Microfluidics: Where Surface Properties Rule

What do inkjet printers and glucose test strips have in common? Both are examples of microfluidic technology. Microfluidics is the science of manipulating very small volumes of liquid through channels measured in microns. At this scale, the surface-area-to-volume ratio is much greater than it is in ordinary plumbing, so surface forces such as capillary action, surface tension, and wetting can matter more than gravity and momentum. By taking advantage of these effects, engineers can create miniature chips that make fluids flow, stop, separate, or mix in a controlled and repeatable way. Inkjets, for example, use precisely engineered nozzles to form and direct ink droplets onto a paper substrate. Glucose test strips use a hydrophilic channel to draw a blood sample across a reaction pad, where the sample interacts with chemicals that produce a measurable response. Many diagnostic devices extend this same concept through a so-called lab-on-a-chip, integrating sample preparation, fluid transport, and detection into a compact platform.

Because microfluidic channels often rely on capillary action to move fluid, their surface properties must be carefully controlled. One of the most commonly used substrates for microfluidic chips is polydimethylsiloxane (PDMS), a flexible and optically clear polymer that is naturally hydrophobic, with a contact angle around 100 degrees. Plasma treatment can temporarily change the PDMS surface so it becomes hydrophilic, with a contact angle between 20 and 30 degrees. The effect has a limited shelf life, however, because polymer chains gradually migrate back to the surface and the material undergoes hydrophobic recovery ¹. Coatings can provide an alternative or complementary strategy when a more durable or application-specific surface is needed. For both approaches, contact angle and surface energy help characterize whether a substrate is ready for coating and whether the finished surface will perform as intended. These measurements can reveal changes in cleanliness, treatment effectiveness, coating uniformity, and long-term stability before a device moves into full production. Monitoring the surface over time is especially important because a treatment that performs well immediately after processing may behave differently after storage, handling, or exposure to air and humidity.

In diagnostic and life-science devices, proteins, cells, and other biological materials can adsorb to channel walls. This biofouling can change wettability, restrict flow, reduce channel performance, and interfere with detection. Low-fouling coatings and surface treatments are used to limit nonspecific adsorption, but their effectiveness must be evaluated over time. Contact-angle measurements provide a practical quality-control method for confirming that a surface remains clean, consistent, and within the desired wetting range.

Understanding surface tension and interfacial tension is also crucial when designing a successful microfluidic system. In droplet-based diagnostics, a carrier fluid can transport small droplets of an aqueous phase containing cells, proteins, or reagents through a channel ². Interfacial tension helps determine the droplets' size, shape, breakup behavior, and stability, while the channel geometry, flow rates, and surface wetting determine how those droplets move. Small changes in surface chemistry can therefore affect whether a droplet remains intact, merges with another droplet, or disperses along the channel wall. Contact-angle hysteresis provides another useful insight: the difference between advancing and receding contact angles can indicate how strongly a droplet is pinned to a surface and how easily it will move. A surface with low hysteresis generally allows more predictable droplet motion, while higher hysteresis may signal chemical or physical heterogeneity. Measuring interfacial tension with a pendant drop or inverted pendant drop technique ³, together with advancing and receding contact angles, gives researchers a quantitative way to compare fluids, optimize formulations, and monitor changes during development or production.

Microfluidics can also be used to simulate geological systems, such as underground sandstone reservoirs ⁴. These miniature models reproduce aspects of the pore networks found in rock and allow researchers to observe multiphase flow under controlled conditions. Interfacial tension is important in this work because it influences how water, oil, and gas move through narrow pores, how easily one phase displaces another, and how much carbon dioxide can dissolve in a liquid. Temperature and pressure can change these relationships, making reliable surface and interfacial tension measurements especially valuable when evaluating carbon dioxide flooding, enhanced oil recovery, and long-term CO2 storage strategies.

ramé-hart goniometers and tensiometers provide practical tools for studying the surface and interfacial properties that govern microfluidic systems. Contact angle measurements can help verify substrate preparation and coating performance, while surface and interfacial tension measurements can support fluid selection, formulation development, and process control. With dependable measurements and a range of instruments and accessories, ramé-hart can help researchers connect surface chemistry to real-world device performance. Contact us to learn more about our instruments and accessories, or request a quote for a specific application.

Notes

1. Forster, S., & McArthur, S. L. (2012). Stable low-fouling plasma polymer coatings on polydimethylsiloxane. Biomicrofluidics, 6. https://doi.org/10.1063/1.4754600
2. Koshy, A., Ray, S., & Das, G. (2021). Between droplets and fluid thread - the role of gravity in meso-scale flow. Physics of Fluids, 33. https://doi.org/10.1063/5.0055384
3. Refer to our technical resource on surface tension.
4. Pradhan, Sushobhan, Khandaker Fahim Anjum, and Prem Bikkina. "Experimental Evaluation of Carbonated Water Flooding Using Microfluidics and Coreflooding for Simultaneous EOR and CO2 Storage in Sandstone Reservoirs." Industrial & Engineering Chemistry Research (2026). https://doi.org/10.1021/acs.iecr.6c01713

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