The role of lubrication film in droplet characteristics: from non-axisymmetric to axisymmetric shape in a rectangular microfluidic channel

This research summary describes the optical measurement of the lubrication film around flowing droplets in a rectangular microfluidic channel. In addition, the study explains droplet characteristics under different conditions.

The paper ‘Droplet transition from non-axisymmetric to axisymmetric shape: Dynamic role of lubrication film in a rectangular microfluidic channel’ by Riccardo Zamboni, Annamaria Zaltron, Davide Ferraro, and Cinzia Sada was published in Physics of Fluids in December 2022.

Abstract

In microfluidics, the confinement of droplets flowing in microfluidic channels is physically ensured by the presence of a thin lubrication film. This film prevents contact between the droplets and the channel walls, maintaining droplet characteristics, even for highly squeezed droplets. The thickness of this lubrication film depends strongly on the dynamics of the entire microfluidic system and thus influences the actual droplet’s shape and velocity^1–3. For these reasons, this film has been studied extensively since the 30s⁴. This research summary presents an effective optical method to measure this lubrication film. This method allowed film characterization during a unique droplet shape transition that occurs in a non-axisymmetric channel.

Introduction | Film thickness and droplet characteristics

The authors present a novel approach to experimentally study the lubricant film thickness in a rectangular channel and its influence on droplet characteristics. This non-axisymmetric configuration is the most common channel used in microfluidics. In addition, the lack of axial symmetry hinders any numerical and theoretical study, thus still resulting in an understudied configuration. This geometric feature also leads to unexpected behaviour in droplet characteristics, such as a shape transition. The droplets change from an axisymmetric to a non-axisymmetric shape depending on the Capillary number⁵.

The optical method is based on the diffraction of a localized light beam in a microfluidic channel. During the flow of droplets in the channel, the light is diffracted due to the presence of the lubrication films. This phenomenon is caused by the localized beam provided by two optical waveguides on the two sides of the channel. The output light signal from the waveguides reveals the light diffraction due to the flow of the droplets, thus providing a valid experimental method to measure the lubrication film.

Aims

• To experimentally characterize the profile of the lubrication film around flowing droplets in a rectangular section channel.
• To measure the lubrication film regimes before and after the droplet shape transition.
• To measure the critical capillary number for the transition.

Experiment setup | A real-time droplet monitoring system

The setup consists of a lithium niobate microfluidic device for droplet generation (Fig 1). The microfluidic circuit is a cross junction for droplet generation and a main rectangular channel, where the measurement is performed.

The channel is equipped on the two side with Ti-indiffused waveguides located on the edges of the channel^6,7. One waveguide carries confined light beam that propagates in the channel and it is recollected with the other waveguide on the other side, providing data on droplet characteristics.

During the flow of the droplets, the transmitted signal depends on the lubrication film. He-Ne laser is pigtailed into the waveguide and the light signal is collected by a photodiode. Water droplets in hexadecane oil with 3% SPAN80 surfactant are generated in the junction. The two liquids are injected using the Elveflow OB1 MK3+ pressure controller connected to the Bronkhorst Coriolis flow sensor.

The flow of the droplets in the channel and droplets characteristics are monitored by a microscope configuration and a camera.

Materials

• Lithium niobate (LN) substrate
• Elveflow OB1 MK3+ pressure controller
• Bronkhorst coriolis flow sensor
• Suspension of Hexadecane oil (Sigma Aldrich) containing 3% SPAN80 (Sigma Aldrich)
• Microscope objective (2 with 20/0.35 and 10/0.45)
• Photodiode
• Camera acA800-510μm

Key findings | Lubrication film has regime-dependent profiles

The lubrication film is extensively studied using the optical method to gain insight into the dynamics of flowing droplets and droplet characteristics. This work provides a comprehensive experimental characterization of the dynamics of the lubrication film during the droplet shape transition.

During the axisymmetric regimes, the profile of the lubrication film reveals a clear minimum, that follows the Bretherton law^8–10. Conversely, the lubrication film is constant during non-axisymmetric and along most of the profile of the droplets (see Fig 1).

A detailed analysis the manuscript also provides an accurate estimation of the critical value of the capillary number for the transition.

Conclusions | Film Dynamics and droplet characteristics

The results obtained in this work are not only consistent with previous literature^5,11, but extend it to a complete description of the lubrication film dynamics and its effects on droplet characteristics. The latter allows, therefore, a better understanding of the overall dynamics of droplet transport in a non-axisymmetric channel.

Finally, the remarkable sensitivity of this optical method could be exploited to study other fluid dynamic phenomena involving the lubrication film, as well as investigating biochemical reactions occurring at the solid-liquid or liquid-liquid interfaces.