accuracy and reliability is paramount for the continued advancement of these technologies.

A key factor in droplet manipulation is the interaction between the liquid droplet and the surface it encounters. Surface properties, such as wettability (the ability of a liquid to spread on a solid surface), play a significant role in determining how a droplet moves, spreads, or remains stationary. Traditionally, researchers have focused on creating surfaces with uniform wettability gradients to guide droplet movement. However, a growing body of evidence suggests that surface defects—tiny imperfections or variations in chemical composition—can also be harnessed to control droplet behavior.

A recent study published in Chemical Engineering Science sheds new light on this phenomenon, exploring how chemical defects on a surface can trap droplets in shear flows. By employing advanced numerical simulations, the researchers uncovered the complex interplay between surface properties, fluid dynamics, and droplet behavior, offering valuable insights for the design of more effective microfluidic devices and other droplet-based technologies.

What Are 'Wetting Defects' and How Do They Trap Droplets?

Microscopic view of water droplets interacting with a chemically imperfect surface.

Imagine a perfectly smooth surface, uniformly coated with a substance that attracts water. Now, introduce a small stripe-like region with a slightly different chemical composition, one that is less attractive to water. This is a wetting defect. These defects act as tiny anchors, creating regions of higher or lower wettability that can either attract or repel droplets. When a droplet encounters such a defect in a shear flow (a flow where different layers of the fluid move at different speeds), it can become trapped.

The trapping mechanism is complex, involving a delicate balance of forces:

Surface Tension: The cohesive forces between liquid molecules that minimize the surface area of the droplet.
Viscous Forces: The resistance to flow within the fluid, which is influenced by the shear flow.
Capillary Forces: Forces arising from the interaction of the liquid, the surrounding fluid, and the solid surface, dictated by the wettability of the surface.

The study reveals that the strength of the wetting defect, defined by the difference in wettability between the surrounding surface and the defect, is a critical factor in determining whether a droplet becomes trapped or escapes. However, the relationship is not straightforward. The researchers identified three distinct equilibrium states for trapped droplets, each characterized by a unique balance of forces at the triple-phase contact line—the point where the liquid, the surrounding fluid, and the solid surface meet.

The Future of Droplet Control: From Lab to Industry

This research provides valuable insights for designing surfaces with controlled wettability patterns to manipulate droplets with unprecedented precision. Imagine microfluidic devices that can sort cells, deliver drugs, or perform chemical reactions with unparalleled accuracy. The ability to trap and release droplets on demand opens up a world of possibilities for various applications, including: chemical sensors, microreactors and industrial coatings.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1016/j.ces.2018.09.041, Alternate LINK

Title: Droplets Trapped By A Wetting Surface With Chemical Defects In Shear Flows

Subject: Applied Mathematics

Journal: Chemical Engineering Science

Publisher: Elsevier BV

Authors: Xinglong Shang, Zhengyuan Luo, Bofeng Bai

Published: 2019-02-01

Everything You Need To Know

What are 'wetting defects' and how do they influence droplet behavior on surfaces?

Wetting defects are small imperfections or variations in chemical composition on a surface that alter its wettability. They can act as tiny anchors, creating regions that either attract or repel droplets. When a droplet encounters a wetting defect in a shear flow, it can become trapped due to the complex interplay of Surface Tension, Viscous Forces, and Capillary Forces. The strength of the wetting defect, defined by the difference in wettability between the defect and the surrounding surface, is crucial in determining droplet trapping.

How do Surface Tension, Viscous Forces, and Capillary Forces collectively determine whether a droplet becomes trapped by a wetting defect?

The trapping mechanism involves a delicate balance of Surface Tension, Viscous Forces, and Capillary Forces. Surface Tension minimizes the droplet's surface area, while Viscous Forces resist flow within the fluid influenced by the shear flow. Capillary Forces arise from the interaction of the liquid, the surrounding fluid, and the solid surface, dictated by the surface's wettability. The study identified three distinct equilibrium states for trapped droplets, each representing a unique balance of these forces at the triple-phase contact line.

What potential applications could arise from the ability to control droplet movement using surface defects?

The ability to trap and release droplets on demand opens up possibilities for applications such as precise microfluidic devices used to sort cells, deliver drugs, or perform chemical reactions with high accuracy. Further applications include the development of improved chemical sensors, more efficient microreactors, and advanced industrial coatings with tailored properties.

How does this research on droplet trapping relate to the advancement of microfluidics and lab-on-a-chip technology?

This research enhances the ability to manipulate droplets with precision, which is crucial for advancing microfluidics and lab-on-a-chip technology. By understanding how wetting defects influence droplet behavior, scientists and engineers can design more effective microfluidic devices for applications like diagnostics and drug delivery systems. The newfound control over droplet movement allows for more complex and precise operations at the microscale.

Why is the study of droplet behavior important for technological advancements, and what challenges are researchers trying to overcome?

Controlling droplet behavior is essential for advancements in fields requiring miniaturization and precision, such as lab-on-a-chip diagnostics and drug delivery systems. Researchers aim to overcome challenges related to manipulating droplets with accuracy and reliability. Traditionally, uniform wettability gradients were used, but recent studies explore harnessing surface defects to control droplet behavior. Understanding the interplay between Surface Tension, Viscous Forces, and Capillary Forces is vital for designing effective droplet-based technologies.