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Jones J.J.,Massachusetts Institute of Technology | Van Der Maarel J.R.C.,Singapore Alliance for Research and Technology Center | Van Der Maarel J.R.C.,National University of Singapore | Doyle P.S.,Massachusetts Institute of Technology | Doyle P.S.,Singapore Alliance for Research and Technology Center
Nano Letters | Year: 2011

We experimentally and numerically study the effects of macromolecular crowding agents on DNA structure when confined to a nanochannel. Curiously, DNA response to crowding is significantly different between bulk phase, nanoslit confinement, and nanotube confinement. Coarse grained Brownian dynamics simulations reproduce trends seen in the experiments and allow us to develop a deeper understanding of the key physics at play in these systems. It is proposed that the occupancy of free volume next to the channel wall by crowders causes an effective reduction in confining dimensions of the channel that initially swells DNA in nanoconfinement. © 2011 American Chemical Society.


Hashemi H.,Massachusetts Institute of Technology | Zhang B.,Singapore Alliance for Research and Technology Center | Joannopoulos J.D.,Massachusetts Institute of Technology | Johnson S.G.,Massachusetts Institute of Technology
Physical Review Letters | Year: 2010

We show that the difficulty of cloaking is fundamentally limited by delay-loss and delay-bandwidth limitations that worsen as the size of the object to be cloaked increases relative to the wavelength, using a simple model of ground-plane cloaking. These limitations must be considered when scaling experimental cloaking demonstrations up from wavelength-scale objects. © 2010 The American Physical Society.


Park K.-C.,Massachusetts Institute of Technology | Choi H.J.,Massachusetts Institute of Technology | Chang C.-H.,Massachusetts Institute of Technology | Chang C.-H.,North Carolina State University | And 4 more authors.
ACS Nano | Year: 2012

Designing multifunctional surfaces that have user-specified interactions with impacting liquids and with incident light is a topic of both fundamental and practical significance. Taking cues fromnature, we use tapered conical nanotextures to fabricate the multifunctional surfaces; the slender conical features result in large topographic roughness, while the axial gradient in the effective refractive index minimizes reflection through adiabatic index-matching between air and the substrate. Precise geometric control of the conical shape and slenderness of the features as well as periodicity at the nanoscale are all keys to optimizing the multifunctionality of the textured surface, but at the same time, these demands pose the toughest fabrication challenges. Here we report a systematic approach to concurrent design of optimal structures in the fluidic and optical domains and a fabrication procedure that achieves the desired aspect ratios and periodicities with few defects and large pattern area. Our fabricated nanostructures demonstrate structural superhydrophilicity or, in combination with a suitable chemical coating, robust superhydrophobicity. Enhanced polarization-independent optical transmission exceeding 98% has also been achieved over a broad range of bandwidth and incident angles. These nanotextured surfaces are also robustly antifogging or self-cleaning, offering potential benefits for applications such as photovoltaic solar cells. © 2012 American Chemical Society.


Zhang B.,Singapore Alliance for Research and Technology Center | Zhang B.,Massachusetts Institute of Technology | Luo Y.,Singapore Alliance for Research and Technology Center | Luo Y.,Massachusetts Institute of Technology | And 3 more authors.
Physical Review Letters | Year: 2011

Invisibility cloaks, a subject that usually occurs in science fiction and myths, have attracted wide interest recently because of their possible realization. The biggest challenge to true invisibility is known to be the cloaking of a macroscopic object in the broad range of wavelengths visible to the human eye. Here we experimentally solve this problem by incorporating the principle of transformation optics into a conventional optical lens fabrication with low-cost materials and simple manufacturing techniques. A transparent cloak made of two pieces of calcite is created. This cloak is able to conceal a macroscopic object with a maximum height of 2 mm, larger than 3500 free-space-wavelength, inside a transparent liquid environment. Its working bandwidth encompassing red, green, and blue light is also demonstrated. © 2011 American Physical Society.


Petruccelli J.C.,Massachusetts Institute of Technology | Tian L.,Massachusetts Institute of Technology | Barbastathis G.,Massachusetts Institute of Technology | Barbastathis G.,Singapore Alliance for Research and Technology Center
Optics Express | Year: 2013

We investigate the measurement of a thin sample's optical thickness using the transport of intensity equation (TIE) and demonstrate a version of the TIE, valid for partially coherent illumination, that allows the measurement of a sample's optical path length by the removal of illumination effects. © 2013 Optical Society of America.


Dai L.,Singapore Alliance for Research and Technology Center | Doyle P.S.,Singapore Alliance for Research and Technology Center | Doyle P.S.,Massachusetts Institute of Technology
Macromolecules | Year: 2013

The similarities and differences between geometric and tensile constraints on polymers have not been fully investigated. Here we use theory (blob models) and simulations to present a comprehensive comparison between polymers in these two situations. For a polymer in good solvent, the effect of tensile force f on extension in the Pincus regime is similar to the effect of cylindrical confinement in the de Gennes regime after mapping the characteristic length kBT/f to the cylindrical diameter D, where kBT is the thermal energy. However, the comparison of the effects of tension and confinement on extension is lacking when kBT/f and D are less than the thermal blob size b, referred to as extended Pincus regime and extended de Gennes regime, respectively. In the extended Pincus regime, force can still segregate the ideal-coils with the size of ∼kBT/f, resulting in the scaling of extension L∥ ∼ (kBT/f) -1. In the extended de Gennes regime, excluded volume interaction is not sufficient to segregate the ideal-coils with the size of D, resulting in the scaling L∥ ∼ D-2/3 different from the scaling in the extended Pincus regime. In addition to the scaling of extension, the scaling of fluctuation in extension σ is also compared in the extended Pincus or extended de Gennes regime. It is found that σ is independent of f and D, which reflects the ideal-chain behavior. All of the above scaling relations are validated by Monte Carlo simulations. Simulation and scaling results are also used to determine the experimental conditions needed to access the extended de Gennes and de Gennes regimes in various single molecule experiments. © 2013 American Chemical Society.


Weymouth G.D.,Singapore Alliance for Research and Technology Center | Triantafyllou M.S.,Massachusetts Institute of Technology
Journal of Fluid Mechanics | Year: 2012

We study numerically the viscous flow around a steadily moving two-dimensional cylinder undergoing a rapid reduction in its diameter as a model problem for force production through shape change which is encountered in the locomotion of certain animals. We consider first the case of a rapidly collapsing circular cylinder in steady translation, starting from an original diameter and reaching a final, smaller diameter under prescribed kinematics. We show that the difference in added mass energy is recovered by the body, and the boundary layer vorticity is reduced through annihilation with opposite-sign vorticity generated during the reduction phase. Next we consider a steadily moving circular cylinder which undergoes rapid but orderly melting, resulting in the same reduction of its diameter but which exhibits radically different flow patterns compared to the collapsing cylinder. The original vorticity in the boundary layer is shed instantaneously and globally in the fluid at the start of the melting phase, and then rapidly rolls up to form a pair of strong vortices, which contain the energy difference between the original and final cylinder states. The formation of the vortices in the melting cylinder takes less than a third of the time required by a rigid translating cylinder to form such vortices. © 2012 Cambridge University Press.


Dai L.,Singapore Alliance for Research and Technology Center | Renner C.B.,Massachusetts Institute of Technology | Doyle P.S.,Singapore Alliance for Research and Technology Center | Doyle P.S.,Massachusetts Institute of Technology
Physical Review Letters | Year: 2015

Recent theoretical progress has explained the physics of knotting of semiflexible polymers, yet knotting of flexible polymers is relatively unexplored. We herein develop a new theory for the size distribution of knots on a flexible polymer and the existence of metastable knots. We show the free energy of a flexible molecule in a tube can be mapped to quantitatively reproduce the free energy distribution of a knot on a flexible chain. The size distribution of knots on flexible chains is expected to be universal and might be observed at a macroscopic scale, such as a string of hard balls. © 2015 American Physical Society.


Sheppard C.J.R.,Italian Institute of Technology | Sheppard C.J.R.,Singapore Alliance for Research and Technology Center
Optics Express | Year: 2013

Pupil filters for cylindrical (two-dimensional) focusing with extended depth of field are investigated. An important application is in generating light sheets with uniform intensity. Filters for spherical (threedimensional) focusing with a flat axial intensity, coupled with weak side lobes are also discussed. © 2013 Optical Society of America.


Sheppard C.J.R.,Italian Institute of Technology | Sheppard C.J.R.,Singapore Alliance for Research and Technology Center
Applied Optics | Year: 2013

Focusing by an aberration-free cylindrical lens is analyzed in the paraxial Fresnel and Debye approximations, and expressions are given. Plots are given for the intensity in the focal region, the defocused optical transfer function (OTF), the generalized OTF, and the ambiguity function and are compared with the case of an aberration-free spherical lens. Nonparaxial lenses are also discussed. © 2013 Optical Society of America.

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