Insights article Viscosity – A Challenge for Low Volume Dispensing?

Overview

In microfluidics and precision dispensing, delivering small volumes of liquid accurately is essential. Whether you’re working with biological samples, reagents, or industrial fluids, the ability to dispense consistent droplets can make or break the performance of your system. One of the most critical—and often underestimated—factors affecting dispensing accuracy is viscosity. This blog explores how viscosity influences fluid delivery, compares two common fluid supply systems, and discusses strategies to overcome viscosity-related challenges in low-volume dispensing.

Viscosity – What?

Viscosity is a measure of a fluid’s resistance to flow. It describes how “thick” or “sticky” a liquid is. Water, for example, has low viscosity and flows easily, while honey has high viscosity and flows slowly. Viscosity is influenced by:

• Temperature: Most fluids become less viscous as temperature increases.
• Composition: Additives, solutes, or polymers can increase viscosity.
• Shear rate: Some fluids (non-Newtonian) change viscosity depending on how fast they are moved.

In microfluidics, even small changes in viscosity can significantly affect flow behaviour, especially when working with volumes in the microliter or nanoliter range.

Dispensing Viscous Liquids: The Science

When dispensing fluids, especially in microfluidic systems, two main types of fluid delivery mechanisms are used:

Pressure-Driven Systems:

These systems apply a constant pressure to push fluid through a channel or nozzle. The flow rate is governed by the equation:

Q=ΔP/R​

Where:

Q: Flow rate
ΔP: Pressure difference
R: Hydrodynamic resistance

For a cylindrical microchannel, resistance R is given by:

R=8ηL/πr^4​

Where:

η: Fluid viscosity
L: Channel length
r: Channel radius

As viscosity increases, resistance increases, and flow rate decreases—unless pressure is adjusted. This makes pressure-driven systems highly sensitive to viscosity changes.

Positive Displacement Pumps

These pumps move a fixed fluid volume per cycle, regardless of viscosity. Examples include piston pumps, diaphragm pumps, and bellows pumps. Since the volume is mechanically defined, the flow rate remains consistent even if the fluid becomes thicker or thinner.

The Case of Two Elevators

Positive Displacement System: Like an elevator programmed to always go up 10 floors, no matter how many people are inside. The mass of the passengers (analogous to fluid viscosity) might affect how hard the motor works, but it doesn’t change the height reached → Dispensed volume stays constant.

Pressure-Driven System: Like an elevator powered by a fixed force. If only one person is inside (low viscosity), it might go up 10 floors. If it’s full of people (high viscosity), it might only reach 5 floors → Dispensed volume varies with viscosity.

Here:

• Height reached = Dispensed volume
• Passenger mass = Fluid viscosity
• Motor force = Applied pressure

Implications and Challenges

Using the flow rate and resistance equations, we see that:

• In pressure-driven systems, viscosity directly affects flow rate.
• In positive displacement systems, viscosity affects mechanical effort, but not the volume dispensed.

This has major implications for monodisperse droplet generation, where uniform droplet size is critical. Inconsistent flow rates due to viscosity changes can lead to:

• Irregular droplet sizes
• Poor reproducibility
• Clogging or delayed response

Strategies for Optimization

To overcome viscosity-related challenges in low-volume dispensing:

• Use positive displacement pumps for fluids with variable or high viscosity.
• Implement feedback control in pressure-driven systems to dynamically adjust pressure.
• Stabilise temperature to reduce viscosity fluctuations.
• Design microfluidic channels to minimise resistance and shear sensitivity.
• Calibrate systems for specific fluid properties and operating conditions.

The Bottom Line

Viscosity plays a central role in fluid dispensing, especially at low volumes. Understanding how it interacts with different supply systems allows engineers and researchers to choose the right technology for their application. While pressure-driven systems offer simplicity, positive displacement pumps provide superior consistency when dealing with viscous or variable fluids. By combining theoretical insights with practical design strategies, we can achieve precise, reliable, and monodisperse droplet generation—even in the face of viscosity challenges.