We study a variety of problems in the field of soft condensed matter and biophysics. Frequently, our research involves the application of continuum mechanics and hydrodynamics to biomaterials ranging in length scale from single proteins to biopolymer networks spanning tens of microns. We are also studying some aspects of the statistical mechanics of neuronal networks, phase transitions in colloidal crystals, and even laser trapping of colloidal particles with more complex shapes.
We benefit from collaborations with a number of experimentalists, working closely with Michael Dennin (UCI), Gabriel Popescu (UIUC), Tony Dinsmore (UMASS), Jeff Urbach (Georgetown), Christoph Schmidt (Göttingen) Giovanni Zocchi (UCLA), Tom Mason (UCLA), Jack Feldman (UCLA), Robijn Bruinsma (UCLA), and Fred MacKintosh (Vrije Universiteit, Amsterdam). Please see their webpages to learn more about their exciting work.
UCLA Department of Chemistry and Biochemistry
607 Charles E. Young Drive, EAST
Los Angeles, CA 90095
Alex J. Levine's Office: Young Hall 3044A
Student Offices: Young Hall 3086,
Last Updated: April 2012
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Office:Young Hall 3044A
Alex Levine completed his Ph.D. in physics at UCLA in 1996 after receiving undergraduate degrees in mathematics and physics. Following postdocs at Exxon Research & Engineering, UPENN, and UCSB, he joined the physics department of the University of Massachusetts, Amherst as an assistant professor. In 2005 he returned to UCLA where he is now a professor of Physics & Astronomy, Chemistry & Biochemistry, and the director of the Center for Biological Physics. His main research interests involve statistical physics, mechanics of disordered elastic systems, and the dynamical phase behavior of interacting neurons. His principal work in biological physics involves the mechanics of the cytoskeleton, hydrodynamics and transport in membranes, and theoretical neuroscience. Here is a copy of Alex's most recent CV (Updated April 2012).
1996 Ph.D., UCLA
1996-1998 Postdoctoral researcher, Exxon Research
1998-2001 Postdoctoral researcher, University of Pennsylvania
2001-2002 Postdoctoral researcher, UCSB
2002-2005 Assistant Professor, UMASS
2005-2008 Assistant Professor, UCLA
2008-2011 Associate Professor, UCLA
2011- present Professor, UCLA.
Office: Geology 4607
Office: Geology 4607
Office: Young Hall 3087
Office: Young Hall 3087
Office: Young Hall 3087
Department of Physics
1126 Lederle Graduate Research Tower
University of Massachusetts
Amherst, MA 01003-9337
Department of Physics
Princeton, NJ 08544
The mechanical properties of semiflexible polymer networks, such as are commonly found in the cytoskeleton, differ from those of more traditional synthetic polymer gels in that the thermal persistence length of the filaments in the cytoskeleton is over an order of magnitude larger than the mean distance between cross-links. While one can think of a flexible gel as cross-linked cooked spaghetti, these cytoskeletal networks are more like bamboo networks or random rattan furniture! We are interested in the relationship of the local network structure and the mechanical properties of the constituent filaments to collective mechanical properties of the network at longer length scales. More recently, we have been studying the role of endogenous molecular motors in the control of the network mechanics. This research, much of which is done with our theoretical collaborator, Fred MacKintosh, is motivated by the remarkable experiments of Christoph Schmidt's group wherein they use the activity of molecular motors to induce hundred-fold increases in the elastic moduli of an F-actin biopolymer network. Here we also work closely with Michael Dennin's group; they build thin F-actin gels associated with a Langmuir monolayer. This system, which is a highly simplified toy model of the the physiological cytoskeleton associated with one leaflet of the lipid bilayer of a cell, allows for precise chemical control and imaging of the network while controlled stresses can be applied. By studying this experimental system, we plan to better understand the stress-strain relation of equilibrium semiflexible networks (in particular the affine/non-affine cross-over) and to explore the action of molecular motors on the network.
Three dimensional hydrodynamics in the inertia-free limit (applying at sufficient low flow velocities and small enough length scales) is a scale-free theory. The hydrodynamics of membranes, however, admits an inherent length scale set by the ratio of the viscosity of the two-dimensional membrane fluid to the viscosity of surrounding three-dimensional fluid in which the membrane is embedded. One can think of this inherent length scale as setting the distance over which membrane momentum is dissipated into the third dimension. We have examined a number of problems exploring how this additional length scale effects the drag on extended particles and how membrane curvature (introducing yet another length into the problem) affects membrane hydrodynamics. Currently, we are also interested in studying the dynamics of real proteins in membranes and turning our attention to the mechanics of red blood cell membranes, motivated by our collaboration with the Gabriel Popescu, who has developed new imaging techniques to study their fluctuations over a wide range of length and time scales.
It has long been accepted that the mechanical deformation of proteins when interacting with other molecules (their substrate) leads to biologically important nonlinearities in which the binding of one molecule by the protein affects the subsequent binding of a second molecule. At the nanometer scale of these individual proteins, the border between mechanics and chemistry becomes porous. Working with our experimental collaborator, Giovanni Zocchi, we are analyzing a new set of experiments on protein mechanics in which they use short DNA oligomers as nanoscale pliers to deform proteins and measure changes in their enzymatic activity. We intend to better understand both the mechanics of individual proteins and that of DNA at short length scales (smaller than one thermal persistence length) from the analysis of these data. With a properly calibrated DNA pliers, we will make a broader study of protein mechanics.
Physicist have long accepted the maxim that "more is different," but, in many complex systems, understanding how complex behavior emerges from a large collection of interacting, but (seemingly) simple objects remains a mystery. The dynamics of collections of interacting neurons provides just one of many classes of examples of this forefront problem in statistical physics. One particular biological system has captured out attention: The pre-Bötzinger complex is a collection of a few hundreds of interacting leaky-integrate-and-fire neurons that collectively generates a metronomic signal that helps to control the breathing rhythm in mammals. Remarkably, it appears possible that none of neurons making up this network can individual produce a rhythmic train of action potential spikes, but rather this periodic dynamics emerges as a collective property of sufficiently large networks of identical neurons. Working closely with Jack Feldman, who developed the basic physiological model for interactions of these neurons, we have developed a theory that predicts that topological aspects of the network determine the stability of its collective metronomic signal. Based on this preliminary work, we are currently exploring new ideas related to how neurons' dendritic trees process incoming information and how changes to the model at the single-neuron level propagate outwards to changes in the collective dynamics of network of many such interacting neurons.
Traditionally, rheology is the study of the flow or deformation of materials. One typically finds that the shear deformation of materials is strongly time (or frequency) dependent. A great example is provided by silly putty -- it behaves elastically when struck rapidly. After all, you can make it into a ball and bounce it! Over long periods of time though (say over night) it will flow like a liquid. Silly putty is not unique in these properties, but rather provides a dramatic example of what is rather commonplace by tuning this transition frequency between an elastic response at high frequencies (or short times) and a viscous one at low frequencies (or low times) into a range that is easily observable by us. More generally, one can study similar behavior many polymeric materials. At short times, the polymer chains are entangled with each other so that the material acts like a permanent (solid) elastic network. At longer times, however, these chains wiggle past each other allowing the material to flow. One can study these collective molecular scale dynamics through rheology -- the study of the (typically) shear response of a material over a wide range of frequencies.
How do you measure the frequency-dependent rheology of the world's most soft and fragile materials? How wide frequency range can one study? How can one measure the mechanics or rheology of very tiny samples that are hard or impossible to grab and shear? When materials are soft enough they have observable strain (solids) or velocity (liquids) fluctuations in thermal equilibrium. The forces associated with Brownian motion are performing your shear experiment for you! Using microrheology experimentalists can probe the mechanics of the world's most fragile materials since they need only remain stable against their own inherent thermal fluctuations! Moreover, since the technique requires one to only observe these random motions, it can be applied to microscopic volumes and be studied over many decades of frequency. Using this technique, people have even measured the mechanics of materials that have no bulk, three-dimensional counter-part like single molecular layers making up Langmuir monolayers and the membrane of red blood cells; they have also explored the mechanics of the interior (cytoskeleton) of other living cells. Recently, we have begun to think about magnetic microrheology of flux liquids and lattices in superconductors.
We have been interested in how one can analyze these fluctuation-based data. We are now exploring how to take the non-equilibrium nature of the cellular interior into account in analyzing microrheological data from living cells. The molecular motors acting on the cellular interior are an extra source of fluctuations -- cell quakes -- in addition to the usual thermal fluctuations. Thus, intracellular microrheology depends on equilibrium statistical mechanics and a quantitative study of the effect of molecular motors.
Semiflexible polymer networks are constructed of stiff filaments that are densely cross-linked on the scale of their thermal persistence length. Due to this microscale architecture, these gels can store elastic energy through both the bending and the stretching of their constituent filaments. In contrast, traditional rubber elasticity theory assumes that all elastic stresses are stored in the stretching of filaments. Such assumptions are reasonable when describing the mechanics of natural rubber or gels that are the product of modern synthetic chemistry. The cytoskeleton, on the other hand, is constructed of extremely stiff protein filaments that are densely cross-linked. This biogel is the source of morphological control and force generation in all eukaryotic cells and is the prototypical semiflexible network. Understanding its mechanical properties requires new ideas. In this talk I discuss the highly nonlinear dependence of the mechanics of this system on network density. In particular I show that there is a abrupt change in the mode of elastic energy storage, the mechanics of this material, and in the geometry of its strain field at a particular network density. We call this the affine-to-nonaffine transition. After discussing our basic understanding of the affine-to-nonaffine transition in semiflexible networks, I then present some data on F-actin gel rheology from Dave Weitz and Margaret Gardel that qualitatively supports the existence of this cross-over. I conclude by discussing briefly the implications of this work for the linear and nonlinear rheology of the cytoskeleton. Powerpoint
Here I consider the hydrodynamics in membranes and at fluid/fluid interfaces. Normal hydrodynamics is a scale-free theory, but, as first noted by Saffmann and Delbruck, membrane hydrodynamics is controlled by new length scale in the system given by the ratio of the two-dimensional membrane viscosity to the usual three-dimensional bulk viscosity of the surrounding fluid. In this talk, I discuss the implications of this new length in the hydrodynamic mobilities of extended and flexible objects. These results have been experimentally tested by the groups of Dinsmore (UMASS) and Weeks (Emory); I present some of their data. Finally, I discuss some newer work done in collaboration with Mark Henle on membrane hydrodynamics on curved surfaces. Powerpoint
What are the mechanical properties of a single alpha-helical polypeptide? Can one develop a meaningful coarse-grained theory of protein mechanics? In this talk I explore the highly nonlinear elastic properties a polypeptide with alpha-helical secondary structure using a simple model. This model consists of a set of tangent vectors to describe the spatial conformations of the polymer chain and a second set of Ising-type variables to describe the local degree of secondary structure. These two sets of degrees of freedom interact through the bending stiffness (persistence length) of the chain. Where the molecule is ordered in its native state (alpha-helix) it is stiff to bending and has a significantly longer thermal persistence length than it does in its disordered state. Using this model we calculate the nanomechanics of a single alpha helix. Because of this coupling of internal degrees of freedom to the conformational degrees of freedom of the polymer backbone, an alpha helix is highly nonlinear in its response to torques and forces. For example, under torque the molecule undergoes a buckling transition associated with a localized break in the hydrogen-bonding pattern in its secondary structure. This nonlinearity may explain how proteins are able to undergo controlled, reproducible conformational changes. We briefly consider ongoing work where we seek to apply these ideas to conformational changes in calmodulin. Powerpoint
How does physical sand differ from the sand typically considered by physicists? In both a geophysical context and from the point of view of processing technologies small amounts of a wetting fluid lead to interparticle attractions. In this talk I discuss a theory developed with Dr. Thomas Halsey (Exxon Mobile) for the dependence of the stability of static sand piles upon both the surface roughness of the particles and the volume of wetting fluid present in the system. This work is strongly supported by a series of experiments by Professor Thomas Mason (UCLA). Finally, I will discuss our more recent work on the dynamics of cohesive sand in the flowing state. Here we find a breakdown of the usual Bagnold constitutive law and the appearance of plug flow near a free surface. I also discuss new attempts to develop a microscopic model for the breakdown of Bagnold scaling. Powerpoint
Jeremy D. Schmit and Alex J. Levine Intermolecular adhesion in semiconducting polymers, Submitted to Physical Review E. (2009).
Chiao-Yu Tseng, Andrew Wang, Giovanni Zocchi, Biljana Rolih, and Alex J. Levine, The elastic energy of protein-DNA chimeras, Submitted to Physical Review Letters (2009).
J.R. Rothenbuhler, J.-R. Huang, B.A. DiDonna, A.J. Levine and T.G. Mason, Diffusion-limited aggregation of colloidal rods and disks, Submitted to Soft Materials (2009).
David Schwab, Robijn F. Bruinsma, and Alex J. Levine, Rhythmogenic Neuronal Networks and k-Core Percolation, Submitted to Physical Review Letters (2008).
Biswaroop Mukherjee and Alex J. Levine, The effect of colored noise on stochastic resonance, Submitted to American Journal of Physics (2008).
YongKeun Park, Catherine Best-Popescu, Kamran Badizadegan, Ramachandra R. Dasari, Michael S. Feld, Tatiana Kuriabova, Mark L. Henle, Alex J. Levine, and Gabriel Popescu, Measurement of red blood cell mechanics during morphological changes, Submitted to the Proceedings of the National Academy of Sciences USA (2008).
Mark L. Henle and Alex J. Levine, Effective Viscosity of a Dilute Suspension of Membrane-bound Inclusions, Physics of Fluids 21, 033106 (2009).
Alex J. Levine and F.C. MacKintosh, The Mechanics and Fluctuation Spectrum of Active Gels, Journal of Physical Chemistry B 113, 3820-3830 (2009). P.-G. de Gennes memorial issue.
Robert Brewster, Gary S. Grest, and Alex J. Levine Effects of cohesion on the surface angle and velocity profiles of granular material in a rotating drum, Physical Review E 79, 011305 (2009).
Mark L. Henle, R. McGorty, A.B. Schofield, A.D. Dinsmore, and A.J. Levine The effect of curvature and topology on membrane hydrodynamics, Europhysics Letters 84, 48001 (2008).
Moumita Das, Alex J. Levine, and F.C. MacKintosh, Buckling and force propagation along intracellular microtubules, Europhysics Letters 84, 18003 (2008).
Robert Brewster, Leonardo Silbert, Gary S. Grest, and Alex J. Levine Relationship between interparticle contact lifetimes and rheology in gravity-driven granular flows, Physical Review E 77, 061302 (2008).
Jeremy D. Schmit and Alex J. Levine Statistical model for Intermolecular Adhesion in conjugated polymers, Physical Review Letters 100, 198303 (2008).
Tatiana Kuribova and Alex J. Levine, Nanorheology of viscoelastic shells: Applications to viral capsids, Physical Review E 77, 031921 (2008).
Robert Walder, Alex J. Levine, and Michael Dennin, Rheology of two-dimensional F-actin networks associated with a lipid interface, Physical Review E 77, 011909 (2008).
F. C. MacKintosh and Alex J. Levine, Non-equilibrium mechanics and dynamics of motor-activated gels, Physical Review Letters 100, 018104 (2008).
Ali Naji, Alex J. Levine, and P.A. Pincus, Corrections to the Saffman-Delbr¨uck mobility for membrane bound proteins, Biophysical Journal: Biophysical Letters 93, L49-L51 (2007).
Leonardo Silbert, Gary S. Grest, Robert Brewster, and Alex J. Levine Rheology and Contact Lifetimes in Dense Granular Flows, Physical Review Letters 99, 068002 (2007).
Moumita Das, F.C. MacKintosh, and Alex J. Levine Effective medium theory of semiflexible filamentous networks, Physical Review Letters 99, 038101 (2007).
M.L. Henle and Alex J. Levine, Erratum: Capillary wave dynamics on supported viscoelastic films: Single and double layers, Physical Review E 75, 059909(E) (2007).
B.A. DiDonna and Alex J. Levine Unfolding cross-linkers as rheology regulators in F-actin networks, Physical Review E 75, 041909 (2007).
Mark L. Henle and Alex J. Levine Capillary wave dynamics on supported viscoelastic films: Single and double layers, Physical Review E 75, 021604 (2007).
J.R. Savage, D. Blair, A.J. Levine, R.A. Guyer, and A.D. Dinsmore Imaging the Sublimation Dynamics of Colloidal Crystallites Science 314(5800), pp. 795-798 (2006).
Buddhapriya Chakrabarti and Alex J. Levine The nonlinear elasticity of an -helical polypeptide: Monte Carlo studies Physical Review E 74, 031903 (2006).
B.A. DiDonna and Alex J. Levine Filamin cross-linked semiflexible networks: Fragility under strain Physical Review Letters 97, 068104 (2006).
M. Atakhorrami, J.I. Sulkowska, K.M. Addas, G.H. Koenderink, J.X. Tang, A.J. Levine, F.C. MacKintosh, and C.F. Schmidt Correlated fluctuations of microparticles in viscoelastic solutions: Quantitative measurement of material properties by microrheology in the presence of optical traps, Physical Review E 73, 061501 (2006).
R. Brewster, J. Landry, G.S. Grest, and A.J. Levine Breakdown of Bagnold scaling in cohesive granular flows, Physical Review E 72, 061301 (2005).
David A. Head, Alex J. Levine, and F.C. MacKintosh Mechanical response of semiflexible networks to localized perturbations. Physical Review E 72, 061914 (2005).
Jeremy D. Schmit and Alex J. Levine Intermolecular adhesion in conducting polymers Physical Review E 71, 051802 (2005).
Buddhapriya Chakrabarti and Alex J. Levine The nonlinear elasticity of an -helical polypeptide Physical Review E 71, 031905 (2004).
Alex J. Levine, David A. Head, and F.C. MacKintosh The elasticity of semiflexible networks, Proceedings of the XIVth Congress on Rheology. ed. The Korean Society of Rheology (2004).
Alex J. Levine, David A. Head, and F.C. MacKintosh The Deformation Field in Semiflexible Networks, Journal of Physics, Condensed Matter 16, S2079 (2004).
Alex J. Levine, T.B. Liverpool, and F.C. MacKintosh Dynamics of rigid and flexible extended bodies in viscous films and membranes, Physical Review Letters 93, 038102 (2004).
Alex J. Levine, T.B. Liverpool, and F.C. MacKintosh The mobility of extended bodies in viscous films and membranes, Physical Review E 69, 021503 (2004).
David A. Head, Alex J. Levine, and F.C. MacKintosh Distinct regimes of elastic response and deformation modes of cross-linked cytoskeletal and semiflexible polymer networks, Physical Review E 68, 061907 (2003).
David A. Head, F.C. MacKintosh, and Alex J. Levine Non–universality of elastic exponents in random bond–bending networks. Physical Review E 68, 025101 (R) (2003).
David A. Head, Alex J. Levine, and F.C. MacKintosh Deformation of cross-linked semiflexible polymer networks, Physical Review Letters 91, 108102 (2003).
Victor Breedveld and Alex J. Levine Shear induced diffusion in dilute suspensions of charged colloids, Soft Materials 1, 235-244 (2003).
D.T. Chen, E.R. Weeks, J.C. Crocker, M.F. Islam, R. Verma, J. Gruber, A.J. Levine, T.C. Lubensky, and A.G. Yodh Rheological Microscopy:Local Mechanical Properties from Microrheology, Physical Review Letters 90, 108301 (2003).
Alex J. Levine and F.C. MacKintosh Dynamics of viscoelastic membranes, Physical Review E 66, 061606 (2002).
Deniz Erta¸s, Thomas C. Halsey, Alex J. Levine, and Thomas G. Mason, Stability of monomer–dimer piles, Physical Review E 66, 051307 (2002).
K.M. Addas, J.X. Tang, A.J. Levine, and C.F. Schmidt Extracting local and bulk viscoelasticity of entangled FD virus solutions by two–bead microrheology, Biophysical Journal 82 (1) 2432 (2002).
KarimM. Addas, Alex J. Levine, Jay X. Tang, and Christoph F. Schmidt One- and Two-Particle Microrheology in Entangled Solutions of fd Virus, Physical Characterization of Biological Materials and Systems Symposium. Boston, MA USA 26 Nov. – 30 Nov. 2001. Warrendale, PA, USA Materials Research Society, (2002).
Alex J. Levine and T.C. Lubensky Two–point microrheology and the electrostatic analogy, Physical Review E 65, 011501 (2001).
R. Bruinsma, F. Rondelez, and A. Levine Flow-Controlled Island Growth in Langmuir Monolayers, European Physics Journal E 6, 191 (2001).
Alex J. Levine and T.C. Lubensky The response function of a sphere in a viscoelastic two–fluid medium, Physical Review E, 63, 041510-1 (2001).
Alex J. Levine and T.C. Lubensky One- and two–particle microrheology, Physical Review Letters 85, 1774 (2000).
Randall D. Kamien and Alex J. Levine Boundary Effects in Chiral Polymer Hexatics, Physical Review Letters 84, 3109 (2000).
T. G. Mason, A. J. Levine, D. Erta¸s, and T. C. Halsey The Critical Angle of Wet Sand Piles, Physical Review E 60, R5044 (1999)
A. Levine, S. Ramaswamy, E. Frey, and R. Bruinsma “Hydrodynamic Screening in Stokesian Fluidized Beds” in Structure and Dynamics of Materials in the Mesoscopic Domain (Prox. 4th Royal Society–Unilever Indo-UK Forum in Materials Science and Engineering), Eds. Moti Lal, R.A. Mashelkar, B.D. Kulkarni, V.M. Naik (Imperial College Press and The Royal Society, 1999) pp. 195–206.
Amy B. Herhold, Deniz Erta¸s, Alex J. Levine, and Hubert E. King Jr. Impurity mediated nucleation in hexadecane-in-water emulsions. , Physical Review E 59, 6946 (1999).
Amy B. Herhold, Deniz Erta¸s, Alex J. Levine, and Hubert E. King Jr. Impurity induced slowing of nucleation in emulsified liquids in Dynamics in Small Confining Systems IV Symposium, Boston, MA USA 30 Nov. – 3 Dec. 1998. Warrendale, PA, USA: Materials Research Society, pp.85–96 (1999).
Alex Levine, Sriram Ramaswamy, Erwin Frey, and Robijn Bruinsma, Screened and Unscreened Phases in Sedimenting Suspensions, Physical Review Letters 80, 5944 (1998).
Alex J. Levine and Scott T. Milner, Star Polymers and the Failure of Time-Temperature Superposition, Macromolecules 31 (24) 8623 – 8637 (1998).
Alex J. Levine and Thomas C. Halsey, How Sandcastles Fall , Physical Review Letters 80, 3141 (1998)
Martin-D. Lacasse, Gary S. Grest, and Alex J. Levine, Capillary-wave and chain-length effects at polymer/polymer interfaces, Physical Review Letters 80, 309 (1998).
S. N. Coppersmith, T. C. Jones, L. P. Kadanoff, A. Levine, J. P.McCarten, S. R. Nagel, S. C. Venkataramani, and Xinlei Wu, Self-Organized Short-Term Memories, Physical Review Letters 78, 3983 (1997).
Shechao Feng, Alex Levine, and Lan Yin, Suppression of the Josephson Effect and Little-Parks Oscillations in the Quantum Hall Effect , in Coulomb and Interference Effects in Small Electronic Structures ed. D. C Glattli, M. Sanquer, and J. Tran Thanh Van, ´ Editions Fronti`eres (1994).
Shechao Feng, Alex Levine, and Lan Yin, Suppression of the Josephson Effect by Quantum Fluctuations in the Fractional Quantum Hall State, Physical Review B. 50, 11045 (1994).
Robijn Bruinsma (UCLA)
Michael Dennin (UCI)
Gabriel Popescu (UIUC)
Tony Dinsmore (UMASS)
Jeff Urbach (Georgetown)
Christoph Schmidt (Göttingen)
Giovanni Zocchi (UCLA)
Tom Mason (UCLA)
Jack Feldman (UCLA)
Fred MacKintosh (Vrije Universiteit, Amsterdam)
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