Confinement effect on the viscoelastic particle ordering in microfluidic flows: Numerical simulations and experiments

Abstract

Strings of equally spaced particles, also called particle trains, have been employed in several applications, including flow cytometry and particle or cell encapsulation. Recently, the formation of particle trains in viscoelastic liquids has been demonstrated. However, only a few studies have focused on the topic, with several questions remaining unanswered. We here perform numerical simulations and experiments to elucidate the effect of the confinement ratio on the self-ordering dynamics of particles suspended in a viscoelastic liquid and flowing on the centerline of a microfluidic channel. For a fixed channel size, the particles self-order on shorter distances as the particle size increases due to the enhanced hydrodynamic interactions. At relatively low linear concentrations, the relative particle velocities scale with the fourth power of the confinement ratio when plotted as a function of the distance between the particle surfaces normalized by the channel diameter. As the linear concentration increases, the average interparticle spacing reduces and the scaling is lost, with an increasing probability to form strings of particles in contact. To reduce the number of aggregates, a microfluidic device made of an array of trapezoidal elements is fabricated and tested. The particle aggregates reduce down to 5% of the overall particle number, significantly enhancing the ordering efficiency. A good agreement between numerical simulations and experiments is found.

Beating Poisson stochastic particle encapsulation in flow-focusing microfluidic devices using viscoelastic liquids

“…The suspension containing particles flowed in a polymethylmethacrylate (PMMA) square-shaped microchannel with lateral side H = 100 mm, width W = 100 mm and length of 30 cm, bonded on a glass coverslip using double-sided tape (Adhesive Research). The channel length was selected such that the 20 AE 2 mm particles (Polysciences Inc.) employed here at a bulk concentration of 0.4 wt% self-ordered in a particle train thanks to the viscoelastic properties of the suspending liquid 18,21,26 before approaching the encapsulation area (Fig. 1a).…”

Viscoelastic particle encapsulation using a hyaluronic acid solution in a T-junction microfluidic device

“…Experiments were performed in a square shaped microfluidic device designed to facilitate the encapsulation of particles with efficiency above the stochastic Poisson value (Figure 1a). Specifically, particles suspended in the polymer solution entered the channel via Inlet 1 where they went through 16 trapezoidal elements where potential aggregates could be broken down to individual particles, as recently shown by Jeyasountharan et al 24 Afterwords, particles travelled along the serpentine channel to facilitate the formation of equally-spaced particle trains, as reported previously. 17,23 The continuous phase entered the microfluidic device via Inlet 2 (Figure 1a), and it met the dispersed phase at the T-junction, leading to the formation of a droplet containing encapsulated particles (Figure 1a).…”

“…(a) Schematic representation of the T-junction device employed in this work. The dispersed viscoelastic phase entered the device via Inlet 1.The trapezoidal structures were added in order to break down potential particle aggregates, in agreement with previous works 24. After the trapezoidal structure, particles first aligned on the channel centreline and then self-ordered before approaching the encapsulation area.…”

“…This is not surprising, as the whole principle of overcoming the Poisson limit is based on the fact that the frequency of droplet formation f d needs to be synchronised to the one of particles approaching the encapsulation area f p . Since for viscoelastic ordering the volumetric flow rate has only a minor effect on the f p value, 17,23,24 there will only be a single value of Q HA that can beat the Poisson limit for a given value of Q Oil . It is also possible that there is no value for which f p = f d , being the value of f d controlled by the combination of the volumetric flow rates of both continuous and dispersed phase (Figure 2d).…”

“…At variance with standard T-junction devices, our microfluidic device featured the addition of several trapezoidal elements required to break the particle aggregates, as recently demonstrated in another work. 24 We employed hyaluronic acid rather than xanthan gum because xanthan gum solutions tend to degrade more easily that the hyaluronic acid. We first studied the viscoelastic droplet formation mechanism and then the viscoelastic encapsulation.…”