CURRICULUM VITAE
Click here to download Nick’s CV.
BRIEF BIOGRAPHY
Growing up on the coast of Maine, Nick was inspired to study physical sciences by the powerful winter cyclones that strike his hometown each year. Northeastern snowstorms are a rare example of how physics through a continuum of scales—microscopic to global—act in coincidence to form a beautiful, yet fleeting, natural structure that affects all five senses of a human being in a profound way. Long histories and mythologies written by colonial American scholars add a poetical and artistic dimension to their study.
Building on this inspiration, Nick enjoys solving challenging problems in physics, with the goal of discovering new mathematical and physical beauty in nature. As a graduate student in the Lipson group at Dartmouth College, Nick has come to find that polymer physics is a realm where such problems are in endless number.
CURRENT PROJECTS
The glass transition in glass-forming liquids and films
Coadvised by Scott Milner, The Pennsylvania State University
Movie of mobile free volume in a bulk system near kinetic arrest. Shading intensity indicates sites that have been mobile very recently (deep) and in the past (faint). White regions contain kinetically-jammed particles. (Click the image to play the movie in full size.)
What happens when a fluid is super-cooled? Particles in a fluid are not uniformly packed, rather there are fluctuations in the amount of unoccupied space in different parts of the system. Particles utilise this space, called “free volume”, as a source of mobility to undergo local relaxation. Environments that lack free volume are constrained from readily relaxing until more free volume is transported into that vicinity.
The paradox is that free volume transport is dependent on particle motion itself. When a particle shifts to occupy adjacent free volume, that free volume moves to where the particle used to reside. Many such particle shifts results in free volume propagating through a fluid, similar to how the empty square of a sliding tile game is inadvertently shifted as one seeks the solution to the puzzle. However, particles become sluggish when a fluid is rapidly cooled, and free volume transport grows dramatically slower. Regions rich in free volume tend to remain rich, and retain their particle mobility. On the other hand, regions lacking free volume are “kinetically arrested”, and the attendant particles cannot relax within the experimental timescale. This results in dynamic heterogeneity, where different localities in the same system exhibit markedly different dynamics and relaxation lifetimes.
A “glass” is the material that results once the entire fluid undergoes kinetic arrest. The temperature at which the substance transforms from fluid to glass is its “glass transition temperature”. Window panes are a common example of a molecular glass, though many other fluids may form glasses as well. A key microscopic characteristic is that the molecules lack the long-range order seen in a crystalline solid phase. Polymers are particularly good glass-formers, as chain entanglement in their liquid-like “melt” phase tends to facilitate kinetic arrest upon cooling. As a result, they are of considerable interest as glassy materials; examples of applications are polymeric thin films for electronics and molecularly-imprinted sensors, as well as polymer blend and copolymeric glasses for vibrational damping.
Many of the questions that arise from experiments on polymeric glasses also remain to be resolved for simple fluidic glasses. For example, what is the spatial nature of free volume transport in a near-glassy substance? How does it facilitate a glass transition as the sample temperature becomes lower?
In my research, we have found that a surprisingly simple kinetic model captures the dynamic heterogeneity of free volume as a fluid nears its glass transition. This leads to a kinetic phase transition from liquid to glass, accompanied by the hallmarks of glassy behaviour: power-law growth of relaxation lifetimes and dynamic length scales. Simulation movies reveal the tendency for free volume to form “colonies” around the edges of large bubble-like regions of glassified fluid. Due to its simplicity, the model can also be readily applied to more complex systems such as thin films of material.
PUBLICATIONS
Lattice Model of Mobility at Interfaces: Free Surfaces, Substrates, and Bilayers. Tito, N. B.; Lipson, J. E. G.; Milner, S. T. Soft Matter 2013, 9, 9403-9413.
Lattice Model of Dynamic Heterogeneity and Kinetic Arrest in Glass-Forming Liquids. Tito, N. B.; Lipson, J. E. G.; Milner, S. T. Soft Matter 2013, 9, 3173-3180
Ball-of-Yarn Conformation of a Linear Gradient Copolymer in a Hompolymer Melt. Tito, N. B.; Milner, S. T.; Lipson, J. E. G. Macromolecules 2012, 45, 7607-7620.
Self-Assembly of Lamellar Microphases in Linear Gradient Copolymer Melts. Tito, N. B.; Milner, S. T.; Lipson, J. E. G. Macromolecules 2010, 43, 10612-10620.
Application of a coarse-grained model for DNA to homo- and heterogeneous melting equilibria. Tito, N. B.; Stubbs, J. M. Chem. Phys. Lett. 2010, 485, 354-359.