This page gives a preview of some of our group's highlighted research. For more information on our group's research, feel free to check our publications page and/or feel free to contact us! We are always interested in discussing interesting ideas and pursuing new collaborations.
Abstract: The dynamics of a thin liquid film on the underside of a curved cylindrical substrate is studied. The evolution of the liquid layer is investigated as the film thickness and the radius of curvature of the substrate are varied. A dimensionless parameter (a modified Bond number) that incorporates both geometric parameters, gravity, and surface tension is identified, and allows the observations to be classified according to three different flow regimes: stable films, films with transient growth of perturbations followed by decay, and unstable films. Experiments and linear stability theory confirm that below a critical value of the Bond number curvature of the substrate suppresses the Rayleigh-Taylor instability.
Publications: P. H. Trinh, H. Kim, N. Hammoud, P. D. Howell, S. J. Chapman, and H. A. Stone, Phys. Fluids 26, 051704 (2014).
Abstract: Control and stabilization of viscous fingering of immiscible fluids impacts a wide variety of pressure-driven multiphase flows. We report theoretical and experimental results on a time-dependent control strategy by manipulating the gap thickness b(t) in a lifting Hele-Shaw cell in the power-law form b(t) = b1t1/7. Experimental results show good quantitative agreement with the predictions of linear stability analysis. By choosing the value of a single time-independent control parameter, we can either totally suppress the viscous fingering instability or maintain a series of nonsplitting viscous fingers during the fluid displacement process. In addition to the gap thickness of a Hele-Shaw cell, time-dependent control strategies can, in principle, also be placed on the injection rate, viscosity of the displaced fluid, and interfacial tension between the two fluids.
Publications: Z. Zheng, H. Kim, and H. A. Stone, Phys. Rev. Lett. 115, 174501 (2015).
Abstract: Glycerol is a viscous liquid widely used in industry and known for its strong hygroscopic nature. While this unusual property has been well documented from the perspective of solution chemistry, its impact on the mechanical properties of glycerol remains largely unknown. We show that a Benard-Marangoni instability in pure glycerol can be spontaneously driven by absorption of water vapor. Even under standard laboratory conditions, ambient humidity is sufficient to drive distinct Benard-Marangoni convection cells for hours. Such an instability is a consequence of diffusive vapor transport process and competition between solutal and thermal Marangoni forces.
Publications: S. Shin, I. Jacobi and H. A. Stone, EPL 113, 24002 (2016).
Abstract: A drop impacting a smooth, solid, dry surfaces form a radially spreading lamella and sometimes results in splashing. In industrial and natural processes, it is common for the drops to impact the surface on an angle or while the surface is moving, yet previous studies mostly focused on the perpendicular impact of drops onto surfaces. We show that the tangential component of the impact can act to trigger or inhibit a splash, and we develop a model to predict this type of behavior. Our model agrees with previous experimental data and with our observations of the effects of tangential velocity.
Publications: J. C. Bird, S. H. Tsai, and H. A. Stone, New Journal of Physics 11 (2009).
Abstract: The surface-tension-driven coalescence of flexible structures is relevant to a number of engineering and biological systems, such as the clumping of hair, the failure of micro devices during wet lithography, or more generally whenever a liquid-air interface is moving through a deformable media. We study the dynamics of wetting of flexible boundaries with a combination of experiments, scaling arguments and theory. We consider three model systems. We investigate the rise and the spontaneous imbibition of a liquid between flexible sheets clamped at one end, and free to deflect at the other end, and study how the deformation of the sheets affects the meniscus speed and entraps the liquid. We also study the behavior of a single drop on a pair of flexible fibers and show that, due to a combination of capillary and elasticity effects, the drop spreads into a long liquid column, and there is an optimal volume at which the wetted length is maximum.
Publications: J. M. Aristoff, C. Duprat and H. A. Stone, Int. J. Non-Linear Mech. 46 (2011).
C. Duprat, J. M. Aristoff and H. A. Stone, J. Fluid Mech. 679 (2011).
C. Duprat, S. Protiere, A. Y. Beebe and and H. A. Stone, Nature 482 (2012).
Abstract: An unusual phenomenon occurs to low density material, and in particular air bubbles, entrained in a fluid when flowing through a T-junction. For a range of Reynolds numbers, the flow develops two symmetric vortices. Air bubbles are forced to the center of the vortex due to the centrifugal force and are then "trapped", i.e. they accumulate inside the vortex. Bubbles eventually oscillate (i.e. "dance") in the vortex when the flow becomes unsteady. Experiments were conducted by generating air bubbles in a variety of T-junction devices. In addition, our 3D numerical simulations have revealed a gradient of pressure, similar to vortex breakdown, that drives the flow towards the center of the T-junction creating two recirculating zones, which trap air bubbles.
Publications: D. Vigolo, S. Radl, and H. A. Stone, PNAS, vol. 111, no. 13, mar 2014, pp. 4770-4775.
Abstract: Flexible fibers, such as DNA and other polymer chains, have sometimes been found to contain knotted regions. While such fibers are not strict, closed knots, they exhibit similar characteristics; the formation of these `open knots' and the effects they have on material properties are the subject of current research. We investigate the possibility that simple flows can generate open knots in sufficiently long and flexible elastic fibers. Using the HYDROMULTIPOLE algorithm, which solves the multipole expansion of Stokes equations, we use numerical simulations to study the time evolution of a bead-spring model fiber in a shear flow. In certain systems, the characteristic tumbling motion of a fiber in shear flow will result in the formation of 3_1 and 5_1 knots, as identified by their Alexander polynomial knot invariants. Investigation of the key factors influencing knotting, as well as the mechanism of knotting, is ongoing.
Publications: S. Kuei, A. M. Słowicka, M. L. Ekiel-Jezewska, E. Wajnryb, H. A. Stone, New J. Phys., vol. 17, no. 5, may 2015, pp. 053009.
Abstract: A slender fibre, if flexible enough, will bend when immersed in a viscous flow. Confining walls affect the dynamics of a variety of real-world fibre systems ranging from industrial fibre suspensions to the biofilm streamers studied by our group. We study a model system that highlights the effect of confinement: a fibre is anchored in a thin channel, perpendicular to the direction of flow, and fluid is pumped through the channel, forcing the fibre to bend. There is a thin gap between the axis of the fibre and the channel wall, and we study the interplay between flow through this gap, flow around the fibre, and the corresponding effect on fibre deformation. Experiments are performed on a fibre that is polymerized directly in a microfluidic channel, and an analytical model is developed to explain the results.
Publications: J. S. Wexler, P. H. Trinh, H. Berthet, N. Quennouz, O. d. Roure, H. E. Huppert, A. Linder, H. A. Stone, J. Fluid Mech., vol. 733, sep 2013, pp. 684.
Abstract: To explore the health consequences of bacterial quorum sensing in the crypts, the researchers experimented with an antagonist to turn off quorum sensing in chambers colonized by methicillin-resistant S. aureus(MRSA), an antibiotic-resistant strain of bacteria that causes human infection. The left-side chamber contained no antagonist. In the right-side chamber, the antagonist molecules spread throughout the crevices, inactivating quorum sensing and indicating a potential strategy for alleviating MRSA virulence.
Publications: M. K. Kim, F. Ingremeau, A. Zhao, B. L. Bassler, H. A. Stone, Nat. Microbiol, vol. 1, no. 1, jan 2016, pp. 15005.
Abstract: To explore how pathogens spread in the host, the researchers designed a conversing branched flow network that mimic host environments such as lung or plant vasculature. Arrow indicates the directions that cell-free medium and bacterial cells were flowed. Wild-type P. aeruginosa (green) and its mutant pilTU (red) cells were initially seeded downstream of the conversing channels. Wild-type cells reached all upstream channels by 17 hours, while the pilTU cells remained the downstream regions. In this fluid flow environments, Wild-type P. aeruginosa cells attach to surfaces using type IV pili. These are localized to the bacterial cell poles such that upon attaching to the surface, flow causes the bacteria to orient with the pili pole pointed in the opposite direction of the flow. The repeated extension and retraction of pili using pilTU motors in this position drives Wild-type P. aeruginosa to move upstream along the surface.
Publications: A. Siryaporn, M. Kim, Y. Shen, H. Stone, Z. Gitai, Curr. Biol., vol. 25, no. 9, may 2015, pp. 1201–1207.
Abstract: When cells are excited by external chemical stimulations, multiple intracellular signaling will take place to regulate necessary cellular functions. However, the chemosensing of individual cells usually come with large fluctuations, and the cells need to employ different strategies to make reliable decisions based on the noisy readouts. One such strategy we are exploring is to utilize inter-cellular communications. We found the cells self-organize their communication channels to form a network that demonstrates various critical behaviors such as long-range correlations and percolation transitions.
Publications: B. Sun, J. Lembong, V. Normand, M. Rogers and H. A. Stone, PNAS 109 (2012).
Abstract: In the presence of a significant flow, mature multispecies biofilms often develop into long filamentous structures called streamers. We show that suspended thread-like biofilms steadily develop in zigzag microchannels. Numerical simulations of a low-Reynolds-number flow around the corners of the channel indicate the presence of a secondary vortical motion whose intensity is related to the bending angle of the turn. We demonstrate that the formation of streamers is directly proportional to the intensity of the secondary flow around the corners. In addition, we show that a model of an elastic filament in a two-dimensional corner flow is able to explain how the streamers can cross fluid streamlines and connect corners located at the opposite sides of the channel.
Publications: R. Rusconi, S. Lecuyer, L. Guglielmini, and H. A. Stone, J R Soc Interface 7 (2010).
R. Rusconi, S. Lecuyer, N. Autrusson, L. Guglielmini, and H. A. Stone Biophys. J. 100 (2011).
L. Guglielmini, R. Rusconi, S. Lecuyer, and H. A. Stone, J. Fluid Mech. 668 (2011).
N. Autrusson, L. Guglielmini, S. Lecuyer, R. Rusconi, and H. A. Stone Phys. Fluids 23 (2011).
Abstract: Although the plasma membrane in cells is usually confined to other sub-cellular structures, the mechanics of confined membranes has rarely been addressed. To mimic the confinement we have developed a simplified membrane model, which couples a lipid bilayer to an elastic sheet (a). We have demonstrated that upon straining the confined membrane is able to regulate passively its area. In particular, by compressing the elastic support, the bilayer reduces its area in the plane by forming lipid protrusions (b); upon expansion, the protrusions are absorbed back into the planar bilayer (a). The shape of the protrusions, spherical and tubular, can be controlled by the strain and the liquid volume, available between the membrane and its support. Our observations closely reproduce membrane shapes and processes found in cells, thus suggesting that mechanics may be a simple and generic organizing principle.
Publications: M. Staykova and H. A. Stone, Communicative & Integrative Biology 4 (2011).
M. Staykova, D. P. Holmes, C. Read, and H. A. Stone, PNAS 108 (2011).
Abstract: The power density output of microbial fuel cells (MFCs) is enhanced by optimizing the continuous flow of nutrient to obtain a constant rate of electricity production, and developing new electrodes material (optimization of surface roughness to increase the effective surface available to accommodate the bacteria). Preliminary results show how increasing the shear stress corresponds to increasing the output voltage generated by the MFCs up to an optimum flow rate. For higher flow rate the bacteria are discouraged to produce electricity and eventually are flushed away. A cheaper, membraneless microbial fuel cell design based on laminar co-flow is at the moment under investigation.
Publications: D. Vigolo, T. T. Al-Housseiny, Y. Shen, F. O. Akinlawon, S. T. Al-Housseiny, R. K. Hobson, A. Sahu, K. I. Bedkowski, T. J. DiChristina, H. A. Stone, Phys. Chem. Chem. Phys., vol. 16, no. 24, 2014, pp. 12535.
Abstract: Surface coatings and patterning technologies are essential for various physicochemical applications. In this Letter, we describe key parameters to achieve uniform particle coatings from binary solutions. First, multiple sequential Marangoni flows, set by solute and surfactant simultaneously, prevent nonuniform particle distributions and continuously mix suspended materials during droplet evaporation. Second, we show the importance of particle-surface interactions that can be established by surface-adsorbed macromolecules. To achieve a uniform deposit in a binary mixture, a small concentration of surfactant and surface-adsorbed polymer (0.05 wt% each) is sufficient, which offers a new physicochemical avenue for control of coatings.
Publications: H. Kim, F. Boulogne, E. Um, I. Jacobi, E. Button, and H. A. Stone, Phys. Rev. Lett. 116, 124501 (2016).
News: B. Verberck, "Fluid dynamics: Spirited away," Nature Physics 12, 291 (2016)
B. Yirka, "Evaporated whisky inspires new type of coating technique," Phys.org (2016)
J. Kemsley, "Why whiskey doesn't put a ring on it," Chemical & Engineering News (2016)
M. Schirber, "Synopsis: Whisky-Inspired Coatings," Physics (2016)
Abstract: Transport of colloids in confined geometries such as dead-end channels is involved in widespread applications including drug delivery and underground oil and gas recovery. In such geometries, Brownian motion may be considered as the sole mechanism that enables transport of colloidal particles into or out of the channels, but it is, unfortunately, an extremely inefficient transport mechanism for microscale particles. We explore the possibility of diffusiophoresis as a means to control the colloid transport in dead-end channels by introducing a solute gradient. We demonstrate that the transport of colloidal particles into the dead-end channels can be either enhanced or completely prevented via diffusiophoresis. In addition, we show that a combination of diffusiophoresis and Brownian motion leads to a strong size-dependent focusing effect such that the larger particles tend to concentrate more and reside deeper in the channel. Our findings have implications for all manners of controlled release processes, especially for site-specific delivery systems where localized targeting of particles with minimal dispersion to the nontarget area is essential.
Publications: S. Shin, E. Um, B. Sabass, J. T. Ault, M. Rahimi, P. B. Warren and H. A. Stone, Size-dependent control of colloid transport in dead-end channels via solute gradients, Proc. Natl. Acad. Sci. U.S.A. 113, 257–261 (2016).
Abstract: Deposition of colloidal material and bacteria is of major concern for membrane separation processes. A particle near a permeable surface experiences a hydrodynamic force, which increases as the surface becomes less permeable. This force may be orders of magnitude larger than the Stokes drag in an unbounded fluid. Shown here is a case where the particle is soft and deforms under this force, bringing it closer to the surface. This may have important implication for the adhesion propensity of soft particles onto membrane surfaces.
Publications: G. Z. Ramon, H. E. Huppert, J. R. Lister, H. A. Stone, Phys. Fluids, vol. 25, no. 7, 2013, pp. 073103.
Abstract: Granular materials do not perform thermally driven Brownian motion, so diffusion is observed in such systems because agitation (flow) causes inelastic collisions between particles. It has been suggested that axial diffusion of granular matter in a rotating drum might be "anomalous" in the sense that the mean squared displacement of particles follows a power law in time with exponent less than unity. We have shown that such a "paradox" can be resolved using Barenblatt's theory of self-similar intermediate asymptotics. Specifically, we found an analytical expression for the instantaneous scaling exponent of a macroscopic concentration profile, as a function of the initial distribution. Then, we incorporated concentration-dependent diffusivity into the model, showing the existence of a crossover from an anomalous scaling (consistent with experimental observations) to a normal diffusive scaling at very long times.
Publications: I. C. Christov, H. A. Stone, PNAS 109 (2012).
Abstract: The impacting event for a given system of particles entrained in a fluid is described in terms of the Reynolds number and the particle Stokes number. Experimental results for the impact in a T-junction are compared with the trajectories predicted by theoretical particle-tracing models for a range of configurations to determine the role of the viscous boundary layer in slowing down the particles and reducing the rate of collision with the substrate. In particular a 2D model based on a stagnation point flow is used together with detailed 3D numerical simulations.
Publications: D. Vigolo, I. M. Griffiths, S. Radl, and H. A. Stone. J. Fluid Mech., Submitted (2012).
Abstract: We present experiments demonstrating the spontaneous generation and traveling of a colloidal pulse in a steady channel flow. When deionized (DI) water with suspended positively-charged particles flows steadily through a single channel, a pulse (unexpected focusing) of particles is generated, which then flows through the channel at a slower speed than the mean flow velocity. With detailed experimental investigations and quantified results, we rationalize our observations by considering CO2 driven diffusiophoresis. The concentration gradient of ions in the liquid phase is created by the leakage of CO2 through the permeable PDMS walls. Mathematical models for early stage particle focusing and the traveling pulse will be compared with the experimental observations.
Publications: Suin Shim and Howard A. Stone. In preparation
Spontaneous pulse generation in a steady channel flow of a colloidal suspension – the role of dissolved gas, 2017 APS DFD Meeting, Nov. 2017, Denver CO
Spontaneous pulse generation in a steady channel flow of a colloidal suspension, APS March Meeting 2019, Mar. 2019, Boston MA
Abstract: This project is focused on the development of a simple microfluidic method for the synthesis of polymeric microfibers of controlled length. We explored the use of valve actuation and UV light modulation to control the length of the microfibers. The valve-based approach, in particular, was developed to synthesize fibers with tunable lengths, which has not previously been demonstrated. We observed good, reproducible control of microfiber length as a function of the valve actuation frequency. We also focused on the synthesis of wavy or crimped polymeric microfibers using a microfluidic method. We trigger a buckling instability through the initiation of a polymerization reaction in a liquid jet when microchannel dimensions increase, and subsequently preserve the buckled morphology when the crosslinking reaction is completed. The resulting microfibers have highly uniform and reproducible morphologies. By changing the UV exposure location in the channel, as well as the flow rates, the degree of waviness of the microfibers can be controlled.
Publications: J. K. Nunes, K. Sadlej, J. I. Tam and H. A. Stone, Lab Chip, 12 (2012).
Abstract: A new technique to control temperature along microchannels using a low viscosity, conductive epoxy as Joule heater was developped. By using this technique we were able to effectively keep a constant temperature or create a temperature gradient across a microfluidic channel. In the latter case we implemented a thermophoretic separator to actually separate (or drive) particles suspended in aqueous solution in a microfluidic lab-on-chip system.
Publications: D. Vigolo, R. Rusconi, R. Piazza, and H. A. Stone. Lab Chip, 10(6):795-798 (2010).
D. Vigolo, R. Rusconi, H. A. Stone, and R. Piazza. Soft Matter, 6(15):3489-3493 (2010).
Abstract: We propose a microfluidic approach for the generation of water droplets containing a high volume fraction of gas bubbles and we provide a design principle for microbubble-based pressure sensing inside channels. We also present a microfluidic approach for the controlled encapsulation of individual gas bubbles in micrometer-diameter aqueous droplets with high gas volume fractions and demonstrate this approach to making a liquid shell, which serves as a template for the synthesis of hollow inorganic particles.
Publications: J. Wan, A. Bick, M. Sullivan, H. A. Stone. Adv. Mater. 20 (2008).
J. Wan, H. A. Stone, Soft Matter. 6 (2010).
J. Wan, H. A. Stone, Langmuir 28 (2012).
Abstract: Magnetic forces are used to manipulate micron-sized paramagnetic beads in a microfluidic device, with applications in medicine and industry. By balancing the magnetic forces against fluid forces at the small scale (viscous drag and interfacial tension), we accomplish a variety of tasks on the serialized platform of a microfluidic device. It is shown that magnetic particles can be sorted by size transversely across a channel, by applying a magnetic field whose gradient is perpendicular to the direction of flow. If an immiscible interface is present at the center of a channel, a similar procedure may be used to coat spheres with a micron-sized coating, produce aggregates of controllable size, and to make measurements of ultra-low surface tension. Since magnetic particles may be functionalized to bind to various biological agents, these materials may be manipulated in a similar manner.
Publications: S. H. Tsai, I. M. Griffiths, and H. A. Stone. Lab on a Chip, 11 (2011).
S. H. Tsai, J. S. Wexler, J. Wan, and H. A. Stone. Applied Physics Letters, 99 (2011).
Abstract: We study theoretically and experimentally how a thin layer of liquid flows along a flexible beam. The flow is modelled using lubrication theory and the substrate is modelled as an elastica which deforms according to the Euler-Bernoulli equation. A constant flux of liquid is supplied at one end of the beam, which is clamped horizontally, while the other end of the beam is free. As the liquid film spreads, its weight causes the beam deflection to increase, which in turn enhances the spreading rate of the liquid. This feedback mechanism causes the front position σ(t) and the deflection angle at the front ϕ(t) to go through a number of different power-law behaviours. For early times, the liquid spreads like a horizontal gravity current, with σ(t) =t4/5 and ϕ(t) = t13/5. For intermediate times, the deflection of the beam leads to rapid acceleration of the liquid layer, with σ (t) = t4 and ϕ (t) = t9. Finally, when the beam has sagged to become almost vertical, the liquid film flows downward with σ(t) = t and ϕ ~ π/2. We demonstrate good agreement between these theoretical predictions and experimental results.
Publications: P. D. Howell, J. Robinson, and H. A. Stone, J. Fluid Mech. 732, 190-213 (2013).
P. D. Howell, H. Kim, M. G. Popova, and H. A. Stone, J. Fluid Mech. accepted (2016).
Abstract: We are developing a family of multiphase microfluidic methods for the controlled synthesis of monodisperse polymeric microfibers where the size, shape, morphology, spatial composition, and the encapsulation of cargoes can be precisely tailored.
Publications: E. Um, J. K. Nunes, T. Pico, H. A. Stone, J. Mater. Chem. B, vol. 2, no. 45, oct 2014, pp. 7866-7871.
J. K. Nunes, C. Wu, H. Amini, K. Owsley, D. D. Carlo, H. A. Stone, Adv. Mater., vol. 26, no. 22, mar 2014, pp. 3712-3717.
J. K. Nunes, H. Constantin and H. A. Stone, Soft Matter, vol. 9, no. 16, 2013, pp. 4227.
J. K. Nunes, K. Sadlej, J. I. Tam, H. A. Stone, Lab. Chip, vol. 12, no. 13, 2012, pp. 2301.
Abstract: When subjected to flow conditions, such as extrusion from a needle, a suspension of flexible high aspect ratio fibers entangles irreversibly and forms a network. This flow-induced process is a simple mechanical approach to hydrogel formation that does not depend on chemical reactions. We study this phenomenon with shear rheology experiments and microscopic visualization. We propose that these microfiber suspensions are potentially useful material candidates for in situ scaffold fabrication in bioengineering applications.
Abstract: Thin, layered materials develop surface undulations or wrinkles when they experience small compressive strain. This response is the result of a complex interplay between deformation of the top layer and its foundation. This periodic wrinkling ﬁnd applications, e.g. in stretchable electronics but can also limit an application’s performance owing to delamination or cracking under load. In particular, because of curvature localization, ﬁnite deformations can cause wrinkles to evolve into folds. Using a two-layer polymeric system under biaxial compressive stress, we show that a repetitive wrinkle-to-fold transition generates a hierarchical network of folds during reorganization of the stress ﬁeld. The folds delineate individual domains, and each domain subdivides into smaller ones over multiple generations. By modifying the boundary conditions and geometry, we demonstrate control over the ﬁnal network morphology. We then exploit these wrinkles and deep folds to guide and retain light within the photoactive regions of photovoltaics and show substantial improvements in light harvesting efficiencies, particularly in the near-infrared region where light absorption is otherwise minimal.
Publications: P. Kim, M. Abkarian, and H. A. Stone, Nature Materials 10 (2011).
J. B. Kim, P. Kim, N. C. Pegard, S. J. Oh, C. R. Kagan, J. W. Fleischer, H. A. Stone, Y. L. Loo, Nature Photonics 6 (2012).