 receptor type and density and different specific/non-specific interaction forces on the cellular uptake of nanoparticles. The significance of this research is reflected by the urgent societal needs to understand both beneficial and hazardous effects of nanotechnology which are projected to produce and release thousands of tons of nanomaterials into the environment in the coming decades. Here, theoretical analysis and molecular dynamics simulations are carried out to investigate the adhesive wrapping of a cylindrical or a spherical elastic vesicle by cell membrane. We show that there exist three distinct wrapping phases - full wrapping, partial wrapping and no wrapping - depending on the vesicle size, the adhesion energy, the surface tension of the membrane, and the bending rigidity ratio between the vesicle and the cell membrane. In particular, the bending rigidity ratio is found to play a critical role. These results are of fundamental importance not only to understanding vesicular transport but also to modeling endocytosis/phagocytosis of elastic particles into cells.
Morphologic Instability of Graphene on Substrates.
Mechanics of Periodic Polymeric Structures: Exploiting Instabilities to Enhance Performance.
Periodic microstructures abound in nature and provide numerous interesting and unique mechanical, photonic, phononic and hydrophobic properties. Advances in block copolymer chemistry, lithography, controlled printing, molding, laser cutting, and other processing techniques enable synthetic production of periodic polymeric microstructures at many different lengthscales. Here, we focus on the mechanics of the finite deformation of periodic polymeric-based microstructures and how the periodic structure can give rise to interesting and novel mechanical behaviors, in many instances involving microstructural instabilities which can be used to trigger changes in either internal periodic morphology or in surface topology. The periodic structures and their switching and/or evolution with deformation can be used to either permanently or dynamically tune other attributes including, for example, phononic bandgaps and hydrophobicity. Several different examples will be presented including: (i) mechanics of periodic porous elastomers which exhibit deformation-induced pattern transformations upon reaching a critical load giving a superelastic stress-strain behavior as well as a tunability in phononic and photonic band gaps; (ii) mechanics of filled polymers where instabilities are used to provide enhanced resiliency; (iii) mechanics of lamellar block copolymers building from “single crystal” lamellar structure mechanics to “polycrystal” lamellar block copolymers; (iiv) mechanics of processing of electrospun polymeric fibers using solvent evaporation to control surface topology; and (iv) mechanics of three-dimensional periodic porous and bicontinuous structures for controlling stiffness, strength, and energy dissipation.
Swelling-induced Surface Pattern Formation and Ordering in Hydrogel Films with Depth-wise Crosslinking Gradients.
Hydrogels undergo extensive three-dimensional volume changes when immersed in water, the degree of which is determined by the network chemical composition and degree of crosslinking. When the hydrogel is attached to a rigid substrate, it swells preferentially perpendicular to the substrate. This anisotropic swelling generates a compressive stress, which drives the formation of surface patterns when exceeding a critical stress. In order to develop an in depth understanding of the mechanism of surface pattern formation in hydrogels, we investigated the dynamic evolution of surface patterns in photocured hydrogel films from poly(2-hydroxyethyl methacrylate) (PHEMA) crosslinked with different concentrations of ethylene glycol dimethacrylate (EGDMA, 0-3 wt%). During curing in the presence of oxygen, a modulus gradient along the film depth was generated due to oxygen inhibition of the radical polymerization near the film surface. Our results showed that the swelling-induced wrinkling pattern formation followed Fickian-type kinetics (λ~t^1/2) at early stages, which was independent of the final pattern morphology. The onset of wrinkling was found at a critical linear expansion of ~1.12, which remained constant with increasing EGDMA concentration but decreased with initial film thickness. Moreover, the equilibrium linear expansion value decreased significantly (from 2.55 to 1.20) with increasing crosslinker concentration (from 0 to 3 wt%), resulting in the transition from random patterns to highly ordered hexagonal patterns. We have found that hydrogel surface instabilities are either in the form of shallow undulations (wrinkling) or in the form of sharp folds (creasing). To understand the mechanism of wrinkling and creasing pattern formation, we investigated the dynamic evolution of surface patterns in a range of pure and mixed solvents. We found that the Hildebrand solubility parameter of the PHEMA films containing 1 wt% or 3 wt% EGDMA is between 26.5 and 29.6 MPa^1/2. The onset of the wrinkling to creasing transition was found to decrease significantly (from 2.0 to 1.3) with increasing crosslinker concentration (from 1 to 3 wt% EGDMA). Hydrogels with controlled surface patterns are useful for a range of applications, including in microdevices, sensors, responsive coatings, and adhesives. As an example, we investigated human mesenchymal stem cell interactions with uniform hydrogels and hydrogels with lamellar and hexagonal wrinkling patterns and showed that the pattern morphology was able to dictate stem cell morphology and differentiation.
Nanoparticle Structure Formation by Confined Drying in Wrinkles.
Controlled Flopping of Vertical Carbon Nanotube Gratings into Multi-directional and Multi-layered Networks.
Vertically aligned “forests” of carbon nanotubes (CNTs) and other one-dimensional nanostructures are often grown on or etched from planar substrates, thus forming networks perpendicular to the substrate. These unique materials typically have a low density along with high physical anisotropy, and can therefore be directionally infiltrated with liquids due to capillary action. We reveal how narrow micro-patterned gratings of vertical CNTs can be transformed into multi-directional and multi-layered lateral networks due to a mechanical “flopping” instability triggered by capillary induced contraction during liquid infiltration and evaporation. Importantly, liquid is introduced by condensation so each grating feature is infiltrated independently without causing capillary bridging among nearby features. The flopping direction of individual micro-gratings, and therefore the final direction of the CNTs after flopping, is controlled by patterning initiating arrows at the ends of the films. By design and patterning of the initiators, we fabricate circuits of flopped CNTs having radial, tangential, rectangular, and multi-directional arrangements of densely packed CNTs with thickness ranging from 500 nm to 5 µm. Parametric variation of the grating and arrow geometry, along with in situ observation of the flopping behavior offers insight about the process dynamics and critical length scales at which these structures form without introducing defects. These insights are also applied to fold orthogonal and curved vertical CNT films into closed 3D micro-tents and twisted micro-pillars. Finally, the attractive properties of the anisotropic CNT networks are demonstrated by a 16:1 ratio of electrical conductivity between the radial and tangential directions, and by directed neurite outgrowth from primary rat dorsal root ganglia.
Elastic Moduli of Thin Film Glasses Elucidated from Surface Wrinkling.
Surface wrinkling has been proven to be a powerful measurement tool to determine the elastic modulus of thin films. A wide range of materials have been examined to date including polymers, porous films, and organic semiconductors as examples. Additionally, the mechanical properties in very thin films determined using surface wrinkling have been shown to vary significantly from the bulk. In this presentation, the thin film moduli of polystyrene and poly(methyl methacrylate) will be discussed with a decrease in the elastic modulus for both polymers observed in ultrathin films. The influence of molecular mass on this thin film behavior is examined. Additionally, surface cross-linking and molecular additives will be explored as routes to engineer the mechanics of polymers at the nanoscale. In order to better understand why the mechanical properties of organic glass thin films are dependent upon their thickness, the moduli of a series of small molecule organic glasses with well defined absorption spectra are determined as a function of film thickness. As the absorption spectrum is related to the molecular packing, any relationship between the molecular density in thin films and their mechanical properties can be explored.
Role of Confinement on Material Flow in Thin Film Geometries for NanoImprint Lithography.
Wrinkling Delamination in Thin Polymer Coatings on Compliant Substrates.
When a thin, stiff coating on a compliant substrate is compressed in the plane of the film, the film may either remain adhered to the substrate and adopt a sinusoidal accordian-like morphology, or simply delaminate from the substrate surface. These phenomena are often referred to as wrinkling and buckle or blister delamination, respectively. Generally speaking, wrinkling is encouraged by factors such as good film adhesion and substrate compliance, while buckles form in systems where the substrate-film interface is weak and/or the the substrate compliance is high enough to make wrinkling energetically unfavorable. Certain systems, however, will initially wrinkle but then subsequently form blisters that act to locally relax the wrinkling instability in the film. We term this intermediate behavior "wrinkling delamination". Through the investigation of wrinkling delamination in thin films of polystyrene on rubbery silicone, we lay a basic framework for understanding the physical parameters governing this phenomenon. Furthermore, we demonstrate how the length scales of wrinkling delamination features can be utilized to measure the adhesion strength of polymer coatings and to pattern microscale features in thin polymer films.
Nanopatterned Polymer Thin Films Through Wrinkling.
Patterning via spontaneous or self-assembly processes have steadily gained attention as interest in smaller, more complex or hierarchical patterns has increased. These patterns, which may be difficult to obtain through traditional patterning techniques, can be created through the proper manipulation of spontaneous events such as block copolymer phase separations, breath figures, etc. Buckling or wrinkling based techniques have been harnessed to create well