Anand Jagota

DNA sequence motifs for structure-specific recognition and separation of carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are a family of molecules that have the same cylindrical shape but different chiralities. Many fundamental studies and technological applications of SWNTs require a population of tubes with identical chirality that current syntheses cannot provide. The SWNT sorting problem—that is, separation of a synthetic mixture of tubes into individual single-chirality components—has attracted considerable attention in recent years. Intense efforts so far have focused largely on, and resulted in solutions for, a weaker version of the sorting problem: metal/semiconductor separation. A systematic and general method to purify each and every single-chirality species of the same electronic type from the synthetic mixture of SWNTs is highly desirable, but the task has proven to be insurmountable to date. Here we report such a method, which allows purification of all 12 major single-chirality semiconducting species from a synthetic mixture, with sufficient yield for both fundamental studies and application development. We have designed an effective search of a DNA library of ∼1060 in size, and have identified more than 20 short DNA sequences, each of which recognizes and enables chromatographic purification of a particular nanotube species from the synthetic mixture. Recognition sequences exhibit a periodic purine–pyrimidines pattern, which can undergo hydrogen-bonding to form a two-dimensional sheet, and fold selectively on nanotubes into a well-ordered three-dimensional barrel. We propose that the ordered two-dimensional sheet and three-dimensional barrel provide the structural basis for the observed DNA recognition of SWNTs.

Design of biomimetic fibrillar interfaces: 1. Making contact

This paper explores the contact behaviour of simple fibrillar interfaces designed to mimic natural contact surfaces in lizards and insects. A simple model of bending and buckling of fibrils shows that such a structure can enhance compliance considerably. Contact experiments on poly(dimethylsiloxane) (PDMS) fibrils confirm the model predictions. Although buckling increases compliance, it also reduces adhesion by breaking contact between fibril ends and the substrate. Also, while slender fibrils are preferred from the viewpoint of enhanced compliance, their lateral collapse under the action of surface forces limits the aspect ratio achievable. We have developed a quantitative model to understand this phenomenon, which is shown to be in good agreement with experiments.

Design of biomimetic fibrillar interfaces: 2. Mechanics of enhanced adhesion.

This study addresses the strength and toughness of generic fibrillar structures. We show that the stress σc required to pull a fibril out of adhesive contact with a substrate has the form σc=σ0Φ(χ). In this equation, σ0 is the interfacial strength, Φ(χ) is a dimensionless function satisfying 0=Φ(χ)=1 and χ is a dimensionless parameter that depends on the interfacial properties, as well as the fibril stiffness and radius. Pull-off is flaw sensitive for χ≫1, but is flaw insensitive for χ<1. The important parameter χ also controls the stability of a homogeneously deformed non-fibrillar (flat) interface. Using these results, we show that the work to fail a unit area of fibrillar surface can be much higher than the intrinsic work of adhesion for a flat interface of the same material. In addition, we show that cross-sectional fibril dimensions control the pull-off force, which increases with decreasing fibril radius. Finally, an increase in fibril length is shown to increase the work necessary to separate a fibrillar interface. Besides our calculations involving a single fibril, we study the concept of equal load sharing (ELS) for a perfect interface containing many fibrils. We obtain the practical work of adhesion for an idealized fibrillated interface under equal load sharing. We then analyse the peeling of a fibrillar surface from a rigid substrate and establish a criterion for ELS.

Mechanics of Adhesion Through a Fibrillar Microstructure

Abstract Many organisms have evolved a fibrillated interface for contact and adhesion as shown by several of the papers in this issue. For example, in the Gecko, this structure appears to give them the ability to adhere and separate from a variety of surfaces by relying only on weak van der Waals forces. Despite the low intrinsic energy of separating surfaces held together by van der Waals forces, these organisms are able to achieve remarkably strong adhesion. To help understand adhesion in such a case, we consider a simple model of a fibrillar interface. For it, we examine the mechanics of contact and adhesion to a substrate. It appears that this structure allows the organism, at the same time, to achieve good, ‘universal’ contact and adhesion. Due to buckling, a carpet of fibrils behaves like a plastic solid under compressive loading, allowing intimate contact in the presence of some roughness. As an adhesive, we conjecture that energy in the fibrils is lost upon decohesion and unloading. This mechanism can add considerably to the intrinsic work of fracture, resulting in good adhesion even with only van der Waals forces. Analysis of the mechanics of adhesion through such a fibrillar interface provides rules for the design of the microstructure for desired performance as an adhesive.

Biologically inspired crack trapping for enhanced adhesion

We present a synthetic adaptation of the fibrillar adhesion surfaces found in nature. The structure consists of protruding fibrils topped by a thin plate and shows an experimentally measured enhancement in adhesion energy of up to a factor of 9 over a flat control. Additionally, this structure solves the robustness problems of previous mimic structures and has preferred contact properties (i.e., a large surface area and a highly compliant structure). We show that this geometry enhances adhesion because of its ability to trap interfacial cracks in highly compliant contact regimes between successive fibril detachments. This results in the requirement that the externally supplied energy release rate for interfacial separation be greater than the intrinsic work of adhesion, in a manner analogous to lattice trapping of cracks in crystalline solids.

Cohesive element modeling of viscoelastic fracture : Application to peel testing of polymers

A computational modeling technique for fracture propagation in viscoelastic materials using cohesive elements for the zone ahead of the crack tip is presented. The computational technique is used to study the problem of increase in fracture energy with peel velocity in peel testing of polymers. A rate-independent phenomenological cohesive zone model is used to model the intrinsic fracture toughness of the interface between the polymer sheets. A dimensional analysis reveals that the macroscopic fracture energy scales with the intrinsic fracture toughness and is a function of peel velocity, and parameters such as the thickness, bulk properties of the polymer sheets, and other cohesive zone properties. The growth of fracture energy as a function of the peel velocity has been studied for polymer sheets characterized by a standard linear viscoelastic solid. Viscoelastic losses in the peel arm vanish in the limits of very slow and rapid peeling. Peak dissipation is obtained at an intermediate velocity, which is related to the characteristic relaxation time and thickness. This behavior is interpreted in terms of the size of elastic and viscous zones near the crack tip. It is found that the total energy dissipated is dependent upon both the intrinsic fracture toughness and the characteristic opening displacement of the cohesive zone model. The computational framework has been used to model experimental data on peeling of Butadiene rubbers. It is found that the usual interpretation of these data, that the macroscopic dissipation equals the rate-independent intrinsic toughness multiplied by a factor that depends on rate of loading, leads to a large quantitative discrepancy between theory and experiment. It is proposed that a model based on a rate-dependent cohesive law be used to model these peel tests.

Crack blunting and the strength of soft elastic solids

When a material is so soft that the cohesive strength (or adhesive strength, in the case of interfacial fracture) exceeds the elastic modulus of the material, we show that a crack will blunt instead of propagating. Large–deformation finite–element model (FEM) simulations of crack initiation, in which the debonding processes are quantified using a cohesive zone model, are used to support this hypothesis. An approximate analytic solution, which agrees well with the FEM simulation, gives additional insight into the blunting process. The consequence of this result on the strength of soft, rubbery materials is the main topic of this paper. We propose two mechanisms by which crack growth can occur in such blunted regions. We have also performed experiments on two different elastomers to demonstrate elastic blunting. In one system, we present some details on a void growth mechanism for ultimate failure, post–blunting. Finally, we demonstrate how crack blunting can shed light on some long–standing problems in the area of adhesion and fracture of elastomers.

Mechanically tunable dry adhesive from wrinkled elastomers

We report a new dry adhesive structure using a rippled poly(dimethylsiloxane) (PDMS) elastomer bilayer film, whose surface roughness and adhesion can be reversibly regulated by applying mechanical strain. It has a set of advantages not offered by other techniques for regulation of adhesion, including real-time tunability, no requirement of specific surface chemistry, operability under ambient conditions, and relative ease of control. To understand the mechanism for adhesion regulation quantitatively, we have modeled the mechanics of adhesion in the limits of small- and large-amplitude ripples, and show good agreement with indentation experiments. We demonstrate the real-time tunability of the new adhesive structure by repeatedly picking and releasing a glass ball simply by modulating the mechanical stretch of the rippled PDMS film.

Micromechanical modeling of powder compacts—I. Unit problems for sintering and traction induced deformation

Abstract A micromechanical model has been developed for the constitutive behavior of powder compacts based on unit models for the interaction between individual particles. In this paper, the first of two companion papers, two unit problems have been formulated for the study of inter-particle behavior. These correspond to the cases of deformation by external tractions and local surface tension forces. The contact area between individual particles is used to characterize the state of the powder compact. Specific numerical results are presented for linear viscous material properties and are compared with existing models. The choice of unit problems for sintering has been discussed at length and some ambiguities in the traditional method of modeling viscous sintering are pointed out. The results of this study are used in the companion paper [Acta metall.36, 2563 (1988)] to develop a model for the sintering and compaction of discrete packings.

Polymer interfacial fracture simulations using cohesive elements

A family of cohesive elements is presented based on cohesive zone models to describe polymer interfacial fracture. Their capabilities are demonstrated in three case studies of interfacial failure. The first is a simulation of the t-peel test for the determination of adhesion between two elastomers. This case is characterized by large, inelastic deformation that is difficult to model using classical fracture mechanics and analytic cohesive zone approaches. The formulation allows simulation of crack growth in the presence of large global strains and the identification of peak viscous loss zones in the peel arms. The second case study is the analysis of a compressive shear test to determine adhesion between a viscoelastic elastomer and a rigid substrate. An experimentally observed transition from stable to unstable fracture is described accurately by the model, providing appropriate cohesive zone parameters are established. The third example treats interfacial failure in a multilayer elasto-plastic polymer system. The approach illustrates a capability to capture crack nucleation and propagation in systems with complex microstructures comprising of multiple layered phases and associated interfaces.