How are Contact Angles and Microfluidics Allowing Us to Miniaturise Diagnostic Testing?

Author: Carla Elizondo

The Science of Microfluidics and Contact Angles

As technology development continues to focus on miniaturisation, the field of microfluidics is becoming increasingly attractive as a means of miniaturising diagnostic tests in biological, chemical and physical fields. Microfluidics is the study of the behaviour of fluids on the micro- and nanometre scale. Currently, microfluidic systems are widely used in many medical applications, including PCR and ELISA point-of-care diagnostic testing. At such a small scale, gravity and other fluid mechanic laws become less significant, whereas surface tension becomes significantly more important than in larger-scale systems. Because of this, a hydrophobic or oleophobic (oil-repelling) surface is preferred in order to reduce the degree to which the fluid is attracted to the respective surface and, consequently, increase fluid flow to optimise the system. When developing a microfluidic system, measurement of contact angle is paramount in assessing the properties of the materials used.

What is Contact Angle?

Contact angle (Θ, theta) quantifies the wettability of a solid surface by a liquid via the Young equation. Wetting is defined as the ability of a liquid to maintain contact with a solid surface. If a material is hydrophobic it repels water and, conversely, if it is hydrophilic it attracts water. A drop of water on a hydrophobic material will appear rounder, with contact angle larger than 90°. This is considered a low degree of wettability. On a hydrophilic surface, however, it will appear flatter, with contact angle smaller than 90°. Here, it is considered a high degree of wettability. In the case of perfect wetting, Θ is equal to 0° and the droplet becomes a flat puddle with a much larger contact with the surface than seen with a hydrophobic surface. This is also applicable for oils and oleophilic or oleophobic surfaces. 

Measuring a Contact Angle with Optical Tensiometers

Optical tensiometers, also referred to as contact angle goniometers, allow researchers to measure a variety of types of contact angles. These are made up of a camera, a means of disposing a drop onto a sample and a light source to illuminate the drop for imaging. Sessile drop is the most used method for measuring contact angle, where the drop is disposed onto the sample stage and an image is recorded for measuring. Changing the liquid, the surface, or both will impact a contact angle. 

A dynamic contact angle is favoured when measuring superhydrophobic surfaces. This can be obtained by measuring the advancing and receding contact angles via a needle method. This involves the use of a needle to deposit a droplet onto the surface, followed by the injection of additional liquid to measure the advancing contact angle as the droplet grows, or the removal of liquid to measure the receding contact angle as the droplet is drawn back into the needle. Images are captured continuously to offer accurate measurement of contact angles at all stages. 

Applications of Microfluidics

Using these techniques to identify the most suitable materials, miniaturisation of diagnostic testing through microfluidics is bringing us one step closer to quick, point-of-care testing. Microfluidic systems decrease sample and reagent consumptions, shorten the time of experiments and reduces the overall costs of applications. Additionally, due to their size, these systems lend themselves well to large-scale production. In the medical domain, the potential for turning lengthy, expensive, laboratory testing into easily accessible ‘lab-on-chip’ testing would significantly improve the timescale for diagnosing and treating patients. Lab-on-chip devices are expected to revolutionise the chemical and biological fields in the same way as integrated circuits did in computers, but the development of these innovative techniques cannot continue to evolve without accurate testing of wetting with tools such as optical tensiometers. 

Integrated Graphene - Contact Angle

We have released Contact Angle, an optical tensiometer, to help enable ground-breaking surface science research and product development by accurately measuring contact angle. Contact Angle is cost-effective, reliable and accurate in a range of tasks ranging from a simple wetting test to the accurate measurement of surface-free energy. In the sessile drop mode, Contact Angle supports the measurement of both advancing and receding drops, allowing for testing a variety of applications. Our product is supported by open-source software and OEM components.

Where will Contact Angle take your innovative research and development? Contact us for more information.