Kamlesh J. Suthar

Role of solvation dynamics and local ordering of water in inducing conformational transitions in poly(N-isopropylacrylamide) oligomers through the LCST.

Conformational transitions in thermo-sensitive polymers are critical in determining their functional properties. The atomistic origin of polymer collapse at the lower critical solution temperature (LCST) remains a fundamental and challenging problem in polymer science. Here, molecular dynamics simulations are used to establish the role of solvation dynamics and local ordering of water in inducing conformational transitions in isotactic-rich poly(N-isopropylacrylamide) (PNIPAM) oligomers when the temperature is changed through the LCST. Simulated atomic trajectories are used to identify stable conformations of the water-molecule network in the vicinity of polymer segments, as a function of the polymer chain length. The dynamics of the conformational evolution of the polymer chain within its surrounding water molecules is evaluated using various structural and dynamical correlation functions. Around the polymer, water forms cage-like structures with hydrogen bonds. Such structures form at temperatures both below and above the LCST. The structures formed at temperatures above LCST, however, are significantly different from those formed below LCST. Short oligomers consisting of 3, 5, and 10 monomer units (3-, 5-, and 10-mer), are characterized by significantly higher hydration level (water per monomer ~ 16). Increasing the temperature from 278 to 310 K does not perturb the structure of water around the short oligomers. In the case of 3-, 5-, and 10-mer, a distinct coil-to-globule transition was not observed when the temperature was raised from 278 to 310 K. For a PNIPAM polymer chain consisting of 30 monomeric units (30-mer), however, there exist significantly different conformations corresponding to two distinct temperature regimes. Below LCST, the water molecules in the first hydration layer (~12) around hydrophilic groups arrange themselves in a specific ordered manner by forming a hydrogen-bonded network with the polymer, resulting in a solvated polymer acting as hydrophilic. Above LCST, this arrangement of water is no longer stable, and the hydrophobic interactions become dominant, which contributes to the collapse of the polymer. Thus, this study provides atomic-scale insights into the role of solvation dynamics in inducing coil-to-globule phase transitions through the LCST for thermo-sensitive polymers like PNIPAM.

Fabrication of Poly(ethylene glycol) Hydrogel Structures for Pharmaceutical Applications using Electron beam and Optical Lithography

Soft-polymer based microparticles are currently being applied in many biomedical applications, ranging from bioimaging and bioassays to drug delivery carriers. As one class of soft-polymers, hydrogels are materials, which can be used for delivering drug cargoes and can be fabricated in controlled sizes. Among the various hydrogel-forming polymers, poly(ethylene glycol) (PEG) based hydrogel systems are widely used due to their negligible toxicity and limited immunogenic recognition. Physical and chemical properties of particles (i.e., particle size, shape, surface charge, and hydrophobicity) are known to play an important role in cell-particle recognition and response. To understand the role of physicochemical properties of PEG-based hydrogel structures on cells, it is important to have geometrically precise and uniform hydrogel structures. To fabricate geometrically uniform structures, we have employed electron beam lithography (EBL) and ultra-violet optical lithography (UVL) using PEG or PEG diacrylate polymers. These hydrogel structures have been characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), optical microscopy, and attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) confirming control of chemistry, size, and shape.

A multi-length-scale USAXS/SAXS facility: 10–50 keV small-angle X-ray scattering instrument

Coupling small-angle X-ray scattering (SAXS) and ultra-small-angle X-ray scattering (USAXS) provides a powerful system of techniques for determining the structural organization of nanostructured materials that exhibit a wide range of characteristic length scales. A new facility that combines high-energy (HE) SAXS and USAXS has been developed at the Advanced Photon Source (APS). The application of X-rays across a range of energies, from 10 to 50 keV, offers opportunities to probe structural behavior at the nano- and microscale. An X-ray setup that can characterize both soft matter or hard matter and high-Z samples in the solid or solution forms is described. Recent upgrades to the Sector 15ID beamline allow an extension of the X-ray energy range and improved beam intensity. The function and performance of the dedicated USAXS/HE-SAXS ChemMatCARS-APS facility is described.

Levitating water droplets formed by mist particles in an acoustic field

Understanding the physics behind levitation and the flow field around suspended liquid droplets is key to enhancing the drying process of pharmaceuticals and food products. Here an acoustic levitator has been successfully integrated at the Advanced Photon Source for In-situ high-energy x-ray diffraction measurements on particles suspended in an acoustic field. It is demonstrated that acoustic levitation can be utilized to mimic the spray drying amorphization process under controlled conditions. Investigating the velocity field around levitating droplets is also important to understand the forces acting upon the droplets during the levitation process. This paper presents experimental results of the flow field in an acoustic field using Particle Imaging Velocimetry and high-speed imaging and using 3D finite element analysis. The finite element analysis was employed to evaluate the required experimental conditions. The finite element results of acoustically levitated droplets from an ultrasonic wave operating at 22 KHz are compared and discussed. The finite element simulation results are in good agreement with the experimental observations.

Liquid heating can cause denaturation of sensing layer in SAW biosensors

The acoustic streaming phenomenon, i.e., fluid motion induced from high intensity sound waves, can be used effectively to remove nonspecifically bound proteins to allow reuse of SAW biosensors. While the streaming effect is clearly beneficial, this longitudinal irradiation into the fluid medium can also be accompanied by a corresponding temperature rise of the fluid near the SAW interface due to viscous dissipation, which can lead to denaturation of biosensing layer. Finite element solution of coupled wave propagation (piezoelectric domain) and Navier-Stokes equation (fluid domain) in conjunction with the energy balance equations were used to model the temperature rise of a liquid loading on top of a SAW device. Based on the temperature profiles, we identify the conditions that preserve the activity of the sensing layer as well as those leading to significant heating of the fluid domain. Our computational study highlights the fact that such increase in temperatures of the interfacial liquid layer can have significant implications for designing reusable and highly sensitive biosensors.

Simulation of the effect of different parameters on the swelling characteristics of a pH-sensitive hydrogel

Hydrogels are 3-D network polymeric materials that exhibit a large volume phase-transition due to a of change in their environment so that the response causes the hydrogel to swell or shrink. Since hydrogels have been found to be useful for chemical sensing and delivery, there is a growing interest in their use for medicine. This ,requires a thorough understanding of the hydrogels characteristic response to pH. The hydrogel response can be explained by various physical equations which are often challenging to solve. We discuss the simulation of such phase-transitions in steady-state conditions emphasizing the response to solvent pH and other environmental stimuli. We demonstrate a method for simulating pH response of hydrogels and describe numerical model and its implementation in detail. Though a few models have been developed for simulation of these hydrogel characteristics, these have been based on custom programs implemented in individual laboratories and often not generally accessible. Hence, our modeling effort is implemented using the generic finite element software COMSOL and the method can be used with any software having similar capabilities. The effect of buffer solution concentration, fixed charge density, the solution pH on the swelling characteristics are studied. Results are compared with published experimental data.

Finite Element Analysis of a Photon Absorber Based on Volumetric Absorption of the Photon Beam

A photon absorber is a device used to protect vacuum chambers and other hardware from synchrotron radiation by intercepting unwanted radiation, converting its energy to heat, and then removing that heat. Design of a photon absorber requires careful consideration of the high temperatures and steep thermal gradients that are possible with highly localized absorption of these photons. For next-generation machines, like the multi-bend achromat envisioned for the APS Upgrade (APS-U), a designer is likely to be simultaneously faced with more intense synchrotron radiation beams, more limited available space, and closer proximity to the particle beam than have been designed for in the past. Volumetric absorption of synchrotron radiation, which makes use of materials that are semi-transparent to x-rays, is an attractive option which gives the designer greater freedom by spreading the heat load due to the absorbed photons. Volumetric absorption may also help to reduce residual gas pressures in a vacuum system by reducing the photon-stimulated desorption that results from photons reflected by the incident absorber surface. This paper describes simulations which were performed to evaluate the benefit of utilizing volumetric absorption for a conceptual “crotch” photon absorber for the APS Upgrade (APSU), so-called because of its location in the vacuum system where the chamber is forked to permit x-ray extraction. Results of these simulations show clear benefits of volumetric photon absorption and suggest that such an approach may help to substantially relax constraints on the size, shape, and materials used for such absorbers. VOLUMETRIC ABSORPTION Two crotch absorbers are planned for each APS-U storage ring sector. These are expected to intercept roughly 13 kW each with power densities, as would be intercepted on a normally-intercepting surface, that approach 100 watts/mm2. Conventionally, photon absorbers are made using high opacity and high thermal conductivity materials such as copper and GlidCopTM which cause the heat load to be concentrated on the absorber surfaces. A designer typically struggles with reducing the incident angle of those surfaces and increasing the efficiency of cooling to those surfaces as much as possible to manage the thermal stresses that result. In addition, the high reflectivity of these materials to x-rays, coupled with the grazing angles, causes scattering of a considerable fraction of incident photons which can then be a significant driver of photon-stimulated gas desorption. Finally, the high water flow rates required to maintain surface temperatures at safe levels can introduce vibration that may upset critical, precisely-aligned hardware in the accelerator. A potential solution is the use of an absorber which is semi-transparent to incident x-rays. Such an arrangement allows the heat load into the absorber to be gradually deposited in the body of the component, reducing the associated thermal stresses. In doing so, a designer can find a solution which is more compact than would otherwise be possible. Another approach is to combine an opaque absorbing body with a more transparent, but thermally conductive, material. Doing so similarly limits the resulting temperature rise of absorber materials by allowing the heat generated on the opaque body to be more effectively conducted away. Such approaches have three distinct advantages over a conventional, surface-absorbing design: 1. The material temperatures and the thermal gradients may be reduced, reducing potentially-damaging thermal stresses, fatigue, and structural softening. 2. Reflection of photons may be reduced, thus reducing the outgassing associated with photon-stimulated desorption. 3. Heat transfer may be more efficient due to overall greater proximity of cooling to the heat load, relaxing water flow requirements. Figure 1: Conceptual APS-U crotch absorber options (a) copper body design (b) beryllium and copper design. * Corresponding author’s email: suthar@anl.gov Figure 2: Power profile distribution (a) SynRad trajectories and power profile for B crotch absorber for APS-U project (b) power distribution and profile.

A Discussion on Utilization of Heat Pipes and Vapour Chamber Technology as a Primary Device for Heat Extraction from Photon Absorber Surfaces

* Corresponding author: Suthar@anl.gov, +1(630) 252-4256 Abstract Heat pipes and vapour chambers work on heat exchange phenomena of two-phase flow and are widely used for industrial and commercial applications. These devices offer very high effective thermal conductivities (5,000-200,000 W/m/K) and are adaptable to various sizes, shapes, and orientations. Although they have been found to be an excellent thermal management solution for laptops, satellites, and many things in-between, heat pipes and vapour chambers have yet to be adopted for use at particle accelerator facilities where they offer the possibility of more compact and more efficient means to remove heat from unwanted synchrotron radiation. As with all technologies, there are inherent limitations. Foremost, they are limited by practicality to serve as local heat transfer devices; heat transfer over long distances is likely best provided by other means. Heat pipes also introduce unique failure modes which must be considered.

Comparison of Newtonian and non-Newtonian fluid dynamics on removal efficiency of non-specifically bound proteins in SAW biosensors

Surface acoustic wave (SAW) devices are finding increasing use in medical diagnostic applications, such as detection of specific proteins in bodily fluids for detection of pathologies. In applications aimed at biological sensing, the sensing medium such as blood exhibits a Non-Newtonian behavior. We have recently shown that SAW induced acoustic streaming which refers to fluid motion induced by high frequency sound waves, is an important phenomenon that can be used for the removal of non-specifically bound (NSB) proteins from the device surface1. The removal efficiencies of NSB can be significantly different when the device interacts with Non-Newtonian fluids. This work reports on the influence of non-Newtonian fluid dynamics on the acoustic streaming and fluid velocity profiles in SAW devices, using a computational fluid-structure interaction finite element model. A two-port SAW device, based on 100 MHz YZ Lithium Niobate substrate, in contact with a fluid film was modeled using a three-dimensional bi-directionally coupled fluid-structure interaction model. Blood is modeled as a Non-Newtonian fluid whose viscosity is defined using the Carreau model. To elucidate the effect of non-Newtonian dynamics on acoustic streaming, results are compared with Newtonian fluid with viscosity at infinite shear rate. A transient analysis of the fluid flow profiles on the SAW device indicates significant differences between fluid velocity patterns, magnitudes of fluid velocities, and wall shear stresses for Non-Newtonian fluid loading on the device when compared to a Newtonian fluid. Our results indicate that the peak fluid velocities decreased for non-Newtonian fluid loading suggesting a significant viscous dissipation of energy as compared to the case of a Newtonian fluid. The extent of induced shear stresses at the piezoelectric device-fluid interface is almost two orders of magnitude higher for Non-Newtonian fluids. These results have implications in biosensing as well as micro-fluidic applications involving Non-Newtonian fluids.

Investigation of delay path modifications of Surface Acoustic Wave sensors

Surface Acoustic Wave (SAW) sensors monitor the interaction between a receptor and its target in real time through changes in the properties of the traveling wave (i.e. frequency or phase, and amplitude). A key sensing parameter is power consumption. To increase power transfer we have developed a SAW sensor with microcavities in the delay path on 36° YX LiTaO3 and 90° ST-X Quartz. Three parameters were identified that determine the effectiveness of microcavities: 1) IDT offset, 2) microcavity depth, and 3) microcavity cross-sectional area. Finite element (FE) simulations were performed using ANSYS to determine the optimum value of each parameter to decrease insertion loss (I.L.). Our results show that the best parameter values are; IDT offset = 10 μm, microcavity depth = 2.5 μm and microcavity cross-sectional area = 10 μm × 10 μm, for a device with center frequency of about 100 MHz.