Microfluidic low-flow liquid flow meter: a review

Liquid flow meters are compulsory elements of microfluidic systems requiring control of the sample volume dispensed and/or, obviously, the sample flow rate (e.g. to avoid cellular stress).

Many liquid flow meters have been developed for large-scale industries like food & beverage, pharmaceuticals, or oil & gas companies, but microfluidics require rather high-accuracy, low-flow liquid flow meters for the microliters and nanoliters per minute range.

Cantilever low-flow liquid flow meters are cantilever microstructures placed perpendicularly within the fluid flow thus measuring the flow rate by the cantilever deflection or inflection. To quantify this displacement, piezoelectric microresistors placed on the cantilever structure are the sensing technology most often used but optical detection can also be employed.

Interestingly, academic devices show a very high potential sensitivity thanks to miniaturization technologies (e.g. 1 nl/min [10]).

On the other hand, like with sensitive thermal low-flow liquid flow meters, dust and other residues may disturb the sensor’s function. In addition, the direct contact between the sensor and potential sensitive species within the sample flow is even more invasive than thermal flow sensors.

A calorimetric thermal low-flow liquid flow meter measures the asymmetry of a temperature profile modulated by the fluid flow by using 2 different heat sensors surrounding a central heating element.

These types of sensors are the ones used for the Elveflow microfluidic liquid flow meter because they are highly sensitive and can measure ultra-low flow rates (e.g. nl/min).

Because of the symmetric position of the sensors, flow in both directions also can be easily quantified.

Those thermal low-flow liquid flow meters measure the effect of the flowing fluid on a resistive element which is used also as the heat sensor. The sensors work in 2 modes:

1. They measure the resistance modification with a constant heating power to then determine the corresponding cooling flow value.
2. They measure the increase of heating power to maintain a constant heater temperature. This mode involves a feedback loop and is thus more complicated to implement, but yields a better resolution and frequency response [1].

These thermal low-flow liquid flow meters measure the transit time of a heat pulse over a known distance. For that, one heater is placed upstream and one sensor downstream.

The heater must be thermally isolated from the sensor.

Compared to the other types of thermal liquid flow meters, thermal time-of-flight liquid flow meters are much less developed. This may be due to the fact that at low flow rates, thermal diffusion can more easily mask the sensor signals [3].

From Kuo et al. Micromachines 2012 [2].

Unlike hot-wire and calorimetric low-flow liquid flow meters, the asymmetry of this type of thermal sensor complicates its use for liquid flowing in both directions.

These low-flow liquid flow meters are based on Bernoulli’s laws from fluid mechanics stating that flow velocity variations are correlated with pressure drops.

Thus, by adding restrictions within the fluid channel, the increased velocity along the restriction induces a pressure drop measured with connected pressure sensors and then correlated with flow velocity and consequently with the flow rate.

This type of low-flow liquid flow meter implies that the sample flow mechanically displaces components with known volumes. By recording the amount of displaced elements, one can then determine the flow volume.

The sensitivity of this type of sensor at a low flow rate is then limited by the components’ dimensions and mechanical sensitivity. This type of system is thus rather useful for large fluidic systems than for microfluidics due to their incapacity to quantify low flow rates (minimum range ∼ 20ml/min). The interaction of the measurement unit with samples may be also problematic for assays with sensitive samples.

Nevertheless, this detection method is very accurate.

The Coriolis flow sensors are actually Coriolis mass flow meters. They are resonators of one or several tubes oscillating thanks to an external exciter (e.g. an electrode).

The sample flowing within these tubes modifies their mass and thus makes them twist in a perpendicular direction, producing a modification detected by an external sensing system (e.g. optics).

This movement is correlated to the mass flowing within the tubes and thus to the flow rate.

From Haneveld et al., IEEE MEMS 2008 [7]. 

From Wang et al., Symposium on Design, Test, Integration, and Packaging of MEMS/MOEMS 2008 [11]. 

Cantilever low-flow liquid flow meters are cantilever microstructures placed perpendicularly within the fluid flow thus measuring the flow rate by the cantilever deflection or inflection. To quantify this displacement, piezoelectric microresistors placed on the cantilever structure are the sensing technology most often used, but optical detection can also be employed.

Interestingly, academic devices show a potentially very high sensitivity thanks to miniaturization technologies (e.g. 1 nl/min [10]).

Vortex flow sensors use a bluff body through a sample flow. This obstacle creates vortexes right behind it and alternatively from each of its sides.

The frequency of these alternating vortexes is correlated to the flow velocity and measured thanks to a mechanical piezoelectric sensor or an ultrasonic beam placed in the vortex’s path.

Acoustic flow meters are actually ultrasonic flow meters. The main ultrasonic technology for flow sensing uses the fact that ultrasonic signals propagate faster in the flow direction than in the opposite direction. These sensors are often called time-of-flight ultrasonic flow sensors or transit-times ultrasonic flow sensors.

Basically, this technique relies on the same Doppler effect as the ultrasonic Doppler flow sensors, but with the detection of laser beam frequency shift. As ultrasonic flow meters, they can be totally non-invasive since they can be placed out of the sample flow.

The much shorter optical wavelengths allow us to analyze low flow rates undetectable with ultrasonic sensors.

Nevertheless, this technique is limited by high background noise and low signals due to multiple scattering.

Particles injected in the sample can be used to measure its velocity and other useful properties.

For this purpose, glass or plastic micrometric beads, with a refractive index different than the medium one, are used with a high-speed camera and an adapted laser.

Since this technique requires a complex setup and is obviously invasive, it is mostly used in research and education. It also can be implemented for microfluidic assays though [13].

Electro-magnetic flow meters use field coils to generate a constant magnetic field across the sample way. Sensing electrodes are placed perpendicularly to these field coils.

A magnetic field applies force to charged particles carried in the sample flow, and so negatively and positively charged particles are separated and thus create a voltage detected by the electrodes. This voltage is proportional to the flow velocity.

Electro-magnetic flow meters can be very precise (0,2% [14]) due to the electronic-magnetic detection technology but it requires to use of conductive samples and carefully isolating the sample channel with the measuring electrodes.

 From efunda website [15].

Conclusion on low-flow liquid flow meters

It is important to carefully pick your flow sensors since using a high-precision microfluidic flow controller remains the most limiting element of the setup. Currently, thermal low-flow liquid flow meters remain the most relevant sensing technique for microfluidics due to their accuracy and large flow rate range, covering especially the low flow rates used in microfluidics.

Optical low-flow liquid sensors still have some practical issues.

Besides, mechanical and acoustic low-flow liquid meters present some great potentialities. Indeed, other techniques based on MEMS and micro technologies like Coriolis, cantilever, and SAW flow sensors are currently developed and could later represent interesting alternatives with better sensitivities and ranges.