In specially coated tubes, the more viscous liquid flows faster

Droplet in a capillary
A schematic illustration of the experimental set-up for imaging motion of tracer particles in a liquid droplet. Credit: Aalto University

The rate of flow of different types of fluids moving along pipes has critical ramifications when you consider physiological functions like the cardiovascular system. The speed of blood flow through the heart and its network is a finely tuned, sensitive balance. There are a wide range of industrial processes that must also factor in the rate of flow of fluids through pipework, like water pipelines, widescale irrigation systems or oil refineries.

It is very well known that high viscosity liquids “flow” along a surface more slowly than a much less viscous liquid, such as water, which simply “runs off”. We have all observed the simple phenomenon of the speed of movement of various fluids. When making a sandwich, we notice that honey, being quite a viscous liquid, flows noticeably more slowly than water does. And no-one would dispute that glycerol, a thick, sticky sugar alcohol also known as glycerine, demonstrates the same slower flow properties compared with water.

However, research has revealed that these widely accepted flow characteristics are completely reversed when liquids flow through chemically coated capillaries. Even more surprising is that liquids even a thousand times more viscous than water will flow ten times faster than water through capillaries.

One method of making a fluid flow through a pipe at a faster rate, is to increase the pressure on it. There are limits on how much pressure you can apply, however, before it reaches a level that risks bursting the pipe. This problem is particularly true for thin, narrow pipes. Thus, increasing the pressure to increase the speed of flow has strict limitations for use in microfluidics.

Studies have been concentrating on how we can increase the rate of flow of liquids through narrow tubes without having to increase the pressure. A paper published in the journal “Science Advances”, reported the unexpected discovery that by coating the inside of narrow pipes with compounds that repel liquids, researchers could make viscous liquids flow faster than liquids with low viscosity. In effect, they could reverse the natural state of liquid flow rates.

Superhydrophobic surfaces are covered in tiny bumps that trap air within the coating which enables a liquid droplet to rest on the surface as if sitting on a cushion of air. A drop of honey and a drop of water placed on a superhydrophobic coated surface behave differently when the surface is tilted and the liquids begin to flow. The low-viscosity water will flow down the tilted surface faster than the honey.

Professor Robin Ras and his research team at Aalto University’s Department of Applied Physics have made a number of valuable discoveries around the topic of extremely water repellent coatings.

They have demonstrated that when a drop of fluid is introduced inside the very narrow type of tube typically used in microfluidics, the flow properties change substantially. The superhydrophobic coating on the inside walls of the tube creates a tiny air gap between the wall of the tube and the exterior of the fluid droplet.

The first author of the research paper, Dr. Maja Vuckovac, explained what they discovered. When a liquid drop was confined to a sealed superhydrophobic capillary, the air gap around the droplet was larger for more viscous fluids. The larger air gap permitted the viscous fluids to travel through the tube faster than liquids of lower viscosity when flowing due to gravity.

What was even more remarkable was the magnitude of the effect. Studies with glycerol droplets, which are a thousand times more viscous than water, flowed through the tube in excess of ten times faster than water. This surprising result is caused by the air layer between the liquid drops and the wall of the tube being thinner than the air layer for non-viscous fluids.

In the studies, the movement of the droplets through the tube were filmed. The researchers tracked the speed of the flow, and also how the liquid flowed inside the actual moving droplet. They discovered that the viscous liquid inside the droplet barely moved at all. However, they observed a fast mixing motion occurring in the droplets of lower viscosity.

This was shown to be a result of the lower viscosity fluids managing to penetrate into the air cushion surrounding the droplet, which created a thinner air gap around them. In effect, the air below a low viscous droplet inside the tube could not move out of the way as fast as it could for the more viscous droplet with a thicker air gap. There was less air managing to squeeze past the low viscosity droplets which therefore moved down the tube at a slower speed than more viscous droplets.

From the results of the studies, the research team was able to develop a fluid dynamics model that can be applied to predict how liquid drops will behave in tubes coated with different superhydrophobic coatings. They are confident that continued work on these fluid dynamics systems could result in significant applications for microfluidics engineers developing new microfluidics systems.

Further information: Viscosity-enhanced droplet motion in sealed superhydrophobic capillaries. Science Advances (2020). DOI: 10.1126/sciadv.aba5197

Keywords: rate of flow; high viscosity; chemically coated capillaries;

microfluidics; Superhydrophobic surfaces; liquid droplet; water repellent

coatings; flow properties; fluid dynamics

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