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The problem of thermal instability of a Walters' (model B H ) viscoelastic¯uid in a porous medium is considered in the presence of a variable gravity ®eld and rotation. It is found that the principle of exchange of stabilities is valid under certain conditions. For stationary convection, the Walters' (model B H ) elastico-viscous¯uid behaves like a Newtonian¯uid. It is found that rotation has stabilizing effect as gravity increases upward and a destabilizing effect as gravity decreases upward, the medium permeability has stabilizing/destabilizing effects depending on the rotation parameter, gravity is considered to be increasing upward from its value g (i.e. ! b 0). The effects of rotation and the medium permeability on thermal instability have also been shown graphically. The suf®cient conditions for the non-existence of overstability are also obtained.

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Thermal instability in a low Prandtl number nanofluid in a porous medium is investigated by using Galerkin weighted residuals method for free-free boundaries. For porous medium, Brinkman-Darcy modelis applied. The model used for the nanofluid describes the effects of Brownian motion and thermophoresis. Linear stability theory based upon normal mode analysis is employed to find the expression for stationary and oscillatory convection. The effects of Prandtl-number, Darcy number, Lewis number and modified diffusivity ratio on the stationary convection are investigated both analytically and graphically. The results indicated that the Prandtl and Darcy numbers have a destabilizing effect while the Lewis number and modified diffusivity ratio have a stabilizing effect for the stationary convection.

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Highly toxic polyallylamine (PA) was reacted with a varying amount of a novel linker, 6-(N,N,N 0 ,N 0 -tetramethylguanidinium chloride) hexanoic acid (Tmg-HA), to prepare a series of tetramethylguanidinium-PA (Tmg-PA) polymers, which were used as vectors for gene transfection. The extent of attachment of the linker, Tmg-HA, to the PA backbone was determined by 2,4,6-trinitrobenzene sulfonic acid assay. The modified polymers (Tmg-PAs), when complexed with pDNA, exhibited good condensation ability. The nanoparticles, so formed, were characterized by their size and zeta potential and were subsequently evaluated for their toxicity and transfection ability on various mammalian cells, viz., HeLa, CHO, and HEK 293 cells. Mobility shift assay revealed that on increasing the percent substitution of Tmg-HA onto PA (from Tmg-PA1 to Tmg-PA6), relatively higher amounts of modified polymers were required to retard the mobility of a fixed amount of DNA. Besides, Tmg-PA polymers provided sufficient protection (ca. 84-88%) to bound DNA against nucleases and one of the formulations, Tmg-PA2 (ca. 15% substitution) displayed the highest transfection efficiency outcompeting the commercial transfection reagent, Lipofectamine TM with minimal cytotoxicity. More impressively, the transfection efficiency increased despite recording a decrease in the buffering capacity of the grafted polymers suggesting that buffering capacity is not the sole parameter in determining the gene delivery efficiency of a vector system.