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Full-Text Articles in Engineering
Rotational Diffusion Of Nonlinear Optical Chromophores In Rubbery And Glassy Polymers: Ï¡-Relaxation Dynamics Probed By Second Harmonic Generation And Dielectric Relaxation, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
In Dynamics In Small Confining Systems Ii, Ali Dhinojwala
In Dynamics In Small Confining Systems Ii, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
Small Molecule Probe Diffusion In Thin Polymer Films Near The Glass Transition: A Novel Approach Using Fluorescence Nonradiative Energy Transfer, Ali Dhinojwala
Small Molecule Probe Diffusion In Thin Polymer Films Near The Glass Transition: A Novel Approach Using Fluorescence Nonradiative Energy Transfer, Ali Dhinojwala
Ali Dhinojwala
A novel experimental approach involving fluorescence nonradiative energy transfer (NRET) is employed to study the Fickian diffusion of small molecules in rubbery polymer films near the glass transition. A theoretical formalism has been developed which directly relates the small molecule translational diffusion coefficient, , to changes in the energy transfer efficiency, E. Values of as low as 5 × 10-16 cm2/s have been measured. In this approach, two thin polymer films are sandwiched together, one labeled with either NRET donor or acceptor chromophores and the second doped with the complementary chromophore. Upon annealing for a time t, dopant chromophore diffusion …
Cobweb-Weaving Spiders Produce Different Attachment Discs For Locomotion And Prey Capture, Vasav Sahni, Jared Harris, Todd Blackledge, Ali Dhinojwala
Cobweb-Weaving Spiders Produce Different Attachment Discs For Locomotion And Prey Capture, Vasav Sahni, Jared Harris, Todd Blackledge, Ali Dhinojwala
Ali Dhinojwala
Spiders' cobwebs ensnare both walking and flying prey. While the scaffolding silk can entangle flying insects, gumfoot silk threads pull walking prey off the ground and into the web. Therefore, scaffolding silk needs to withstand the impact of the prey, whereas gumfoot silk needs to easily detach from the substrate when contacted by prey. Here we show that spiders accomplish these divergent demands by creating attachment discs of two distinct architectures using the same pyriform silk. A 'staple-pin' architecture firmly attaches the scaffolding silk to the substrate and a previously unknown 'dendritic' architecture weakly attaches the gumfoot silk to the …
In Dynamics In Small Confining Systems Iii, Ali Dhinojwala
In Dynamics In Small Confining Systems Iii, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
Adsorption, Ali Dhinojwala
New Approaches To Measure Interfacial Rheology Of Confined Fluids, Ali Dhinojwala
New Approaches To Measure Interfacial Rheology Of Confined Fluids, Ali Dhinojwala
Ali Dhinojwala
Several instrumental developments to examine the rheology of ultrathin confined liquids are described. These include capacitance sensing of the film thickness, concurrent measurement of normal and shear forces by piezoelectric methods, application of periodic forces in the normal direction to determine dynamic oscillatory shear moduli, and quantification of apparatus misalignment. The method to separate apparatus from sample contributions is stressed.
Translation-Rotation Paradox For Diffusion In Glass-Forming Polymers: The Role Of The Temperature Dependence Of The Relaxation Time Distribution, Ali Dhinojwala
Translation-Rotation Paradox For Diffusion In Glass-Forming Polymers: The Role Of The Temperature Dependence Of The Relaxation Time Distribution, Ali Dhinojwala
Ali Dhinojwala
Comparisons are made of the translational and rotational diffusion of small-molecule probes in a polymer near its glass transition temperature, Tg. In the rubbery state, 1.1Tg>T>Tg, translational diffusion is much less temperature dependent than rotational reorientation; in a ``quenched'' glass, translation and rotation have similar temperature dependencies. This is explained to be a consequence of the fact that in the rubbery state near Tg the breadth of the polymer relaxation distribution is strongly temperature dependent, while in the quenched glass it is temperature invariant.
Novel Experimental Protocol For Studying Temporal Decay Of Second-Order Nonlinear Optical Polymer Properties: Quantitative Analysis Of Coupling To Polymer Dynamics And Conditions For Temporal Stability, Ali Dhinojwala
Ali Dhinojwala
A new experimental approach has been developed which allows characterization of rotational dynamics of NLO chromophores in amorphous polymers over 12 decades in time, from 10-6 to 106 sec. Measurements over such a broad distribution in relaxation times are important not only in determining how the chromophore orientational dynamics are coupled to polymer dynamics but also in making accurate predictions of the temporal stability of technological interest. In particular, accurate characterization over a broad range of time scales reveals that NLO polymers meet technological requirements more easily than is predicted using a less complete characterization.
Optorheological Studies Of Sheared Confined Fluids With Mesoscopic Thickness, Ali Dhinojwala
Optorheological Studies Of Sheared Confined Fluids With Mesoscopic Thickness, Ali Dhinojwala
Ali Dhinojwala
Fluids of mesoscopic thickness can be sheared and their molecular orientation probed concurrently with the new instrument described in this paper. The fluid is confined between parallel optically flat windows whose spacing is controlled, using piezoelectric inchworms, from submicrometer thickness to 500 μm, with no essential lower limit apart from surface roughness. Capacitance sensors or optical interferometry is used to monitor spacing between the windows with submicrometer accuracy. Piezoelectric bimorphs are used to apply periodic shear displacements with amplitude 0.l−10 μm and frequency 0.1−700 Hz. Shear-induced molecular alignment during sinusoidal shear cycles is determined, with up to 5 μs time …
Rotational Dynamics In Polystyrene Above And Below The Glass Transition Temperature: Second Harmonic Generation And Dielectric Relaxation Measurements, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
Implementing Carbon Nanotube Thermal Interface Material Onto Heat Sink Involves Growing Nanotubes On Sink By Chemical Vapor Deposition; And Compressing Nanotube Onto Metallic Surface To Increase Contact Surface Area Between Nanotube/Surface, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
Probe Translational And Rotational Diffusion In Polymers Near Tg: Role Of Probe Size, Shape, And Secondary Bonding In Deviations From Debye-Stokes-Einstein Scaling, Ali Dhinojwala
Ali Dhinojwala
Rotational and translational dynamics of a variety of probes are compared in several polymers near the glass transition temperature, Tg. Second harmonic generation was used to measure the rotational relaxation distribution and average rotational relaxation time, 〈τ〉, and fluorescence nonradiative energy transfer was used to measure Dtrans, the translational diffusion coefficient. Dtrans is affected greatly by probe size and shape, typically with a temperature dependence in the rubbery state near Tg which violates Debye–Stokes–Einstein (DSE) scaling and an apparent enhancement in translational diffusion; in contrast, 〈τ〉 is largely unaffected by probe size and shape (for the ranges studied), as long …
Humidity Responsive Material Composition For Fibrous Body Or System, And Biomimetic Fiber Product, Comprises Aqueous Solution Of Silk Protein Formed To Have Predetermined Contraction/Relaxation Characteristics, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
Relaxation Time Of Confined Water, Ali Dhinojwala
Relaxation Time Of Confined Water, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
Generalized Effects In Confined Fluids: New Friction Map For Boundary Lubrication, Ali Dhinojwala
Generalized Effects In Confined Fluids: New Friction Map For Boundary Lubrication, Ali Dhinojwala
Ali Dhinojwala
Recent advances in measuring the rheology and tribology of thin liquid films between shearing surfaces have enabled previously-inaccessible parameters to be measured accurately during frictional sliding. These include the real area of contact, the local asperity load and pressure, and the sheared film thickness. The results show striking non-continuum, non-bulk like effects when the thicknesses of sheared films approach molecular dimensions as occurs under most tribological conditions. Based on these new results, we assess the validity of current presentations of friction processes, such as the Stribeck curve, and propose new constitutive relations and a dynamic friction map, including an alternative …
Molecular Structure Of Hydrophobic Alkyl Side Chains At Comb Polymer-Air Interface, Keshav Gautam, Ali Dhinojwala
Molecular Structure Of Hydrophobic Alkyl Side Chains At Comb Polymer-Air Interface, Keshav Gautam, Ali Dhinojwala
Ali Dhinojwala
Surface sensitive sum-frequency generation (SFG) spectroscopic technique has been used to study the orientation of alkyl side chains for comb polymers at polymer-air interface. The SFG spectra at 24oC for thick spin-coated polymer films and adsorbed layer (on mica substrate) show strong signals associated with methyl asymmetric and symmetric vibrations indicating ordered methyl terminal groups. However, the ratio of methylene to methyl stretch vibrations is higher for the adsorbed layer than that observed for spin-coated films. This spectroscopic evidence of poor packing of alkyl chains with enhanced gauche defects correlates well with low water contact angles measured for the adsorbed …
Apparent Hydrodynamic Thickness Of Densely Grafted Polymer Layers In A Theta Solvent, Ali Dhinojwala
Apparent Hydrodynamic Thickness Of Densely Grafted Polymer Layers In A Theta Solvent, Ali Dhinojwala
Ali Dhinojwala
We study the drainage of a near-theta solvent through densely grafted polymer layers and compare to recent notions that these layers display little permeabil- ity to solvent flow at surface separations less than a ''hydrodynamic thickness.'' The solvent is trans-decalin (a near-theta solvent at the experimental temperature of 247C). The polymer is polystyrene (PS) end-attached to two opposed mica surfaces via the selective adsorption of the polyvinylpyridine (PVP) block of a PS-PVP diblock copoly- mer. The experimental probe was a surface forces apparatus modified to apply small- amplitude oscillatory displacements in the normal direction. Out-of-phase responses reflected viscous flow of …
Shear-Induced Dilation Of Confined Liquid Films, Ali Dhinojwala, Sung Chul Bae, Steve Granick
Shear-Induced Dilation Of Confined Liquid Films, Ali Dhinojwala, Sung Chul Bae, Steve Granick
Ali Dhinojwala
We demonstrate that sheared molecularly-thin fluid films dilate at the point of stick-to-slip (which is the transition from static to kinetic friction), indicating that density decreases when sliding occurs. This contrasts with incompressibility characteristic of bulk fluids when they are deformed. The magnitude of dilation was less than the size of the molecule and was larger in a polymer system (large molecules) than for small-molecule fluids. The experiments employed a surface forces apparatus modified to measure, using piezoelectric methods, sub-angstrom variations of film thickness during dynamic shear excitations that were performed at rates too rapid to allow fluid to enter …
Surface Forces In The Tapping Mode: Rh Of Adsorbed Polymer Brushes, Ali Dhinojwala
Surface Forces In The Tapping Mode: Rh Of Adsorbed Polymer Brushes, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
How Polyelectrolyte Adsorption Depends On History: A Combined Fourier Transform Infrared Spectroscopy In Attenuated Total Reflection And Surface Forces Study, Ali Dhinojwala
Ali Dhinojwala
We present a systematic study of how adsorption history affects the thickness, surface forces, and interfacial rheology of a model cationic polymer. The polymer was quaternized poly-4-vinylpyridine, QPVP (weight-average degree of polymerization nw = 325 and 98% quaternized with ethyl bromide). The main comparisons concerned one-step adsorption from solution at a variable salt concentration up to 0.5 M NaCl, versus two-step adsorption (initial adsorption from buffer solution without added salt, then NaCl added later). The aqueous solutions were buffered at pH = 9.2 such that the surfaces (mica in the case of surfaces forces (SFA) experiments, oxidized silicon in the …
Direct Measurement Of Fluid Mechanical Properties In Micro Space; Liquid Crystal Orientation Under Shear, Ali Dhinojwala
Direct Measurement Of Fluid Mechanical Properties In Micro Space; Liquid Crystal Orientation Under Shear, Ali Dhinojwala
Ali Dhinojwala
The shear-induced molecular orientation of a nematic liquidcrystal, 5CB (4-pentyl-4'-cyanobiphenyl) was studied in anewly-developed apparatus in which the liquid crystal was confinedbetween optical windows at a film thickness that is continuouslyvariable. The molecular orientation was measured in situduring shear in the presence and absence of DC electrical bias betweenthe two surfaces. In this preliminary study we present findings for aspacing of 10 µm, which is comparable to the spacing ofactual liquid crystal display technology. Fourier transform infraredtime resolved spectroscopy (FTIR-TRS) synchronized with the shearmotion provided the molecular orientation as a function of time duringthe shear cycle. Only a slight orientation …
Optical Storage Medium For Audio, Compact And Digital Versatile Disks, Has Substrate Comprising Polycarbonate Containing Units Derived From Spiro(Bis)Indene, Resorcinol Carbonate And Bisphenol Carbonate, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
Carbon Nanotube Composite Material, Useful E.G. In Dry Adhesives And Self-Cleaning Applications, Comprises A Substrate, An Adhesive Coating On The Substrate And Many Carbon Nanostructures Formed Into A Predetermined Architecture, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
Relaxation Dynamics Of Rubbed Polystyrene Films, Ali Dhinojwala
Relaxation Dynamics Of Rubbed Polystyrene Films, Ali Dhinojwala
Ali Dhinojwala
Optical retardation measurements were used to probe the chain relaxation dynamics in rubbed polystyrene films of varying thicknesses on glass substrates. A model based on Kohlrausch-Williams-Watts relaxation was developed and used to determine the glass transition temperature (Tg) of the films. Results showed reductions of 15–20 K in Tg for thin films of thicknesses comparable to the radius of gyration as well as for cast films rubbed with different strengths. These results provide evidence of a faster relaxation dynamics relative to the polymer-substrate interface for thinner films and enhanced chain mobility at the polymer-air interface.
Thermodynamic Study Of A Novel Surface Ordered Phase Above The Bulk Melting Temperature In Alkyl Side Chain Comb Polymers, Ali Dhinojwala
Thermodynamic Study Of A Novel Surface Ordered Phase Above The Bulk Melting Temperature In Alkyl Side Chain Comb Polymers, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
Manufacturing Method For Fabricating Cathodic Portion Of Field Emission Display Involves Embedding Carbon Nanotubes In Polymer Matrix That Extends To Plane Of Attachment Of Nanotubes To Planar Substrate, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
Micron-Gap Rheo-Optics With Parallel Plates, Ali Dhinojwala
Micron-Gap Rheo-Optics With Parallel Plates, Ali Dhinojwala
Ali Dhinojwala
Solid planar plates with area up to several square centimeters can be made parallel at controllable separations from ca. 0.1 (if airborne dust is eliminated) to >500 μm. Apart from dust and surface roughness, which set the lower bound of plate-plate separation, there is no other fundamental constraint on the type of surface (metallic or dielectric; opaque or translucent) that can be studied. When conducting plates are employed, it is possible to apply an electric field in the direction normal to the plates and observe the competition between shear fields and electric fields in orthogonal directions. The large surface area …
Viscoelastic Solid Explains Spider-Web Stickiness, Vasav Sahni, Todd Blackledge, Ali Dhinojwala
Viscoelastic Solid Explains Spider-Web Stickiness, Vasav Sahni, Todd Blackledge, Ali Dhinojwala
Ali Dhinojwala
Modern orb-weaving spiders have evolved well-designed adhesives to capture preys. This adhesive is laid on a pair of axial silk fibres as micron-sized glue droplets that are composed of an aqueous coat of salts surrounding nodules made of glycoproteins. In this study, we measure the adhesive forces required to separate a small microscopic probe after bringing it in contact with a single glue droplet. These forces are highly rate-dependent and are two orders of magnitude higher than the capillary forces. The glycoproteins in the glue droplets behave as a viscoelastic solid and the elasticity is critical in enhancing adhesion caused …
A Review On Spider Silk Adhesion, Ali Dhinojwala
A Review On Spider Silk Adhesion, Ali Dhinojwala
Ali Dhinojwala
Spiders employ clever behavioral strategies combined with almost invisible custom-made adhesive silk fibers to spin prey capture webs. The adhesives used in these webs evolved over millions of years into a class of natural materials with outstanding properties. Here, we review how spiders use different adhesives to capture prey. We show how spiders take advantage of the elasticity of both the capture silk and the glue to enhance adhesive forces, thereby providing important insights in designing new synthetic adhesives.