Flowering in bursting bubbles with viscoelastic interfaces.

The lifetime of bubbles, from formation to rupture, attracts attention because bubbles are often present in natural and industrial processes, and their geometry, drainage, coarsening, and rupture strongly affect those operations. Bubble rupture happens rapidly, and it may generate a cascade of small droplets or bubbles. Once a hole is nucleated within a bubble, it opens up with a variety of shapes and velocities depending on the liquid properties. A range of bubble rupture modes are reported in literature in which the reduction of a surface energy drives the rupture against inertial and viscous forces. The role of surface viscoelasticity of the liquid film in this colorful scenario is, however, still unknown. We found that the presence of interfacial viscoelasticity has a profound effect in the bubble bursting dynamics. Indeed, we observed different bubble bursting mechanisms upon the transition from viscous-controlled to surface viscoelasticity-controlled rupture. When this transition occurs, a bursting bubble resembling the blooming of a flower is observed. A simple modeling argument is proposed, leading to the prediction of the characteristic length scales and the number and shape of the bubble flower petals, thus paving the way for the control of liquid formulations with surface viscoelasticity as a key ingredient. These findings can have important implications in the study of bubble dynamics, with consequences for the numerous processes involving bubble rupture. Bubble flowering can indeed impact phenomena such as the spreading of nutrients in nature or the life of cells in bioreactors.

Oscillation Dynamics of Dielectric Polymer Droplets during Electrohydrodynamic Jetting in a Wide Range of Viscosities.

The electrohydrodynamic (EHD) jetting of fluids is used for several applications such as inkjet printing, atomization of analyte in mass spectrometry, liquid metal alloy ion sources, and electrospinning of polymer fibers. Historically, the bulk of research has focused on nonviscous, highly conductive fluids which are most suitable for EHD spray and printing, while there is relatively little experimental work on EHD jetting of highly viscous liquid dielectrics. We studied the dynamics of oscillation and pulsating jetting from a suspended drop of polydimethylsiloxane (PDMS) polymers in an electric field, with particular attention to the viscosity dependence of the oscillation period and meniscus elongation and contraction time over a wide viscosity range (102-105 cSt). The reported results could help the appropriate design of EHD processes and may open new possibilities for the rheological characterization of liquid polymers using small volumes at the scale of nanoliters.

Effect of wall slip on the viscoelastic particle ordering in a microfluidic channel.

The formation of a line of equally spaced particles at the centerline of a microchannel, referred as "particle ordering," is desired in several microfluidic applications. Recent experiments and simulations highlighted the capability of viscoelastic fluids to form a row of particles characterized by a preferential spacing. When dealing with non-Newtonian fluids in microfluidics, the adherence condition of the liquid at the channel wall may be violated and the liquid can slip over the surface, possibly affecting the ordering efficiency. In this work, we investigate the effect of wall slip on the ordering of particles suspended in a viscoelastic liquid by numerical simulations. The dynamics of a triplet of particles in an infinite cylindrical channel is first addressed by solving the fluid and particle governing equations. The relative velocities computed for the three-particle system are used to predict the dynamics of a train of particles flowing in a long microchannel. The distributions of the interparticle spacing evaluated at different slip coefficients, linear particle concentrations, and distances from the channel inlet show that wall slip slows down the self-assembly mechanism. For strong slipping surfaces, no significant change of the initial microstructure is observed at low particle concentrations, whereas strings of particles in contact form at higher concentrations. The detrimental effect of wall slip on viscoelastic ordering suggests care when designing microdevices, especially in case of hydrophobic surfaces that may enhance the slipping phenomenon.

An Experimental and Numerical Investigation on Bubble Growth in Polymeric Foams.

The cellular morphology of thermoplastic polymeric foams is a key factor for their performances. Three possible foam morphologies exist, namely, with closed cells, interconnected cellular structure, and open cells. In the gas foaming technology, a physical blowing agent, e.g., CO2 or N2, is used to form bubbles at high pressure in softened/melted polymers. As a consequence of a pressure quench, the bubbles grow in the liquid matrix until they impinge and possibly break the thin liquid films among them. If film breakage happens, the broken film may retract due to the elastic energy accumulated by the polymeric liquid during the bubble growth. This, in turn, determines the final morphology of the foam. In this work, we experimentally study the growth of CO2 bubbles in a poly(e-caprolactone) (PCL) matrix under different pressure conditions. In addition, we perform three-dimensional direct numerical simulations to support the experimental findings and rationalize the effects of the process parameters on the elastic energy accumulated in the liquid at the end of the bubble growth, and thus on the expected morphology of the foam. To do that, we also extend the analytic model available in the literature for the growth of a single bubble in a liquid to the case of a liquid with a multi-mode viscoelastic constitutive equation.

Design of a microfluidic device for the measurement of the elastic modulus of deformable particles.

A microfluidic technique recently proposed in the literature to measure the interfacial tension between a liquid droplet and an immiscible suspending liquid [Hudson et al., Appl. Phys. Lett., 2005, 87, 081905], [Cabral and Hudson, Lab Chip, 2006, 6, 427] is suitably adapted to the characterization of the elastic modulus of soft particles in a continuous-flow process. A microfluidic device consisting of a cylindrical pipe with a reduction in cross-section is designed, and the deformation and velocity of incompressible elastic particles suspended in a Newtonian liquid are tracked as they move along the centerline through the constriction. Kinematic and shape information is exploited to calculate the particle's elastic modulus by means of the theory of elastic particle deformation in extensional flow. The approach is validated for different orders of magnitude of the elastic capillary number through experiments and numerical simulations.

Dissolution of concentrated surfactant solutions: from microscopy imaging to rheological measurements through numerical simulations.

Concentrated aqueous solutions of surfactants, often referred to as pastes, experience complex phase and rheology changes upon dissolution in water, which is a typical step in the production of liquid detergents. During the dilution process, depending on water content, surfactant molecules can arrange in different morphologies, such as lamellar or cubic and hexagonal structures. These phases are characterized by different physico-chemical properties, such as viscosity or diffusivity, which lead to non-simple transport mechanisms during the dissolution process. In this work, we investigate the dissolution of concentrated Sodium Lauryl Ether Sulfate (SLES) pastes in water under quiescent conditions by coupling different experimental techniques. A thorough rheological characterization of the system showed non-monotonic changes of several orders of magnitude in its viscosity and viscoelastic moduli as a function of water content. Time-lapse microscopy allowed us to image the dynamic evolution of the phase changes as water penetrated in a disk-shaped sample (with the same geometry used in rheological tests). Numerical simulation, based on a simple diffusion-based multi-parameter model is shown to describe satisfactorily SLES dissolution data.

Elasticity in Bubble Rupture.

When a Newtonian bubble ruptures, the film retraction dynamics is controlled by the interplay of surface, inertial, and viscous forces. In case a viscoelastic liquid is considered, the scenario is enriched by the appearance of a new significant contribution, namely, the elastic force. In this paper, we investigate experimentally the retraction of viscoelastic bubbles inflated at different blowing rates, showing that the amount of elastic energy stored by the liquid film enclosing the bubble depends on the inflation history and in turn affects the velocity of film retraction when the bubble is punctured. Several viscoelastic liquids are considered. We also perform direct numerical simulations to support the experimental findings. Finally, we develop a simple heuristic model able to interpret the physical mechanism underlying the process.

Highly sensitive detection of the neurodegenerative biomarker Tau by using the concentration effect of the pyro-electrohydrodynamic jetting.

It is largely documented that neurodegenerative diseases can be effectively treated only if early diagnosed. In this context, the structural changes of some biomolecules such as Tau, seem to play a key role in neurodegeneration mechanism becoming eligible targets for an early diagnosis. Post-translational modifications are responsible to drive the Tau protein towards a transition phase from a native disorder conformation into a preaggregation state, which then straight recruits the final fibrillization process. Here, we show for the first time the detection of pre-aggregated Tau in artificial urine at femto-molar level, through the concentration effect of the pyro-electrohydrodynamic jet (p-jet) technique. An excellent linear calibration curve is demonstrated at the femto-molar level with a limit of detection (LOD) of 130 fM. Moreover, for the first time we show here the structure stability of the protein after p-jet application through a deep spectroscopic investigation. Thanks to the small volumes required and the relatively compact and cost-effective characteristics, this technique represents an innovative breakthrough in monitoring the early stage associated to neurodegeneration syndromes in different scenarios of point of care (POC) and such as for example in long-term human space exploration missions.

Holographic flow scanning cytometry overcomes depth of focus limits and smartly adapts to microfluidic speed.

Space-time digital holography (STDH) maps holograms in a hybrid space-time domain to achieve extended field of view, resolution enhanced, quantitative phase-contrast microscopy and velocimetry of flowing objects in a label-free modality. In STDH, area sensors can be replaced by compact and faster linear sensor arrays to augment the imaging throughput and to compress data from a microfluidic video sequence into one single hybrid hologram. However, in order to ensure proper imaging, the velocity of the objects in microfluidic channels has to be well-matched to the acquisition frame rate, which is the major constraint of the method. Also, imaging all the flowing samples in focus at the same time, while avoiding hydrodynamic focusing devices, is a highly desirable goal. Here we demonstrate a novel processing pipeline that addresses non-ideal flow conditions and is capable of returning the correct and extended focus phase contrast mapping of an entire microfluidic experiment in a single image. We apply this novel processing strategy to recover phase imaging of flowing HeLa cells in a lab-on-a-chip platform even when severely undersampled due to too fast flow while ensuring that all cells are in focus.

Label-free liquid biopsy through the identification of tumor cells by machine learning-powered tomographic phase imaging flow cytometry.

Image-based identification of circulating tumor cells in microfluidic cytometry condition is one of the most challenging perspectives in the Liquid Biopsy scenario. Here we show a machine learning-powered tomographic phase imaging flow cytometry system capable to provide high-throughput 3D phase-contrast tomograms of each single cell. In fact, we show that discrimination of tumor cells against white blood cells is potentially achievable with the aid of artificial intelligence in a label-free flow-cyto-tomography method. We propose a hierarchical machine learning decision-maker, working on a set of features calculated from the 3D tomograms of the cells' refractive index. We prove that 3D morphological features are adequately distinctive to identify tumor cells versus the white blood cell background in the first stage and, moreover, in recognizing the tumor type at the second decision step. Proof-of-concept experiments are shown, in which two different tumor cell lines, namely neuroblastoma cancer cells and ovarian cancer cells, are used against monocytes. The reported results allow claiming the identification of tumor cells with a success rate higher than 97% and with an accuracy over 97% in discriminating between the two cancer cell types, thus opening in a near future the route to a new Liquid Biopsy tool for detecting and classifying circulating tumor cells in blood by stain-free method.

Microfoamed Strands by 3D Foam Printing.

We report the design, production, and characterization of microfoamed strands by means of a green and sustainable technology that makes use of CO2 to create ad-hoc innovative bubble morphologies. 3D foam-printing technology has been recently developed; thus, the foaming mechanism in the printer nozzle is not yet fully understood and controlled. We study the effects of the operating parameters of the 3D foam-printing process to control and optimize CO2 utilization through a maximization of the foaming efficiency. The strands' mechanical properties were measured as a function of the foam density and explained by means of an innovative model that takes into consideration the polymer's crystallinity content. The innovative microfoamed morphologies were produced using a bio-based and compostable polymer as well as polylactic acid and were then blown with CO2. The results of the extensive experimental campaigns show insightful maps of the bubble size, density, and crystallinity as a function of the process parameters, i.e., the CO2 concentration and temperature. A CO2 content of 15 wt% enables the acquirement of an incredibly low foam density of 40 kg/m3 and porosities from the macro-scale (100-900 µm) to the micro-scale (1-10 µm), depending on the temperature. The foam crystallinity content varied from 5% (using a low concentration of CO2) to 45% (using a high concentration of CO2). Indeed, we determined that the crystallinity content changes linearly with the CO2 concentration. In turn, the foamed strand's elastic modulus is strongly affected by the crystallinity content. Hence, a corrected Egli's equation was proposed to fit the strand mechanical properties as a function of foam density.

Instant in situ formation of a polymer film at the water-oil interface.

The water-oil interface is an environment that is often found in many contexts of the natural sciences and technological arenas. This interface has always been considered a special environment as it is rich in different phenomena, thus stimulating numerous studies aimed at understanding the abundance of physico-chemical problems that occur there. The intense research activity and the intriguing results that emerged from these investigations have inspired scientists to consider the water-oil interface even as a suitable setting for bottom-up nanofabrication processes, such as molecular self-assembly, or fabrication of nanofilms or nano-devices. On the other hand, biphasic liquid separation is a key enabling technology in many applications, including water treatment for environmental problems. Here we show for the first time an instant nanofabrication strategy of a thin film of biopolymer at the water-oil interface. The polymer film is fabricated in situ, simply by injecting a drop of polymer solution at the interface. Furthermore, we demonstrate that with an appropriate multiple drop delivery it is also possible to quickly produce a large area film (up to 150 cm2). The film inherently separates the two liquids, thus forming a separation layer between them and remains stable at the interface for a long time. Furthermore, we demonstrate the fabrication with different oils, thus suggesting potential exploitation in different fields (e.g. food, pollution, biotechnology). We believe that the new strategy fabrication could inspire different uses and promote applications among the many scenarios already explored or to be studied in the future at this special interface environment.

Microfluidic formation of crystal-like structures.

Crystal-like structures find application in several fields ranging from biomedical engineering to material science. For instance, droplet crystals are critical for high throughput assays and material synthesis, while particle crystals are important for particles and cell encapsulation, Drop-seq technologies, and single-cell analysis. Formation of crystal-like structures relies entirely on the possibility of manipulating with great accuracy the micrometer-size objects forming the crystal. In this context, microfluidic devices offer versatile tools for the precise manipulation of droplets and particles, thus enabling fabrication of crystal-like structures that form due to hydrodynamic interactions among droplets or particles. In this review, we aim at providing an holistic representation of crystal-like structure formation mediated by hydrodynamic interactions in microfluidic devices. We also discuss the physical origin of these hydrodynamic interactions and their relation to parameters such as device geometry, fluid properties, and flow conditions.

Axisymmetric bare freestanding films of highly viscous liquids: Preparation and real-time investigation of capillary leveling.

HYPOTHESIS: Thin liquid films are important in many scientific fields. In particular, films with both the surface layers exposed to a different fluid phase, known as freestanding films, are relevant in the ambit of foams and emulsions. Hence, there is a great interest in developing novel techniques allowing to form large and stable freestanding liquid films and to follow their dynamics. EXPERIMENTS: We develop a novel opto-mechanical tool allowing to perform and study the preparation and the capillary leveling flow of axisymmetric bare freestanding liquid films. The tool is composed by a customized motorized iris diaphragm and by an innovative joint imaging setup combining digital holography and white light color interferometry that enables real-time measurement of film thickness over a large field of view. The dynamics of films made of a model Newtonian fluid, i.e., high-viscosity silicone oil, is studied. Direct numerical simulations and a hydrodynamic model based on the lubrication theory are used to support the experimental results. FINDINGS: Iris opening induces the formation of large circular freestanding films with a stepped profile. Once iris opening is stopped, the films undergo a capillary leveling flow tending to flatten their profile. The leveling flow follows the theoretical scaling given by Ilton et al. [1]. We prove through numerical simulations that an equi-biaxial extensional flow occurs at the film center. Furthermore, we observe the formation and dynamics of dimples in bare freestanding films for the first time.

Dehydration of plant cells shoves nuclei rotation allowing for 3D phase-contrast tomography.

Single-cell phase-contrast tomography promises to become decisive for studying 3D intracellular structures in biology. It involves probing cells with light at wide angles, which unfortunately requires complex systems. Here we show an intriguing concept based on an inherent natural process for plants biology, i.e., dehydration, allowing us to easily obtain 3D-tomography of onion-epidermal cells' nuclei. In fact, the loss of water reduces the turgor pressure and we recognize it induces significant rotation of cells' nuclei. Thanks to the holographic focusing flexibility and an ad-hoc angles' tracking algorithm, we combine different phase-contrast views of the nuclei to retrieve their 3D refractive index distribution. Nucleolus identification capability and a strategy for measuring morphology, dry mass, biovolume, and refractive index statistics are reported and discussed. This new concept could revolutionize the investigation in plant biology by enabling dynamic 3D quantitative and label-free analysis at sub-nuclear level using a conventional holographic setup.

Assembling and rotating erythrocyte aggregates by acoustofluidic pressure enabling full phase-contrast tomography.

The combined use of ultrasound radiation and microfluidics is a promising tool for aiding the development of lab-on-a-chip devices. In this study, we show that the rotation of linear aggregates of micro-particles can be achieved under the action of acoustic field pressure. This novel manipulation is investigated by tracking polystyrene beads of different sizes through the 3D imaging features of digital holography (DH). From our analysis it is understood that the positioning of the micro-particles and their aggregations are associated with the effect of bulk acoustic radiation forces. The observed rotation is instead found to be compatible with the presence of acoustic streaming patterns as evidenced by our modelling and the resulting numerical simulation. Furthermore, the rotation frequency is shown to depend on the input voltage applied on the acoustic device. Finally, we demonstrate that we can take full advantage of such rotation by combining it with quantitative phase imaging of DH for a significant lab-on-a-chip biomedical application. In fact, we demonstrate that it is possible to put in rotation a linear aggregate of erythrocytes and rely on holographic imaging to achieve a full phase-contrast tomography of the aforementioned aggregate.

Quantitative imaging of the complexity in liquid bubbles' evolution reveals the dynamics of film retraction.

The dynamics and stability of thin liquid films have fascinated scientists over many decades. Thin film flows are central to numerous areas of engineering, geophysics, and biophysics and occur over a wide range of lengths, velocities, and liquid property scales. In spite of many significant developments in this area, we still lack appropriate quantitative experimental tools with the spatial and temporal resolution necessary for a comprehensive study of film evolution. We propose tackling this problem with a holographic technique that combines quantitative phase imaging with a custom setup designed to form and manipulate bubbles. The results, gathered on a model aqueous polymeric solution, provide unparalleled insight into bubble dynamics through the combination of a full-field thickness estimation, three-dimensional imaging, and a fast acquisition time. The unprecedented level of detail offered by the proposed methodology will promote a deeper understanding of the underlying physics of thin film dynamics.

Full-angle tomographic phase microscopy of flowing quasi-spherical cells.

We report a reliable full-angle tomographic phase microscopy (FA-TPM) method for flowing quasi-spherical cells along microfluidic channels. This method lies in a completely passive optical system, i.e. mechanical scanning or multi-direction probing of the sample is avoided. It exploits the engineered rolling of cells while they are flowing along a microfluidic channel. Here we demonstrate significant progress with respect to the state of the art of in-flow TPM by showing a general extension to cells having almost spherical shapes while they are flowing in suspension. In fact, the adopted strategy allows the accurate retrieval of rotation angles through a theoretical model of the cells' rotation in a dynamic microfluidic flow by matching it with phase-contrast images resulting from holographic reconstructions. So far, the proposed method is the first and the only one that permits to get in-flow TPM by probing the cells with full-angle, achieving accurate 3D refractive index mapping and the simplest optical setup, simultaneously. Proof of concept experiments were performed successfully on human breast adenocarcinoma MCF-7 cells, opening the way for the full characterization of circulating tumor cells (CTCs) in the new paradigm of liquid biopsy.

Is microrheometry affected by channel deformation?

Microrheometry is very important for exploring rheological behaviours of several systems when conventional techniques fail. Microrheometrical measurements are usually carried out in microfluidic devices made of Poly(dimethylsiloxane) (PDMS). Although PDMS is a very cheap material, it is also very easy to deform. In particular, a liquid flowing in a PDMS device, in some circumstances, can effectively deform the microchannel, thus altering the flow conditions. The measure of the fluid relaxation time might be performed through viscoelasticity induced particle migration in microfluidics devices. If the channel walls are deformed by the flow, the resulting measured value of the relaxation time could be not reliable. In this work, we study the effect of channel deformation on particle migration in square-shaped microchannel. Experiments are carried out in several PolyEthylene Oxyde solutions flowing in two devices made of PDMS and Poly(methylmethacrylate) (PMMA). The relevance of wall rigidity on particle migration is investigated, and the corresponding importance of wall rigidity on the determination of the relaxation time of the suspending liquid is examined.

Rheometry-on-a-chip: measuring the relaxation time of a viscoelastic liquid through particle migration in microchannel flows.

A novel method to estimate the relaxation time of viscoelastic fluids, down to milliseconds, is here proposed. The adopted technique is based on the particle migration phenomenon occurring when the suspending viscoelastic fluid flows in microfluidic channels. The method is applied to measure the fluid relaxation times of two water-glycerol polymer solutions in an ample range of concentrations. A remarkable improvement in the accuracy of the measure of the relaxation time is found, as compared with experimental data obtained from shear or elongational experiments available in the literature. Good agreement with available theoretical predictions is also found. The proposed method is reliable, handy and does not need a calibration curve, opening an effective way to measure relaxation times of viscoelastic fluids otherwise not easily detectable by conventional techniques.