Abstract
Viscoelastic polymer solutions have been widely employed as suspending liquids for a myriad of microfluidic applications including particle and cell focusing, sorting, and encapsulation. It has been recently shown that viscoelastic solutions can drive the formation of equally spaced particles called "particle trains" as a result of the viscoelasticity-mediated hydrodynamic interactions between adjacent particles. Despite their potential impact on applications such as droplet encapsulation and flow cytometry, only limited experimental studies on viscoelastic ordering are currently available. In this work, we demonstrate that a viscoelastic shear-thinning aqueous xanthan gum solution drives the self-assembly of particle trains on the centerline of a serpentine microfluidic device with a nearly circular cross section. After focusing, the flowing particles change their mutual distance and organize in aligned structures characterized by a preferential spacing, quantified in terms of distributions of the interparticle distance. We observe the occurrence of multi-particle strings, mainly doublets and triplets, that interrupt the continuity of the particle train. To account for the fluctuations in the number of flowing particles in the experimental window, we introduce the concept of local particle concentration, observing that an increase of the local particle concentration leads to an increase of doublets and triplets. We also demonstrate that using only a single tube to connect the sample to the microfluidic device results in a drastic reduction of doublets/triplets, thus leading to a more uniform particle train. Our findings establish the foundation for optimized applications such as deterministic droplet encapsulation in viscoelastic liquids and optimized flow cytometry.
Highlights
The migration of particles and cells transversally to the flow direction due to internal forces generated within the bulk of the flow has been widely exploited in microfluidics for applications ranging from flow focusing to cell separation.[1]
We demonstrated that a viscoelastic shearthinning aqueous xanthan gum (XG) solution 0.1 wt % promoted the selfassembly of particle trains on the centerline of a serpentine microfluidic device
We found that the preferential distance observed through the distributions of the interparticle distances depended on the particle concentration and the Deborah number
Summary
The migration of particles and cells transversally to the flow direction due to internal forces generated within the bulk of the flow has been widely exploited in microfluidics for applications ranging from flow focusing to cell separation.[1]. The hyaluronic acid solutions employed by Del Giudice et al.[8] presented a large zero-shear viscosity, which can cause problems during particle or cell mixing Previous studies, both numerical and experimental, failed to fully characterize the impact of doublets or triplets of attached particles on the formation of a stable train. XG in water was chosen as the suspending liquid because of its strongly shear-thinning properties at relatively low mass concentrations of the polymer (meaning smaller zero-shear viscosity values), its low cost compared to other polymers such as hyaluronic acid, and because of the recent interest about using XG for different microfluidic applications.[24−26] we observed the formation of doublets and triplets of attached particles that disturbed the continuity of the train and we quantified their impact on the train formation. Inter-particle distance histograms were evaluated with (dimensionless) binning size equal to 1 and the two boundary ends were set to 1 and 64, which is the total length of the observation window (see Figure 4b)
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