Imperial College London


Faculty of EngineeringDepartment of Chemical Engineering

Visiting Professor







Ms Sevgi Thompson +44 (0)20 7594 1478




ACE ExtensionSouth Kensington Campus





Publication Type

66 results found

Saint-Michel B, Garbin V, 2020, Bubble dynamics for broadband microrheology of complex fluids, Publisher: arXiv

Bubbles in complex fluids are often desirable, and sometimes simplyinevitable, in the processing of formulated products. Bubbles can rise bybuoyancy, grow or dissolve by mass transfer, and readily respond to changes inpressure, thereby applying a deformation to the surrounding complex fluid. Thedeformation field around a stationary, spherical bubble undergoing a change inradius is simple and localised, thus making it suitable for rheologicalmeasurements. This article reviews emerging approaches to extract informationon the rheology of complex fluids by analysing bubble dynamics. The focus is onthree phenomena: changes in radius by mass transfer, harmonic oscillationsdriven by an acoustic wave, and bubble collapse. These phenomena cover a broadrange of deformation frequencies, from $10^{-4}$ to $10^6$ Hz, thus paving theway to broadband microrheology using bubbles as active probes. The outstandingchallenges that need to be overcome to achieve a robust technique are alsodiscussed

Working paper

Garbin V, Baresch D, 2020, Acoustic trapping of microbubbles in complex environments and controlled payload release, Proceedings of the National Academy of Sciences of USA, Vol: 117, Pages: 15490-15496, ISSN: 0027-8424

Contactless manipulation of microparticles using acoustic waves holds promise for applications ranging from cell sorting to three-dimensional (3D) printing and tissue engineering. However, the unique potential of acoustic trapping to be applied in biomedical settings remains largely untapped. In particular, the main advantage of acoustic trapping over optical trapping, namely the ability of sound to propagate through thick and opaque media, has not yet been exploited in full. Here we demonstrate experimentally the use of the recently developed technique of single-beam acoustical tweezers to trap microbubbles, an important class of biomedically relevant microparticles. We show that the region of vanishing pressure of a propagating vortex beam can confine a microbubble by forcing low-amplitude, nonspherical, shape oscillations, enabling its full 3D positioning. Our interpretation is validated by the absolute calibration of the acoustic trapping force and the direct spatial mapping of isolated bubble echos, for which both find excellent agreement with our theoretical model. Furthermore, we prove the stability of the trap through centimeter-thick layers of bio-mimicking, elastic materials. Finally, we demonstrate the simultaneous trapping of nanoparticle-loaded microbubbles and activation with an independent acoustic field to trigger the release of the nanoparticles. Overall, using exclusively acoustic powering to position and actuate microbubbles paves the way toward controlled delivery of drug payloads in confined, hard-to-reach locations, with potential in vivo applications.

Journal article

Saha S, Saint-Michel B, Leynes V, Binks BP, Garbin Vet al., 2020, Stability of bubbles in wax-based oleofoams: decoupling the effects of bulk oleogel rheology and interfacial rheology, Rheologica Acta: an international journal of rheology, Vol: 59, Pages: 255-266, ISSN: 0035-4511

Oleofoams are dispersions of gas bubbles in a continuous oil phase and can be stabilized by crystals of fatty acids or waxes adsorbing at the oil-air interface. Because excess crystals in the continuous phase form an oleogel, an effect of the bulk rheology of the continuous phase is also expected. Here, we evaluate the contributions of bulk and interfacial rheology below and above the melting point of a wax forming an oleogel in sunflower oil. We study the dissolution behaviour of single bubbles using microscopy on a temperature-controlled stage. We compare the behaviour of a bubble embedded in an oleofoam, which owes its stability to both bulk and interfacial rheology, to that of a bubble extracted from the oleofoam and resuspended in oil, for which the interfacial dilatational rheology alone provides stability. We find that below the melting point of the wax, bubbles in the oleofoam are stable whereas bubbles that are only coated with wax crystals dissolve. Both systems dissolve when heated above the melting point of the wax. These findings are rationalized through independent bulk rheological measurements of the oleogel at different temperatures, as well as measurements of the dilatational rheological properties of a wax-coated oil-air interface.

Journal article

Udoh CE, Garbin V, Cabral JT, 2019, Polymer nanocomposite capsules formed by droplet extraction: spontaneous stratification and tailored dissolution, Soft Matter, Vol: 15, Pages: 5287-5295, ISSN: 1744-683X

We report the formation of polymeric and nanocomposite capsules via droplet solvent extraction, focusing on the interplay between solvent exchange and removal, demixing and directional solidification kinetics. We investigate a model system of sodium poly(styrene sulfonate), NaPSS and silica nanoparticles in aqueous solution, whose phase behaviour is experimentally measured, and examine a series of selective extraction solvents (toluene, butyl acetate, ethyl acetate and methyl ethyl ketone), ranging from 0.04 to 11% v/v water solubility. Tuning the rate of solvent exchange is shown to provide an effective means of decoupling demixing and solidification timescales, and thereby tunes the internal microstructure of the capsule, including hollow, microporous, core–shell, and bicontinuous morphologies. In turn, these determine the capsule dissolution mechanism and kinetics, ranging from single to pulsed release profiles of nanoparticle clusters (at intermediate solubilities), to minimal dissolution (at either extremes). These findings provide facile design and assembly strategies for functional capsules with time-varying release profiles.

Journal article

De Corato M, Saint-Michel B, Makrigiorgos G, Dimakopoulos Y, Tsamopoulos J, Garbin Vet al., 2019, Oscillations of small bubbles and medium yielding in elastoviscoplastic fluids, Physical Review Fluids, Vol: 4, Pages: 073301-1-073301-21, ISSN: 2469-990X

We investigate the radial oscillations of small gas bubbles trapped in yield-stress fluids and driven by an acoustic pressure field. We model the rheological behavior of the yield-stress fluid using the recently developed elastoviscoplastic constitutive equation that takes into account the elastic and viscoplastic deformations of the material [Saramito, J. Non-Newton. Fluid Mech. 158, 154 (2009)]. Assuming that the bubble remains spherical during the pressure driving, we reduce the problem to a set of ordinary differential equations and an integrodifferential equation, which we solve numerically for the case of two yield-stress fluids, i.e., a soft Carbopol gel and a stiffer kaolin suspension. We find that depending on the amplitude and frequency of the pressure field, the radial oscillations of the bubble produce elastic stresses that may or may not suffice to yield the surrounding material. We evaluate the critical amplitude of the acoustic pressure required to achieve yielding and we find a good agreement between numerical simulations and an analytical formula derived under the assumption of linear deformations. Finally, we examine the bubble oscillation amplitude for a very wide range of applied pressures both below and above the critical value to assess the impact of yielding on the bubble dynamics. This analysis could be used to identify a signature of yielding in experiments where the radial dynamics of a bubble is measured. More generally, these results can be used to rationalize the optimal conditions for pressure-induced bubble release from yield-stress fluids, which is relevant to various biomedical and industrial applications, including the oil industry and food processing.

Journal article

Jamburidze A, Huerre A, Baresch D, Poulichet V, De Corato M, Garbin Vet al., 2019, Nanoparticle-coated microbubbles for combined ultrasound imaging and drug delivery, Langmuir, Vol: 35, Pages: 10087-10096, ISSN: 0743-7463

Biomedical microbubbles stabilized by a coating of magnetic or drug-containing nanoparticles show great potential for theranostics applications. Nanoparticle-coated microbubbles can be made to be stable, to be echogenic, and to release the cargo of drug-containing nanoparticles with an ultrasound trigger. This Article reviews the design principles of nanoparticle-coated microbubbles for ultrasound imaging and drug delivery, with a particular focus on the physical chemistry of nanoparticle-coated interfaces; the formation, stability, and dynamics of nanoparticle-coated bubbles; and the conditions for controlled nanoparticle release in ultrasound. The emerging understanding of the modes of nanoparticle expulsion and of the transport of expelled material by microbubble-induced flow is paving the way toward more efficient nanoparticle-mediated drug delivery. This Article highlights the knowledge gap that still remains to be addressed before we can control these phenomena.

Journal article

Garbin V, 2019, Collapse mechanisms and extreme deformation of particle-laden interfaces, Current Opinion in Colloid and Interface Science, Vol: 39, Pages: 202-211, ISSN: 1359-0294

Particle-laden interfaces are at the basis of many advanced materials, such as bijels and dry water. While the final properties of these materials can generally be controlled, their response to deformation during processing and use is still poorly understood. In particular, the dynamics of particle-laden interfaces in relevant flow conditions is receiving increasing attention. These conditions are typically highly dynamic and can involve unsteady flow or large deformations. This paper gives an overview of the remarkable phenomena of particle-laden interfaces undergoing deformations of large amplitude and at high strain rate, in other words extreme deformation. Upon large-amplitude compression, a monolayer of particles can collapse by buckling or by expelling particles in the liquid. The criteria for buckling or expulsions are discussed, as well as recent experiments in highly dynamic conditions showing that these criteria can depend also on the rate of deformation. The emerging use of ultrasound-driven bubbles as an experimental platform for controlled deformation of particle-laden interfaces at high strain rate is also discussed. The ability to control the fate of particles at interfaces during dynamic deformation of droplets or bubbles ultimately underpins a variety of applications from controlled release to catalysis.

Journal article

Dollet B, Marmottant P, Garbin V, 2019, Bubble Dynamics in Soft and Biological Matter, Publisher: ANNUAL REVIEWS


Huerre A, De Corato M, Garbin V, 2018, Dynamic capillary assembly of colloids at interfaces with 10,000g accelerations, Nature Communications, Vol: 9, ISSN: 2041-1723

High-rate deformation of soft matter is an emerging area central to our understanding of far-from-equilibrium phenomena during shock, fracture, and phase change. Monolayers of colloidal particles are a convenient two-dimensional model system to visualise emergent behaviours in soft matter, but previous studies have been limited to slow deformations. Here we probe and visualise the evolution of a monolayer of colloids confined at a bubble surface during high-rate deformation driven by ultrasound. We observe the emergence of a transient network of strings, and use discrete particle simulations to show that it is caused by a delicate interplay of dynamic capillarity and hydrodynamic interactions between particles oscillating at high frequency. Remarkably for a colloidal system, we find evidence of inertial effects, caused by accelerations approaching 10,000g. These results also suggest that extreme deformation of soft matter offers new opportunities for pattern formation and dynamic self-assembly.

Journal article

De Corato M, Garbin V, 2018, Capillary interactions between dynamically forced particles adsorbed at a planar interface and on a bubble, Journal of Fluid Mechanics, Vol: 847, Pages: 71-92, ISSN: 0022-1120

We investigate the dynamic interfacial deformation induced by micrometric particles exerting a periodic force on a planar interface or on a bubble, and the resulting lateral capillary interactions. Assuming that the deformation of the interface is small, neglecting the effect of viscosity and assuming point particles, we derive analytical formulas for the dynamic deformation of the interface. For the case of a planar interface the dynamic point force simply generates capillary waves, while for the case of a bubble it excites shape oscillations, with a dominant deformation mode that depends on the bubble radius for a given forcing frequency. We evaluate the lateral capillary force acting between two particles, by superimposing the deformations induced by two point forces. We find that the lateral capillary forces experienced by dynamically forced particles are non-monotonic and can be repulsive. The results are applicable to micrometric particles driven by different dynamic forcing mechanisms such as magnetic, electric or acoustic fields.

Journal article

Lin S, Zhang G, Jamburidze A, Chee M, Leow CH, Garbin V, Tang M-Xet al., 2018, Imaging of vaporised sub-micron phase change contrast agents with high frame rate ultrasound and optics., Phys Med Biol, Vol: 63, Pages: 065002-065002

Phase-change ultrasound contrast agent (PCCA), or nanodroplet, shows promise as an alternative to the conventional microbubble agent over a wide range of diagnostic applications. Meanwhile, high-frame-rate (HFR) ultrasound imaging with microbubbles enables unprecedented temporal resolution compared to traditional contrast-enhanced ultrasound imaging. The combination of HFR ultrasound imaging and PCCAs can offer the opportunity to observe and better understand PCCA behaviour after vaporisation captures the fast phenomenon at a high temporal resolution. In this study, we utilised HFR ultrasound at frame rates in the kilohertz range (5-20 kHz) to image native and size-selected PCCA populations immediately after vaporisation in vitro within clinical acoustic parameters. The size-selected PCCAs through filtration are shown to preserve a sub-micron-sized (mean diameter  <  200 nm) population without micron-sized outliers (>1 µm) that originate from native PCCA emulsion. The results demonstrate imaging signals with different amplitudes and temporal features compared to that of microbubbles. Compared with the microbubbles, both the B-mode and pulse-inversion (PI) signals from the vaporised PCCA populations were reduced significantly in the first tens of milliseconds, while only the B-mode signals from the PCCAs were recovered during the next 400 ms, suggesting significant changes to the size distribution of the PCCAs after vaporisation. It is also shown that such recovery in signal over time is not evident when using size-selective PCCAs. Furthermore, it was found that signals from the vaporised PCCA populations are affected by the amplitude and frame rate of the HFR ultrasound imaging. Using high-speed optical camera observation (30 kHz), we observed a change in particle size in the vaporised PCCA populations exposed to the HFR ultrasound imaging pulses. These findings can further the understanding of PCCA

Journal article

Ahmmed SM, Suteria NS, Garbin V, Vanapalli SAet al., 2018, Hydrodynamic mobility of confined polymeric particles, vesicles, and cancer cells in a square microchannel, Biomicrofluidics, Vol: 12, ISSN: 1932-1058

The transport of deformable objects, including polymer particles, vesicles, and cells, has been a subject of interest for several decades where the majority of experimental and theoretical studies have been focused on circular tubes. Due to advances in microfluidics, there is a need to study the transport of individual deformable particles in rectangular microchannels where corner flows can be important. In this study, we report measurements of hydrodynamic mobility of confined polymeric particles, vesicles, and cancer cells in a linear microchannel with a square cross-section. Our operating conditions are such that the mobility is measured as a function of geometric confinement over the range 0.3 < λ < 1.5 and at specified particle Reynolds numbers that are within 0.1 < Rep < 2.5. The experimental mobility data of each of these systems is compared with the circular-tube theory of Hestroni, Haber, and Wacholder [J. Fluid Mech. 41, 689–705 (1970)] with modifications made for a square cross-section. For polymeric particles, we find that the mobility data agrees well over a large confinement range with the theory but under predicts for vesicles. The mobility of vesicles is higher in a square channel than in a circular tube, and does not depend significantly on membrane mechanical properties. The mobility of cancer cells is in good agreement with the theory up to λ ≈ 0.8, after which it deviates. Comparison of the mobility data of the three systems reveals that cancer cells have higher mobility than rigid particles but lower than vesicles, suggesting that the cell membrane frictional properties are in between a solid-like interface and a fluid bilayer. We explain further the differences in the mobility of the three systems by considering their shape deformation and surface flow on the interface. The results of this study may find potential applications in drug del

Journal article

Udoh C, CABRAL J, Garbin V, 2017, Nanocomposite capsules with directional, pulsed nanoparticle release, Science Advances, Vol: 3, ISSN: 2375-2548

The precise spatiotemporal delivery of nanoparticles from polymeric capsules is required for applications ranging from medicine to materials science. These capsules derive key performance aspects from their overall shape and dimensions, porosity, and internal microstructure. To this effect, microfluidics provide an exceptional platform for emulsification and subsequent capsule formation. However, facile and robust approaches for nanocomposite capsule fabrication, exhibiting triggered nanoparticle release, remain elusive because of the complex coupling of polymer-nanoparticle phase behavior, diffusion, phase inversion, and directional solidification. We investigate a model system of polyelectrolyte sodium poly(styrene sulfonate) and 22-nm colloidal silica and demonstrate a robust capsule morphology diagram, achieving a range of internal morphologies, including nucleated and bicontinuous microstructures, as well as isotropic and non-isotropic external shapes. Upon dissolution in water, we find that capsules formed with either neat polymers or neat nanoparticles dissolve rapidly and isotropically, whereas bicontinuous, hierarchical, composite capsules dissolve via directional pulses of nanoparticle clusters without disrupting the scaffold, with time scales tunable from seconds to hours. The versatility, facile assembly, and response of these nanocomposite capsules thus show great promise in precision delivery.

Journal article

huerre, Cacho-Nerin F, udoh, Poulichet V, de corato, Garbin Vet al., 2017, Dynamic organization of ligand-grafted nanoparticles during adsorption and surface compression at fluid-fluid interfaces, Langmuir, Vol: 34, Pages: 1020-1028, ISSN: 0743-7463

Monolayers of ligand-grafted nanoparticles at fluid interfaces exhibit a complex response to deformation due to an interplay of particle rearrangements within the monolayer, and molecular rearrangements of the ligand brush on the surface of the particles. We use grazing-incidence small-angle X-ray scattering (GISAXS) combined with pendant drop tensiometry to probe in situ the dynamic organization of ligand-grafted nanoparticles upon adsorption at a fluid–fluid interface, and during monolayer compression. Through the simultaneous measurements of interparticle distance, obtained from GISAXS, and of surface pressure, obtained from pendant drop tensiometry, we link the interfacial stress to the monolayer microstructure. The results indicate that, during adsorption, the nanoparticles form rafts that grow while the interparticle distance remains constant. For small-amplitude, slow compression of the monolayer, the evolution of the interparticle distance bears a signature of ligand rearrangements leading to a local decrease in thickness of the ligand brush. For large-amplitude compression, the surface pressure is found to be strongly dependent on the rate of compression. Two-dimensional Brownian dynamics simulations show that the rate-dependent features are not due to jamming of the monolayer, and suggest that they may be due to out-of-plane reorganization of the particles (for instance expulsion or buckling). The corresponding GISAXS patterns are also consistent with out-of-plane reorganization of the nanoparticles.

Journal article

Lazarus C, Pouliopoulos AN, Tinguely M, Garbin V, Choi JJet al., 2017, Clustering dynamics of microbubbles exposed to low-pressure 1-MHz ultrasound, Journal of the Acoustical Society of America, Vol: 142, Pages: 3135-3146, ISSN: 0001-4966

Ultrasound-driven microbubbles have been used in therapeutic applications to deliver drugs acrosscapillaries and into cells or to dissolve blood clots. Yet the performance and safety of these applica-tions have been difficult to control. Microbubbles exposed to ultrasound not only volumetricallyoscillate, but also move due to acoustic radiation, or Bjerknes, forces. The purpose of this work wasto understand the extent to which microbubbles moved and clustered due to secondary Bjerknesforces. A microbubble population was exposed to a 1-MHz ultrasound pulse with apeak-rarefactional pressure of 50–100 kPa and a pulse length of 20 ms. Microbubbles exposed tolow-pressure therapeutic ultrasound were observed to cluster at clustering rates of 0.01–0.02 micro-bubbles per duration (in ms) per initial average inter-bubble distance (inlm), resulting in 1 to 3clustered microbubbles per initial average inter-bubble distance (inlm). Higher pressures causedfaster clustering rates and a larger number of clustered microbubbles. Experimental data revealedclustering time scales, cluster localizations, and cluster sizes that were in reasonable agreementwith simulations using a time-averaged model at low pressures. This study demonstrates that clus-tering of microbubbles occurs within a few milliseconds and is likely to influence the distributionof stimuli produced in therapeutic applications.

Journal article

Lin S, Zhang G, Jamburidze A, Chee M, Leow CH, Garbin V, Tang M-Xet al., 2017, High Frame Rate Ultrasound Imaging of Vaporised Phase Change Contrast Agents, IEEE International Ultrasonics Symposium (IUS), Publisher: IEEE, ISSN: 1948-5719

Conference paper

Garbin V, 2017, Dynamics of coated microbubbles in ultrasound, The Micro-World Observed by Ultra High-Speed Cameras: We See What You Don't See, Pages: 357-374, ISBN: 9783319614908

© Springer International Publishing AG 2018. All rights reserved. The stability and dynamics of microbubbles coated with an interfacial layer of adsorbed material, ranging from phospholipids, to proteins and nanoparticles, are central to food products, biomedical imaging applications, and controlled release. The dynamics of coated microbubbles in ultrasound fields are of particular relevance to food production and biomedical imaging. High-speed imaging has proven to be an invaluable tool to reveal micromechanical phenomena of the coating during ultrasound-driven microbubble dynamics, so as to gain a fundamental understanding of the factors affecting microbubble durability and performance. This Chapter includes an introduction to the basic concepts of microbubble stability (Sect. 1), and to the dynamics of coated microbubbles in ultrasound (Sect. 2). An overview of recent research advances is then provided, focusing on the following topics: Dynamics of biomedical microbubbles in ultrasound studied by combined optical trapping and ultra-high speed imaging (Sect. 3); Buckling and expulsion of coating material from ultrasound-driven microbubbles (Sect. 4); Shape oscillations of coated bubbles (Sect. 5).

Book chapter

Jamburidze A, De Corato M, Huerre A, Pommella A, Garbin Vet al., 2017, High-frequency linear rheology of hydrogels probed by ultrasound-driven microbubble dynamics, Soft Matter, Vol: 13, Pages: 3946-3953, ISSN: 1744-6848

Ultrasound-driven microbubble dynamics are central to biomedical applications, from diagnostic imaging to drug delivery and therapy. In therapeutic applications, the bubbles are typically embedded in tissue, and their dynamics are strongly affected by the viscoelastic properties of the soft solid medium. While the behaviour of bubbles in Newtonian fluids is well characterised, a fundamental understanding of the effect on ultrasound-driven bubble dynamics of a soft viscoelastic medium is still being developed. We characterised the resonant behaviour in ultrasound of isolated microbubbles embedded in agarose gels, commonly used as tissue-mimicking phantoms. Gels with different viscoelastic properties were obtained by tuning agarose concentration, and were characterised by standard rheological tests. Isolated bubbles (100–200 μm) were excited by ultrasound (10–50 kHz) at small pressure amplitudes (<1 kPa), to ensure that the deformation of the material and the bubble dynamics remained in the linear regime. The radial dynamics of the bubbles were recorded by high-speed video microscopy. Resonance curves were measured experimentally and fitted to a model combining the Rayleigh–Plesset equation governing bubble dynamics, with the Kelvin–Voigt model for the viscoelastic medium. The resonance frequency of the bubbles was found to increase with increasing shear modulus of the medium, with implications for optimisation of imaging and therapeutic ultrasound protocols. In addition, the viscoelastic properties inferred from ultrasound-driven bubble dynamics differ significantly from those measured at low frequency with the rheometer. Hence, rheological characterisation of biomaterials for medical ultrasound applications requires particular attention to the strain rate applied.

Journal article

Achakulwisut K, Tam C, Huerre A, Sammouti R, Binks BP, Garbin Vet al., 2017, Stability of clay particle-coated microbubbles in alkanes against dissolution induced by heating, Langmuir, Vol: 33, Pages: 3809-3817, ISSN: 1520-5827

We investigated the dissolution and morphological dynamics of air bubbles in alkanes stabilized by fluorinated colloidal clay particles when subjected to temperature changes. A model for bubble dissolution with time-dependent temperature reveals that increasing the temperature enhances the bubble dissolution rate in alkanes, opposite to the behavior in water, because of the differing trends in gas solubility. Experimental results for uncoated air bubbles in decane and hexadecane confirm this prediction. Clay-coated bubbles in decane and hexadecane are shown to be stable in air-saturated oil at constant temperature, where dissolution is driven mainly by the Laplace pressure. When the temperature increases from ambient, the particle-coated bubbles are prone to dissolution as the oil phase becomes undersaturated. The interfacial layer of particles is observed to undergo buckling and crumpling, without shedding of clay particles. Increasing the concentration of particles is shown to enhance the bubble stability by providing a higher resistance to dissolution. When subjected to complex temperature cycles, for which the effect of time-dependent temperature is dominant, the clay-coated bubbles can resist long-term dissolution in conditions under which uncoated bubbles dissolve completely. These results underpin the design of ultrastable oil foams stabilized by solid particles with improved shelf life under changing environmental conditions.

Journal article

Pouliopoulos A, Caiqin L, Tinguely M, Garbin V, Tang M, Choi JJet al., 2016, Rapid short-pulse sequences enhance the spatiotemporal uniformity of acoustically driven microbubble activity during flow conditions, Journal of the Acoustical Society of America, Vol: 140, ISSN: 0001-4966

Despite the promise of microbubble-mediated focused ultrasound therapies, in vivo findings have revealed over-treated and under-treated regions distributed throughout the focal volume. This poor distribution cannot be improved by conventional pulse shapes and sequences, due to their limited ability to control acoustic cavitation dynamics within the ultrasonic focus. This paper describes the design of a rapid short-pulse (RaSP) sequence which is comprised of short pulses separated by μs off-time intervals. Improved acoustic cavitation distribution was based on the hypothesis that microbubbles can freely move during the pulse off-times. Flowing SonoVue® microbubbles (flow velocity: 10 mm/s) were sonicated with a 0.5 MHz focused ultrasound transducer using RaSP sequences (peak-rarefactional pressures: 146–900 kPa, pulse repetition frequency: 1.25 kHz, and pulse lengths: 5–50 cycles). The distribution of cavitation activity was evaluated using passive acoustic mapping. RaSP sequences generated uniform distributions within the focus in contrast to long pulses (50 000 cycles) that produced non-uniform distributions. Fast microbubble destruction occurred for long pulses, whereas microbubble activity was sustained for longer durations for shorter pulses. High-speed microscopy revealed increased mobility in the direction of flow during RaSP sonication. In conclusion, RaSP sequences produced spatiotemporally uniform cavitation distributions and could result in efficient therapies by spreading cavitation throughout the treatment area.

Journal article

Poulichet V, Huerre A, Garbin V, 2016, Shape oscillations of particle-coated bubbles and directional particle expulsion, Soft Matter, Vol: 13, Pages: 125-133, ISSN: 1744-6848

Bubbles stabilised by colloidal particles can find applications in advanced materials, catalysis anddrug delivery. For applications in controlled release, it is desirable to remove the particles fromthe interface in a programmable fashion. We have previously shown that ultrasound waves excitevolumetric oscillations of particle-coated bubbles, resulting in precisely timed particle expulsiondue to interface compression on a ultrafast timescale [Poulichet et al., Proc. Natl. Acad. Sci.USA, 2015, 112, 5932]. We also observed shape oscillations, which were found to drive directionalparticle expulsion from the antinodes of the non-spherical deformation. In this paper weinvestigate the mechanisms leading to directional particle expulsion during shape oscillations ofparticle-coated bubbles driven by ultrasound at 40 kHz. We perform high-speed visualisation ofthe interface shape and of the particle distribution during ultrafast deformation at a rate of upto 105 s−1. The mode of shape oscillations is found to not depend on the bubble size, in contrastwith what has been reported for uncoated bubbles. A decomposition of the non-sphericalshape in spatial Fourier modes reveals that the interplay of different modes determines the locationsof particle expulsion. The n-fold symmetry of the dominant mode does not always leadto desorption from all 2n antinodes, but only those where there is favourable alignment with thesub-dominant modes. Desorption from the antinodes of the shape oscillations is due to different,concurrent mechanisms. The radial acceleration of the interface at the antinodes can be upto 105 − 106 ms−2, hence there is a contribution from the inertia of the particles localised at theantinodes. In addition, we found that particles migrate to the antinodes of the shape oscillation,thereby enhancing the contribution from the surface pressure in the monolayer.

Journal article

Udoh C, Garbin V, Cabral JP, 2016, Microporous polymer particles via phase inversion in microfluidics: impact of non-solvent quality, Langmuir, Vol: 32, Pages: 8131-8140, ISSN: 0743-7463

We investigate the impact of ternary phase behavior on the microstructure of porous polymer particles produced by solvent extraction of polymer solution droplets by a nonsolvent. Microfluidic devices fabricated by frontal photopolymerization are employed to produce monodisperse polymer (P)/solvent (S) droplets suspended in a carrier (C) phase before inducing solvent extraction by precipitation in a nonsolvent (NS) bath. Model systems of sodium poly(styrenesulfonate) (P), water (S), hexadecane (C), and either methyl ethyl ketone (MEK) or ethyl acetate (EA) as NS are selected. Extraction across the liquid–liquid interface results in a decrease in the droplet radius and also an ingress of nonsolvent, leading to droplet phase demixing and coarsening. As the concentration of the polymer-rich phase increases, droplet shrinkage and solvent exchange slow down and eventually cease, resulting in microporous polymer particles (of radius ≃50–200 μm) with a smooth surface. The internal structure of these capsules, with pore sizes of ≃1–100 μm, is found to be controlled by polymer solution thermodynamics and the extraction pathway. The ternary phase diagrams are measured by turbidimetry, and the kinetics of phase separation is estimated by stopped-flow small-angle neutron scattering. The higher solubility of water in MEK results in faster particle-formation kinetics than in EA. Surprisingly, however, the lower polymer miscibility with EA/water results in a deeper quench inside the phase boundary and small phase sizes, thus yielding particles with small pores (of narrow distribution). The effects of droplet size, polymer content, and nonsolvent quality provide comprehensive insight into porous particle and capsule formation by phase inversion, with a range of practical applications.

Journal article

Tinguely M, Hennessy MG, Pommella A, Matar OK, Garbin Vet al., 2016, Surface waves on a soft viscoelastic layer produced by an oscillating microbubble, Soft Matter, Vol: 12, Pages: 4247-4256, ISSN: 1744-6848

Ultrasound-driven bubbles can cause significant deformation of soft viscoelastic layers, for instance in surface cleaning and biomedical applications. The effect of the viscoelastic properties of a boundary on the bubble–boundary interaction has been explored only qualitatively, and remains poorly understood. We investigate the dynamic deformation of a viscoelastic layer induced by the volumetric oscillations of an ultrasound-driven microbubble. High-speed video microscopy is used to observe the deformation produced by a bubble oscillating at 17–20 kHz in contact with the surface of a hydrogel. The localised oscillating pressure applied by the bubble generates surface elastic (Rayleigh) waves on the gel, characterised by elliptical particle trajectories. The tilt angle of the elliptical trajectories varies with increasing distance from the bubble. Unexpectedly, the direction of rotation of the surface elements on the elliptical trajectories shifts from prograde to retrograde at a distance from the bubble that depends on the viscoelastic properties of the gel. To explain these behaviours, we develop a simple three-dimensional model for the deformation of a viscoelastic solid by a localised oscillating force. By using as input for the model the values of the shear modulus obtained from the propagation velocity of the Rayleigh waves, we find good qualitative agreement with the experimental observations.

Journal article

Tinguely M, Matar OK, Garbin V, 2015, Tracking the deformation of a tissue phantom induced by ultrasound-driven bubble oscillations, 9th International Symposium on Cavitation (CAV2015), Publisher: IOP Publishing Ltd, ISSN: 1742-6588

Conference paper

Poulichet V, Garbin V, 2015, Cooling particle-coated bubbles: destabilization beyond dissolution arrest, Langmuir, Vol: 31, Pages: 12035-12042, ISSN: 1520-5827

Emulsions and foams that remain stable under varying environmental conditions are central in the food, personal care, and other formulated products industries. Foams stabilized by solid particles can provide longer-term stability than surfactant-stabilized foams. This stability is partly ascribed to the observation that solid particles can arrest bubble dissolution, which is driven by the Laplace pressure across the curved gas–liquid interface. We studied experimentally the effect of changes in temperature on the lifetime of particle-coated air microbubbles in water. We found that a decrease in temperature destabilizes particle-coated microbubbles beyond dissolution arrest. A quasi-steady model describing the effect of the change in temperature on mass transfer suggests that the dominant mechanism of destabilization is the increased solubility of the gas in the liquid, leading to a condition of undersaturation. Experiments at constant temperature confirmed that undersaturation alone can drive destabilization of particle-coated bubbles, even for vanishing Laplace pressure. We also found that dissolution of a particle-coated bubble can lead either to buckling of the coating or to gradual expulsion of particles, depending on the particle-to-bubble size ratio, with potential implications for controlled release.

Journal article

Ja'afar F, Leow CH, Garbin V, Sennoga CA, Tang M-X, Seddon JMet al., 2015, Surface Charge Measurement of SonoVue, Definity and Optison: A Comparison of Laser Doppler Electrophoresis and Micro-Electrophoresis, Ultrasound in Medicine and Biology, Vol: 41, Pages: 2990-3000, ISSN: 0301-5629

Microbubble (MB) contrast-enhanced ultrasonography is a promising tool for targeted molecular imaging. It is important to determine the MB surface charge accurately as it affects the MB interactions with cell membranes. In this article, we report the surface charge measurement of SonoVue, Definity and Optison. We compare the performance of the widely used laser Doppler electrophoresis with an in-house micro-electrophoresis system. By optically tracking MB electrophoretic velocity in a microchannel, we determined the zeta potentials of MB samples. Using micro-electrophoresis, we obtained zeta potential values for SonoVue, Definity and Optison of −28.3, −4.2 and −9.5 mV, with relative standard deviations of 5%, 48% and 8%, respectively. In comparison, laser Doppler electrophoresis gave −8.7, +0.7 and +15.8 mV with relative standard deviations of 330%, 29,000% and 130%, respectively. We found that the reliability of laser Doppler electrophoresis is compromised by MB buoyancy. Micro-electrophoresis determined zeta potential values with a 10-fold improvement in relative standard deviation.

Journal article

Pommella A, Brooks NJ, Seddon JM, Garbin Vet al., 2015, Selective flow-induced vesicle rupture to sort by membrane mechanical properties, Scientific Reports, Vol: 5, ISSN: 2045-2322

Journal article

Garbin V, Jenkins I, Sinno T, Crocker JC, Stebe KJet al., 2015, Erratum: Interactions and Stress Relaxation in Monolayers of Soft Nanoparticles at Fluid-Fluid Interfaces (Physical Review Letters (2015) 114:108301), Physical Review Letters, Vol: 115, ISSN: 0031-9007

Journal article

Poulichet V, Garbin V, 2015, Ultrafast desorption of colloidal particles from fluid interfaces., Proceedings of the National Academy of Sciences, ISSN: 1091-6490

The self-assembly of solid particles at fluid-fluid interfaces is widely exploited to stabilize emulsions and foams, and in materials synthesis. The self-assembly mechanism is very robust owing to the large capillary energy associated with particle adsorption, of the order of millions of times the thermal energy for micrometer-sized colloids. The microstructure of the interfacial colloid monolayer can also favor stability, for instance in the case of particle-stabilized bubbles, which can be indefinitely stable against dissolution due to jamming of the colloid monolayer. As a result, significant challenges arise when destabilization and particle removal are a requirement. Here we demonstrate ultrafast desorption of colloid monolayers from the interface of particle-stabilized bubbles. We drive the bubbles into periodic compression-expansion using ultrasound waves, causing significant deformation and microstructural changes in the particle monolayer. Using high-speed microscopy we uncover different particle expulsion scenarios depending on the mode of bubble deformation, including highly directional patterns of particle release during shape oscillations. Complete removal of colloid monolayers from bubbles is achieved in under a millisecond. Our method should find a broad range of applications, from nanoparticle recycling in sustainable processes to programmable particle delivery in lab-on-a-chip applications.

Journal article

Garbin V, 2015, Remotely triggered colloidal disassembly from particle-laden microbubble, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727

Conference paper

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