A Review of Microfluidic Devices for Rheological Characterisation

The rheological characterisation of liquids finds application in several fields ranging from industrial production to the medical practice. Conventional rheometers are the gold standard for the rheological characterisation; however, they are affected by several limitations, including high costs, large volumes required and difficult integration to other systems. By contrast, microfluidic devices emerged as inexpensive platforms, requiring a little sample to operate and fashioning a very easy integration into other systems. Such advantages have prompted the development of microfluidic devices to measure rheological properties such as viscosity and longest relaxation time, using a finger-prick of volumes. This review highlights some of the microfluidic platforms introduced so far, describing their advantages and limitations, while also offering some prospective for future works

Simultaneous measurement of rheological properties in a microfluidic rheometer

Microfluidic rheometry is considered to be a potential alternative to conventional rheometry for the rheological characterization of viscoelastic solutions having relatively low viscoelastic properties. None of the microfluidic platforms introduced so far, however, can be used for the measurements of multiple rheological properties in the same device. In this work, I present the first microfluidic platform, named the “μ-rheometer,” which allows for the simultaneous measurement of zero-shear viscosity η0 and longest shear relaxation time λ. This is achieved by transforming the original “flow rate controlled” platform presented by Del Giudice et al., “Rheometry-on-a-chip: Measuring the relaxation time of a viscoelastic liquid through particle migration in microchannel flows,” Lab Chip 15, 783–792 (2015) into a “pressure drop controlled” microfluidic device, by replacing a syringe pump with a pressure pump. The novel device has been tested by measuring both η0 and λ for a number of polyethylene oxide solutions in glycerol–water 25 wt. % and pure water, respectively. Its effectiveness has been corroborated by means of a direct comparison with a conventional rotational rheometer

Viscoelastic Particle Train Formation in Microfluidic Flows Using a Xanthan Gum Aqueous Solution

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

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

The encapsulation and co-encapsulation of particles in microfluidic flows is essential in applications related to single-cell analysis and material synthesis. However, the whole encapsulation process is stochastic in nature, and its efficiency is limited by the so-called Poisson limit. We here demonstrate particle encapsulation in microfluidic devices having flow-focusing geometries with efficiency up to 2-fold larger than the stochastic limit imposed by the Poisson statistics. To this aim, we exploited the recently observed phenomenon of particle train formation in viscoelastic liquids, so that particles could approach the encapsulation area with a constant frequency that was subsequently synchronised to the constant frequency of droplet formation. We also developed a simplified expression based on the experimental results that can guide optimal design of the microfluidic encapsulation system. Finally, we report the first experimental evidence of viscoelastic co-encapsulation of particles coming from different streams

Viscoelastic Particle Encapsulation Using a Hyaluronic Acid Solution in a T-Junction Microfluidic Device

The encapsulation of particles and cells in droplets is highly relevant in biomedical engineering as well as in material science. So far, however, the majority of the studies in this area have focused on the encapsulation of particles or cells suspended in Newtonian liquids. We here studied the particle encapsulation phenomenon in a T-junction microfluidic device, using a non-Newtonian viscoelastic hyaluronic acid solution in phosphate buffer saline as suspending liquid for the particles. We first studied the non-Newtonian droplet formation mechanism, finding that the data for the normalised droplet length scaled as the Newtonian ones. We then performed viscoelastic encapsulation experiments, where we exploited the fact that particles self-assembled in equally-spaced structures before approaching the encapsulation area, to then identify some experimental conditions for which the single encapsulation efficiency was larger than the stochastic limit predicted by the Poisson statistics

Farm Size Adjustment and Contract Regulation (I. #203/82): Evidence From an Italian Case Study

In Italy, the structure of farm has always shown remarkable elements of weakness. Among these, the small dimension, in terms of arable land, has represented one of the most difficult to resolve. The absence of a legislation that could favour jointness of the property have remarkably reduced the market of the land. In this scenario, a new law n. 203/1982 was lunched. Now farmers are considering rent land a possible strategy to increase hectares. The object of this paper is to analyse the situation of land contract in Campania Region. A better understanding of these topics should improve public policies for a better adjustment process.Adjustment process, contract regulation, farm structure, rented land, Farm Management,

A Multidimensional Evaluation Approach for the Natural Parks Design

The design of a natural park is generated by the need to protect and organize, for conservation and/or for balanced growth, parts of the territory that are of particular interest for the quality of the natural and historical–cultural heritage. The necessary tool to support the decision-making process in the design of a natural park are the financial and economic evaluations, which intervene in three successive steps: in the definition of protection and enhancement levels of the park areas; in the choice of the interventions to be implemented for the realization of these levels of protection and enhancement; in determining and verifying the economic and financial results obtainable from the project execution. This contribution deals with aspects and issues relating to the economic and financial evaluation of natural park projects. In particular, an application of the “Complex Social Value” to a concrete case of environmental design is developed on the basis of the elements that can be deduced from a feasibility study of a natural park: the levels of protection and enhancement of the homogeneous areas of the natural park are preliminarily defined, and the choice of the design alternative to be implemented is, therefore, rationalized with multicriteria analysis

Rapid Temperature-Dependent Rheological Measurements of Non-Newtonian Solutions Using a Machine-Learning Aided Microfluidic Rheometer

Biofluids such as synovial fluid, blood plasma, and saliva contain several proteins which impart non-Newtonian properties to the biofluids. The concentration of such protein macromolecules in biofluids is regarded as an important biomarker for the diagnosis of several health conditions, including cardiovascular disorders, joint quality, and Alzheimer’s. Existing technologies for the measurements of macromolecules in biofluids are limited; they require a long turnaround time, or require complex protocols, thus calling for alternative, more suitable, methodologies aimed at such measurements. According to the well-established relations for polymer solutions, the concentration of macromolecules in solutions can also be derived via measurement of rheological properties such as shear-viscosity and the longest relaxation time. We here introduce a microfluidic rheometer for rapid simultaneous measurement of shear viscosity and longest relaxation time of non-Newtonian solutions at different temperatures. At variance with previous technologies, our microfluidic rheometer provides a very short turnaround time of around 2 min or less thanks to the implementation of a machine-learning algorithm. We validated our platform on several aqueous solutions of poly(ethylene oxide). We also performed measurements on hyaluronic acid solutions in the clinical range for joint grade assessment. We observed monotonic behavior with the concentration for both rheological properties, thus speculating on their use as potential rheo-markers, i.e., rheological biomarkers, across several disease states

Controlled viscoelastic particle encapsulation in microfluidic devices

The encapsulation of particles in droplets using microfluidic devices finds application across severalfields ranging from biomedical engineering to materials science. The encapsulation process, however, isoften affected by poor single encapsulation efficiency, quantified by the Poisson statistics, with dropletscontaining more than one particle or with several empty droplets. We here demonstrate that viscoelasticaqueous solutions of xanthan gum enable controlled single particle encapsulation in microfluidic deviceswith a single encapsulation efficiency up to 2-fold larger than the one predicted by the Poissonstatistics. We achieved such a result by identifying viscoelastic xanthan gum aqueous solutions thatcould drive particle ordering before approaching the encapsulation area and simultaneously formuniform droplets. This is the first experimental evidence of viscoelastic encapsulation in microfluidicdevices, the existing literature on the subject being focused on Newtonian suspending liquids. We firststudied the process of viscoelastic droplet formation, and found that the droplet length normalised bythe channel diameter scaled as predicted for Newtonian solutions. At variance with Newtonian solutions,we observed that the droplet formation mechanism became unstable above critical values of theWeissenberg number, which quantifies the elasticity of the xanthan gum solutions carrying the particles.In terms of controlled encapsulation, we discovered that the single encapsulation efficiency was largerthan the Poisson values in a specific range of xanthan gum mass concentrations. Finally, we introducedan empirical formula that can help the design of controlled viscoelastic encapsulation systems

Viscoelastic focusing of polydisperse particle suspensions in a straight circular microchannel

Flow cytometry is a technique for the analysis of cells and particles by coupling a sheath flow assisted focusing microfluidic device to an optical or electrical reader. In this context, polymer solutions can be employed to drive particle and cell focusing on the centreline of simple straight microfluidic channels. However, the change of the focusing efficiency due to polydispersity, which is a rule rather than the exception across cell populations, has not been extensively studied yet. In this work, the effect of particle polydispersity on the viscoelastic focusing in a straight cylindrical microchannel was studied by preparing two particle suspensions, containing different concentrations of particles with average diameters of 10μm , 15μm and 20μm , suspended in three solutions of polyethylene oxide with mass concentrations of 0.1 wt%, 0.25 wt% and 0.5 wt%. When the fluid explored the constant-viscosity region of the rheological curve, up to 95% of the particles were aligned on the channel centreline. When the fluid explored the shear-thinning region of the rheological curve, centreline focusing efficiency decreased, with the highest value of aligned particles being of 60%. For both mixtures, it was also observed that the fraction of aligned particles in a polydisperse system was not equivalent to that derived from the estimate of independent experiments with monodisperse particles