Designing new thermoreversible gels by molecular tailoring of hydrophilic-hydrophobic interactions

We have shown that the lattice fluid hydrogen bond (LFHB) model can successfully quantify the first-order volume transition in hydrogels. The model predicts that a critical balance of hydrophilic and hydrophobic interactions is required for a gel to exhibit a discontinuous volume transition. In this work we will report the swelling behavior of a new thermoreversible copolymer hydrogel, which has been synthesized from two monomers, whose homopolymers do not show any volume transition in water in the observable range of temperatures. The discontinuous volume transition phenomenon in the copolymer gel was observed only at a critical balance of hydrophilic-hydrophobic interactions. The discontinuous nature of the volume transition is lost with a subtle change in the hydrophilic-hydrophobic balance. The copolymer gel was synthesized from 2-acrylamido 2-methyl propane sulfonic acid (AMPS), which is a hydrophilic monomer, and N-tertiary butylacrylamide (N-t-BAm), which is a hydrophobic monomer. The hydrophilic-hydrophobic balance in the gel was altered by either changing the composition of the co-monomers or by substituting the N-t-BAm with another hydrophobic monomer, N-isopropylacrylamide (NIPAm)

Deformation induced hydrophobicity: implications in spider silk formation

A theoretical framework, which considers the effect of strong inter-polymer associations on phase separation of a deforming polyacrylamide solution is presented. It is shown that deformation induces effective hydrophobicity in the stretched polymer chains resulting in the formation of strong cooperative hydrogen bonding between the polymer chains. This finding has implications in providing insights into the mechanisms of the way spiders spin silk by rapidly deforming a freshly secreted protein solution. It has been argued that in a manner analogous to the model system showed here, it is likely that a rapid deformation induces phase separation of the solution into a polymer rich phase, which eventually forms the fiber. This work also naturally provides strategic hints for making silk like strong fibers by synthetic means at mild conditions

A unified wall slip model

A unified slip model is developed, which predicts wall slip by either a disentanglement mechanism or by debonding mechanism, depending upon the adhesive energy of the wall-polymer pair. The model is based on the transient network theory, in which the activation processes of adsorption and desorption are considered to occur at the wall in parallel to the stretching of the adsorbed chains. It is shown that the stick-slip transition occurs due to the local non-monotonic flow behavior near the wall irrespective of the mechanism of slip. The model predictions of the critical wall shear stress are in good agreement with experimentally observed values of the critical stress for various adhesive energies of wall polymer pair. Another important prediction of the model is that the temperature dependence of the critical wall shear stress for debonding is different than that of disentanglement mechanism under certain experimental conditions. This may be useful for discerning the correct mechanism of slip. The unified model encompasses different systems (viz. entangled solutions and melts) and diverse mechanisms (viz. disentanglement and debonding) in a common mathematical framework

Slipping fluids: a unified transient network model

Wall slip in polymer solutions and melts play an important role in fluid flow, heat transfer and mass transfer near solid boundaries. Several different physical mechanisms have been suggested for wall slip in entangled systems. We look at the wall slip phenomenon from the point of view of a transient network model, which is suitable for describing both, entangled solutions and melts. We propose a model, which brings about unification of different mechanisms for slip. We assume that the surface is of very high energy and the dynamics of chain entanglement and disentanglement at the wall is different from those in the bulk. We show that severe disentanglement in the annular wall region of one radius of gyration thickness can give rise to non-monotonic flow curve locally in that region. By proposing suitable functions for the chain dynamics so as to capture the right physics, we show that the model can predict all features of wall slip, such as flow enhancement, diameter-dependent flow curves, discontinuous increase in flow rate at a critical stress, hysteresis in flow curves, the possibility of pressure oscillations in extrusion and a second critical wall shear stress at which another jump in flow rate can occur

Towards the prediction of supersonic jet noise predictions using a unified asymptotic approximation for the adjoint vector Green's function

In this paper we continue efforts aimed at modeling jet noise using self-consistent analytical approaches within the generalized acoustic analogy (GAA) formulation. The GAA equations show that the far-field pressure fluctuation is given by a convolution product between a propagator tensor that depends on the (true) non-parallel jet mean flow and a generalized fluctuating stress tensor that is a stationary random function of time and includes the usual fluctuating Reynolds’ stress tensor as well as enthalpy fluctuation components. Here, we focus on approximating the propagator tensor by determining an appropriate asymptotic solution to the adjoint vector Green’s function that it depends on by using an asymptotic approach at all frequencies of interest for jet noise prediction. The Green’s function is then rationally approximated by a composite formula in which the GSA (Goldstein-Sescu-Afsar, J. Fluid Mech., vol. 695, pp. 199-234, 2012) non-parallel flow Green’s function asymptotic solution is used at low frequencies and the O(1) frequency parallel flow Green’s function is used for all frequencies thereafter. The former solution uses the fact that non-parallelism will have a leading order effect on the Green’s function everywhere in the jet under a distinguished scaling in which the jet spread rate is of the same order as the Strouhal number for a slowly-diverging mean flow expansion. Since this solution, however, is expected to apply up to the peak frequency, the latter O(1) frequency Green’s function in a parallel flow must be used at frequencies thereafter. We investigate the predictive capability of the composite Green’s function for the prediction of supersonic axi-symmetric round jets at fixed jet Mach number of 1.5 and two different temperature ratios (isothermal & heated) using Large-eddy simulation data. Our results show that, in the first instance, excellent jet noise predictions are obtained using the non-parallel flow asymptotic approach, remarkably, up to a Strouhal number of 0.5. This is true for both heated and un-heated jets. Furthermore, we develop the analytical approach required to extend this solution by appropriate asymptotic approximation to O(1) frequencies

Role of pressure diffusion in non-homogeneous shear flows

A non-local model is presented for approximating the pressure diffusion in calculations of turbulent free shear and boundary layer flows. It is based on the solution of an elliptic relaxation equation which enables local diffusion sources to be distributed over lengths of the order of the integral scale. The pressure diffusion model was implemented in a boundary layer code within the framework of turbulence models based on both the kappa-epsilon-(bar)upsilon(exp 2) system of equations and the full Reynolds stress equations. Model computations were performed for mixing layers and boundary layer flows. In each case, the pressure diffusion model enabled the well-known free-stream edge singularity problem to be eliminated. There was little effect on near-wall properties. Computed results agreed very well with experimental and DNS data for the mean flow velocity, the turbulent kinetic energy, and the skin-friction coefficient

Modeling intermittent wavepackets and their radiated sound in a turbulent jet

We use data from a new, carefully validated, Large Eddy Simulation (LES) to investigate and model subsonic, turbulent, jet noise. Motivated by the observation that sound-source dynamics are dominated by instability waves (wavepackets), we examine mechanisms by which their intermittency can amplify their noise radiation. Two scenarios, both involving wavepacket evolution on time-dependent base flows, are investigated. In the first, we consider that the main effect of the changing base flow consists in different wavepacket ensembles seeing different steady mean fields, and having, accordingly, different acoustic efficiencies. In the second, the details of the base-flow time dependence also play a role in wavepacket sound production. Both short-time-averaged and slowly varying base flows are extracted from the LES data and used in conjunction with linearized wavepacket models, namely, the Parabolized Stability Equations (PSE), the One-Way Euler Equations (OWE), and the Linearized Euler Equations (LEE). All results support the hypothesized mechanism: wavepackets on time-varying base flows produce sound radiation that is enhanced by as much as 20dB in comparison to their long-time-averaged counterparts, and ensembles of wavepackets based on short-time-averaged base flows display similar amplification. This is not, however, sufficient to explain the sound levels observed in the LES and experiments. Further work is therefore necessary to incorporate two additional factors in the linear models, body forcing by turbulence and realistic inflow forcing, both of which have been identified as potentially important in producing the observed radiation efficiency

Molecular tailoring of thermoreversible copolymer gels: some new mechanistic insights

We earlier reported the role of hydrophobic and hydrogen bonding interactions on the transition temperatures of thermoreversible copolymer gels. We show here that the chemical structure of the hydrophobe and its concentration determine the transition temperatures [lower critical solution temperature (LCST)] and the heat of transition of new hydrophobically modified poly(N-isopropyl acrylamide) [PNIPAm] copolymer gels. The gels, prepared by copolymerizing NIPAm monomer with hydrophobic comonomers containing increasing lengths of alkyl side groups and a terminal carboxyl acid group, showed lower LCST and lower heat of transition when compared to pure PNIPAm gel. The experimental results were also compared with theoretical calculations based on a lattice-fluid-hydrogen-bond [LFHB] model. We show experimentally and theoretically that a linear correlation exists between the transition temperature and length of the hydrophobic alkyl side group. Also, in apparent contradiction to previous work, we found a reduction in the heat of transition with increasing hydrophobicity. We propose that the presence of the terminal carboxyl acid group on the hydrophobic side chain of the comonomer prevents the association of water molecules around the hydrophobe, thereby causing a reduction in the heat of transition. The LFHB model supports this argument