We use microengineering to explore the physico-chemical and dynamical properties of fluids and living tissues.  The summaries below present current and recent topics of study in the group.  

Synthetic Trees - science and engineering of water under tension:

Vascular plants commonly manipulate liquid water at large tensions (negative pressures).  By mimicking the physiology of plants in synthetic systems, we study the properties of this metastable state of water, investigate how we can use it in technological contexts, and ask what strategies plants exploit to use it reliably.  

Synthetic trees. 
How do plants reliably use liquid water under tension?  What are the physiological properties of the xylem (vascular structure) that enables this trick?  What are the physical principles that govern and limit this function?  Can we do this too?  We pursue these questions by building and characterizing synthetic systems that operate on the principles that are believed to govern transpiration in vascular plants.  We have shown that such synthetic trees can recapitulate the basic processes involved in transpiration, namely the steady state pumping of liquid water at large negative pressures (down to ~ -100 bars) and extraction of liquid from sub-saturated atmospheres ("dry soils").  Indeed, in some cases, synthetic trees can out perform living ones!  We are currently asking how plants manage this fragile, metastable state of liquid and regenerate the liquid state when it cavitates or boils.  These studies aim to lay a foundation for understanding the physical principles of plant function and for developing technologies that exploit liquids under tension.  Figure:  We formed our first generation of synthetic trees as microfluidic structures in an organic hydrogel.  (Wheeler and Stroock, Nature, 2008)

 The metastable states of liquid water.
Open questions remain about the structure of the phase diagram of water and its relationship to the anomalous properties of its liquid phase.  The development of synthetic trees opens new possibilities for the study of liquid water in parts of its phase diagram that have been explored only sparsely:  the metastable regions of stretch superheating (ssh - liquid would like to boil) at negative pressures and the doubly metastable region of supercooling (sc - liquid would like to freeze) and stretched superheat.  Theory and simulation suggest that thermodynamic and dynamic properties of the water in these regions could explain water's oddities.  We are developing experimental techniques based on synthetic trees to measure these properties for the first time.  Figure:  Sketch of phase diagram showing known and hypothesize features such as a second critical point (c') and a re-entrant spinodal (s).  (Caupin et al., Journal of Physics, 2012)

 Technologies that exploit water under tension.
Plants demonstrate that liquid water can be used as a tensile element with which to transmit stress efficiently over long distances.  Operation in this metastable regime can have significant advantages for the transfer of mass and heat and the manipulation of sub-saturated phases of a water.  It could enable near-isothermal heat transfer over large distances for the management of thermal energy in buildings and vehicles.  It could allow for the extraction of pure liquid water from partially dry soils or even from air.  It  could be used to control the mechanical properties and motion of soft materials.  In collaboration with plant scientists (N.M. Holbrook and M. Zwieniecki), we are developing design principles and experimental methods to make such applications feasible.  Figure:  To avoid catastrophic failure in the event of cavitation (boiling) within their vessels, xylem is divided into segments by nanoporous membranes.  We have formed a synthetic mimic of such a structure in silicon (A) that allows for independent cavitation events in coupled cavities.  We are studying the interesting dynamics of this process of drying in which acoustic signals accompany each cavitation event (B) and spatial correlations emerge (C) without temporal ones (D).  (Sessoms, unpublished)

 Communication with plants and the  environment.  
Plants form a complex interface between the soil and the atmosphere. In this role, plants mediate and respond to – in their growth, yield, and seed maturation – the mass and energy fluxes that are defined by these boundaries. The thermodynamic state of water within plants represents the most important variable in determining the biological and physical dynamics at this interface. Yet, conventional methods of measuring the chemical potential of water ("water potential") require manual intervention and have low temporal and spatial resolution. In collaboration with horticulturists (A. Lakso), we are developing implantable microtensiometers with which to measure water potential to provide continuous, local data in soils and directly within the plants. This technology has the potential to revolutionize the methods of both growers (e.g., in viticulture) and ecologists.  Figure: Across natural (e.g., forests) and managed (e.g., staple and specialty crops) ecologies, embedded sensors of water status (*) could provide a basis for improved modeling and management of plant-environment interactions (top). A prototype of our MEMS-based tensiometer is shown with a schematic diagram of the proposed use within a wireless network of sensors (bottom). (Pagay and Sessoms, unpublished) 

selected publications:
  • Vincent, O., Szenicer, A., & Stroock, A.D., Capillarity-Driven Flows at the Continuum Limit. (submitted 2015). (pdf) (Supplemental Materials).
  • Pagay, V.,  Santiago, M.,  Sessoms, D.A., Huber, E.J., Vincent, O., Pharkya, A., Corso, T.N., Lakso, A.N. & Stroock, A.D. A microtensiometer capable of measuring water potentials below -10 MPa. Lab-on-a-Chip, 14, 2806-2817 (2014). (pdf)
  • Vincent, O., Sessoms, D.A., Huber, E.J., Guioth, J. & Stroock, A.D. Drying by cavitation and poroelastic relaxations in extreme ink-bottle porous media. Physical Review Letters, 113, 134501 (2014). (pdf). (Supplemental Materials - pdf) (Supplemental Movie - avi) (News & Views article by Philip Ball)
  • Chen, I.-T., Pharkya, A. & Stroock, A. D. Analysis of Superheated Loop Heat Pipes Exploiting Nanoporous Wick Membranes. AICHE Journal, 60, 762-777 (2014) (pdf)
  • Stroock, A. D., Pagay, V. V., Zwieniecki, M. A. & Holbrook, N. M. The physicochemical hydrodynamics of vascular plants. Annual Review of Fluid Mechanics, 46, 615-642 (2014). (pdf)
  • Rockwell, F. E., Holbrook, N. M. & Stroock, A. D. Leaf hydraulics I: Scaling transport properties from single cells to tissues. Journal of Theoretical Biology, 340. 251–266 (2014). (pdf)
  • Rockwell, F. E., Holbrook, M. N. & Stroock, A. D. Leaf hydraulics II: Vascularized tissues. Journal of Theoretical Biology 340. 267–284 (2014). (pdf)
  • Rockwell, F. E., Holbrook, N. M. & Stroock, A. D. The competition between liquid and vapor transport in transpiring leaves. Plant Physiology, 164, 1741-1758 (2014). (pdf)
  • Santiago, M., Pagay, V. V. & Stroock, A. D. Impact of Electroviscosity on the Hydraulic Conductance of the Bordered Pit Membrane: a Theoretical Investigation. Plant Physiology, 163, 999-1011 (2013). (pdf)
  • Caupin, F. and Stroock, A.D., The stability limit and other open questions on water at negative pressure. Advances in Chemical Physics (accepted). (pdf)
  • Wheeler, T.D. and Stroock, A.D. Stability Limit of Liquid Water in Metastable Equilibrium with Subsaturated Vapors. Langmuir. 25, 7609-7622 (2009).  (pdf)
  • Wheeler, T.D. and Stroock, A.D., Transpiration at negative pressures in a Synthetic Tree.  Nature, 455, 208-212 (2008). (pdf)
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Microvascular Biomaterials:

The vascular system is a pervasive organ that mediates mass transfer and cellular communication in the tissues of higher organisms.  Locally, microvascular structure controls development and homeostasis in healthy tissues, and participates in the progression of pathologies such as cancer and diabetes.  We are developing methods to study and control the growth of vascular structure within synthetic scaffolds.  The methods form a basis with which to study angiogenesis in the context of solid tumors (with Prof. Claudia Fischbach) and form pre-vascularized tissues for regenerative medicine (with Prof. Jason Spector, M.D.).  

Bottom-up growth of vascular structure - vasculogenesis.

In embryonic development, progenitors of endothelial cells self-organize into an initial network of capillaries, a process called vasculogenesis. This capillary bed serves as the initial vascular system that, upon perfusion by the heart, undergoes morphogenesis into the hierarchy of vessels that form the mature vascular system. Interesting, a version of this vasculogenic process occurs in vitro. We are interested in the biophysical basis of this self-organization: how do the individual cells communicate and interact with each other and their local environment to define an extensive network of tubes? Insights into vasculogenesis will inform methods for the growth of functional vascular systems in vitro for applications in regenerative medicine. Figure: Primary human endothelial cells spontaneously form a network of interconnected tubes (top). We are dissecting the dynamics of this process at the level of individual cells (bottom). (Cross and Diaz, unpublished)

Top-down fabrication of microvascular structure - an initial condition for angiogenesis.

Beyond the vasculogenic step (above), the vascular system evolves by angiogenesis in which changes of the existing vascular structure occur by sprouting and changes of caliber and connectivity. Angiogenesis plays a central role in development, healing, and pathologies such as cancer. We have developed a route to form functional microvessels within a 3-D tissue scaffold. Importantly, this scaffold supports remodeling by both the tissue and endothelial cells such that these fabricated vessels can act as an initial condition for angiogenic processes. We are using this system to study the role of biophysical (e.g., perfusion stresses) and biochemical (e.g., pro-angiogenic growth factors) on angiogenesis in both healthy and pathological scenarios. These studies aim at the generation of realistic models of vascularized tissues for basic biology, pharmacology, and regenerative medicine. Figure: Fluorescence confocal micrograph of a microvascular network with endothelialized channels that have undergone angiogenic sprouting into the surrounding matrix that was seeded with perivascular cells. (Zheng et al., PNAS, 2012)

Control of the cellular microenvironment in Microfluidic Scaffolds.  

The development of scaffolds for 3-D culture of mammalian cells has focused on defining the chemistry and mechanics of the material to which the cells are exposed. We have developed a complementary approach in which we form microfluidic structure directly within hydrogels that are appropriate for 3-D culture. These microfluidic vessels provide convective paths into the bulk of the scaffold through which we can control and monitor the soluble chemistry in the local environment of the cells. Such microfluidic scaffolds provide a basis for directing the evolution of tissue growth and analyzing the profiles of secreted factors during in vitro culture. Figure: Fluorescence micrographs of a microfluidic scaffold during sequential perfusion with two different dyes. (Choi et al., Nature Materials, 2007)

Directing the growth of tissue and vascular structure in Microstructured Tissue Templates.

The use of porous biomaterials to template the regrowth of tissues has been one of the most successful approaches in regenerative medicine (e.g., Integra®). In collaboration with the reconstructive surgery group of Dr. Jason Spector, we have developed tissue templates with well-defined internal pore structure defined by lithography. These microstructured tissue templates (MTTs) allow us to investigate the mechanisms by which microstructure influences the invasion of tissue and vascular structure into the template. In an animal wound model, we have shown that appropriate, microfabricated pores can lead to increased rates of invasion and vascularization relative to that achieved in conventional, random porous matices. We are investigating the use of MTTs for the treatment of hard-to-heal wounds in which rapid vascularization is a priority. Figure: Micrograph of an MTT formed in collagen with multiple levels of lithographically defined pores. (Zheng et al. Biomatierials, 2011)

selected publications:
  • Diaz-Santana, A., Shan, M., & Stroock, A.D. Endothelial cell dynamics during anastomosis in vitro. Integrative Biology, 7, 454 - 466 (2015). (pdf)
  • Morgan, J.P., Delnero, P.F., Zheng, Y., Verbridge, S.S., Chen, J., Craven, M., Choi, N., Diaz-Santana, A., Kermani, P., Hempstead, B., López, J.A., Corso, T.N., Fischbach-Teschl, C., Stroock, A.D.Formation of Microvascular Networks in vitro.  Nature Protocols, 8, 1820-1836 (2013). (pdf)
  • Verbridge, S. S., DelNero, P., Chakrabarti, A., Kwee, B., Varner, J. D., Stroock, A. D. & Fischbach, C. Physicochemical regulation of endothelial sprouting in a 3-D microfluidic angiogenesis model. Journal of Biomedical Materials Research: Part A. 101, 2948-2956 (2013).
  • Choi, N. W., Verbridge, S. S., Williams, R. M., Chen, J., Kim, J.-Y., Schmehl, R., Farnum, C. E., Zipfel, W. R., Fischbach, C. & Stroock, A. D. Phosphorescent nanoparticles for quantitative measurements of oxygen profiles in vitro and in vivo. Biomaterials 33, 2710–2722 (2012). (pdf
  • Zheng, Y., Chen, J., Craven, M., Choi, N. W., Totorica, S., Diaz-Santana, A., Kermanie, P., Hempstead, B., Fischbach, C., López, J. A. & Stroock, A. D. In vitro microvessels for the study of angiogenesis and thrombosis. Proceedings of the National Academy of Sciences. 109, 9342–9347 (2012). (pdf)
  • Zheng, Y., Henderson, P.W., Choi, N.W., Bonassar, L.J., Spector, J.A., and Stroock, A.D., Microstructured templates for directed growth and vascularization of soft tissue in vivoBiomaterials 32, 5391-5401 (2011).   (pdf)
  • Stroock, A.D., Fischbach, C., Microfluidic culture models of tumor angiogenesis, Tissue Engineering Part A, 16, 2143-2146 (2010). (pdf)
  • Verbridge, S.S., Choi, N., Zheng, Y., Brooks, D.J., Stroock, A.D., Fischbach, C., Oxygen-controlled 3-D cultures to analyze tumor angiogenesis, Tissue Engineering Part A, 16, 2157-2159 (2010). (pdf)
  • Cross, V.L., Zheng, Y., Choi, N.W., Verbridge, S.S., Sutermaster, B.A., Bonassar, L.J., Fischbach, C., and Stroock, A.D., Dense type I collagen matrices that support cellular remodeling and microfabrication for studies of tumor angiogenesis and vasculogenesis in vitroBiomaterials31, 8596-8607 (2010). (pdf)
  • Choi, N. W.; Cabodi, M.; Held, B.; Gleghorn, J. P.; Bonassar, L. J.; Stroock, A. D., Microfluidic scaffolds for tissue engineeringNature Materials 6, 908-915 (2007).  (pdf)
  • Cabodi, M.; Choi, N. W.; Gleghorn, J. P.; Lee, C. S.; Bonassar, L. J.; Stroock, A. D.  A microfluidic biomaterialJ. Am. Chem. Soc.127, 13788-13789 (2005). 
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Microfluidic Transport Phenomena:

The emergence of microfluidic technologies for chemical process requires the development of new methods to manipulate fluids on small scales and of a theoretical basis with which to analyze and design these systems.  In our work, we focus on the impact of chaotic dynamics of mass transfer processes in low Reynolds number flows that are typical of microfluidic systems.  We work on this theme with Profs. Koch and Abruña.  

Fundamentals of transport phenomena in chaotic flows.  

The potential for chaotic advection to accelerate mixing  has long been appreciated.  Less attention has been paid to the impact of chaos on interfacial transfer.  We have used numerical simulation, theory, and experiment to study how the presence of chaotic streamlines in the bulk of a flow can have a dramatic impact on the rates of interfacial transfer to a boundary of the flow.  Figure:  predictions of numerical simulation of interfacial transfer from a chaotic (A) and non-chaotic (B) flow to a no-slip boundary.  Closed symbols in (C) show that rates of transfer can be increased many fold by the presence of chaos.  (Kirtland et al., Physics of Fluids, 2006)
 Reversibility in low Reynolds number flows.
A long-standing debate exists regarding the relationship between chaos and the loss of reversibility in statistical mechanics and transport processes. We have explored this question in depth in the context of a simple reversal experiment involving stirring and un-stirring of a mixture of two solutes. We have developed a unified theoretical treatment that is capable of treating this process for both chaotic and non-chaotic flows. This development clarifies how and when chaotic dynamics leads to distinct behavior in the decay of reversibility. We are pursuing experiments to test our predictions and explore an application to perform molecular separation without a membrane. Figure: Predictions to state of a mixtures after stirring (left) and unstirring (right) for non-chaotic (top) and non-chaotic (left) flow. (Sundararajan et al., 2012)
 Microfuel cells.  
Low temperature fuel cells could provide a clean alternative to internal combustion engines in vehicles and a high performance replacement for batteries in portable electronics. Technical (e.g., membrane stability) and fundamental (e.g., kinetic and transport limitations) have hindered the maturation of this technology. We have worked on alternative designs for fuel cells that eliminate the need to a chemically selective membrane and exploit chaotic flow to improve transport limited current densities. In collaboration with Prof. Abruña (Chemistry, Cornell), we have demonstrated a low temperature fuel cell with among the highest power densities ever reported. Figure: Power curve of a microfluidic fuel cell operated on Borohydride and Cerium with and without chaotic advection generated by the electrodes.  (DeMota et al., JACS, 2012)

selected publications:
  • Deseigne, J., Cottin-Bizonne, C., Stroock, A. D., Bocquet, L., & Ybert, C. How a" pinch of salt" can tune chaotic mixing of colloidal suspensions. Soft Matter, 10, 4795-4799 (2014). (pdf)
  • Sundararajan, P., Kirtland, J.D., Koch, D.L., Stroock, A.D.  Impact of chaos and Brownian diffusion on irreversibility in Stokes flowsPhys. Rev. E 79, 046319 (2009). (pdf)
  • Da Mota, N., Kirtland, J. D., Finkelstein, D. A., Rodriguez, C. A., Stroock, A. D. & Abruña, H. D. Membraneless, Room-Temperature, Direct Borohydride /Cerium Fuel Cell with Power Density of Over 0.25 W/cm2Journal of the American Chemical Society. 134 (14), 6076–6079(2012). (pdf)
  • Sudararajan, P. & Stroock, A. D. Transport phenomena in Chaotic Laminar FlowsAnnual Reviews of Chemical and Biomolecular Engineering. Vol. 3, 473-496. (pdf)
  • Kirtland, J. D.; McGraw, G. J.; Stroock, A. D. Mass transfer to reactive boundaries from steady three-dimensional flows in microchannelsPhysics of Fluids, 18, 073602 (2006). (pdf)
  • Stroock, A. D.; McGraw, G. J.  Investigation of the staggered herringbone mixer with a simple analytical modelPhil. Trans. Roy. Soc. A -Math. Phys. Eng. Sci.362, 971-986 (2004). (pdf)
  • Stone, H. A.; Stroock, A. D.; Ajdari, A. Engineering flows in small devices: Microfluidics toward a lab-on-a-chipAnnual Review of Fluid Mechanics36, 381-411 (2004). (pdf)
  • Ferrigno, R.; Stroock, A. D.; Clark, T. D.; Mayer, M.; Whitesides, G. M. Membraneless vanadium redox fuel cell using laminar flowJ. Am. Chem. Soc.124, 12930-12931 (2002). Addition: J. Am. Chem. Soc., 125, 2014-2014 (2003). (pdf)
  • Stroock, A. D.; Dertinger, S. K. W.; Ajdari, A.; Mezic, I.; Stone, H. A.; Whitesides, G. M. Chaotic mixer for microchannelsScience295, 647-651 (2002). (pdf)
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Customized Colloidal Hydrodynamics and Assembly:

Research in colloidal science has been dominated by the study of simple shapes, particularly spheres.  New synthetic routes and fabrication strategies open the possibility of more complex shapes.  These tailored shapes could be used to engineer both the dynamics and the thermodynamics of suspensions of these particles.  We work on experimental means to form and characterize custom particles and on theoretical and numerical models of their behavior.  We collaborate with Profs. Koch and Escobedo on this theme.  

 Non-tumbling particles. 
As a rule, particles tumble in flow with non-uniform velocity: they rotate with the vorticity. Yet, we have identified an exception to this rule, a class of particles whose shape generates a torque opposing that of the vorticity. This discovery could have interesting consequences for the rheology of suspensions and self assembly of particles. We are pursuing an optimization of these shapes and experiments to characterize their behavior. Figure: Schematic diagram of a ring-shaped particle whose cross-section generates a torque counter to the vorticity (top). Predicted trajectories of the orientation, p of such a particle showing stable points (bottom).  (Singh et al., unpublished)

 Shape-directed self assembly of fabricated colloids. 
On the colloidal scale as on the molecular scale, shape plays an important role in defining particle-particle interactions.  We have developed a lithographic route to form colloidal particles of well-defined, non-spherical shape and shown that shape and surface structure can lead to highly directional interactions.  This work add to the toolbox of methods for a "chemistry of colloids" with which to design self assembled materials from the level of individual particles.  Figure: A silicon wafer covered with lithographically defined particles (top-left).  Photo of a stable suspension of such particles (top-right).  SEM of individual particles formed in this manner (bottom-left).  Optical micrograph of rod-like assemblies of these particles in a suspension that was destabilized by non-adsorbing polymers (bottom-right).  (Badaire et al., JACS, 2005; Badiare et al., Langmuir, 2008)

selected publications:

  • Singh, V., Koch, D.L., and Stroock, A.D., Rigid ring shaped particles that align in simple shear flow. Journal of Fluid Mechanics, 722, 121-158 (2013). (pdf)
  • Singh, V.; Koch, D. L. & Stroock, A. D. Ideal Rate of Collision of Cylinders in Simple Shear Flow. Langmuir 27, 11813-11823 (2011). (pdf)
  • Badaire, S.; Cottin-Bizonne, C.; Stroock, A.D., Experimental Investigation of Selective Colloidal Interactions Controlled by Shape, Surface Roughness, and Steric Layers.  Langmuir. 24, 11451 - 11463 (2008).  (pdf)
  • Badaire S.; Cottin-Bizonne, C. ; Woody, J. W.; Yang, A.; Stroock, A. D. Shape selectivity in the assembly of lithographically designed colloidal particles.  J. Am. Chem. Soc.129, 40-41 (2007). (pdf)
  • John, B. S.; Stroock, A.; Escobedo, F. A. Cubatic liquid-crystalline behavior in a system of hard cuboids. J. Chem. Phys., 120, 9383-9389 (2004). (pdf)
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Transport processes in the geological sub-surface:

Geological sequestration of carbon dioxide is one option for the management of this important greenhouse gas.  This process involves a diversity of transport phenomena from the microscopic scale of individual pores in the rock to the macroscopic scale (kilometers) of an entire reservoir.  We are developing numerical and theoretical tools with which to explore new proposals for how the injection process can be done more effectively than by conventional methods.  

Stirring the sub-surface for CO2 sequestration.
An effective means of stabilizing CO2 injected into a brine aquifer would be to allow it to dissolve into the brine and settle to the bottom of the formation.  We are exploring means of generating efficient stirring flows during the injection process to accelerate this process dramatically.  We are also interested in understanding pore-scale processes that impact convection and mass transfer during the injection process.  Graduate student, Erik Huber is the lead on this project.  Prof. Koch is a collaborator.  Support comes from Cornell's Earth Energy IGERT.  Figure:  Schematic diagram of a time-dependent injection process that could accelerate by a factor of 103 the evolution toward stable sequestration.  

selected publications:
  • Huber, E.J, Stroock, A.D., & Koch, D.L., Analysis of a time dependent injection strategy to accelerate the residual trapping of sequestered CO2 in the geologic subsurface. International Journal of Greenhouse Gas Control (in press, 2015). (pdf)