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The negative charge of the plasma membrane (PM) severely affects the nature of moieties that may enter or leave the cells and controls a large number of ion-interaction-mediated intracellular and extracellular events. In this letter, we report our discovery of a most fascinating scenario, where one interface (e.g., membrane-cytosol interface) of the negatively charged PM shows a positive surface (or ζ) potential, while the other interface (e.g., membrane-electrolyte interface) still shows a negative ζ potential. Therefore, we encounter a completely unexpected situation where an interface (e.g., membrane-cytosol interface) that has a negative surface charge density demonstrates a positive ζ potential. We establish that the attainment of such a property by the membrane can be ascribed to an interplay of the nature of the membrane semi-permeability and the electrostatics of the electric double layer established on either side of the charged membrane. We anticipate that such a membrane property can lead to such capabilities of the cell (in terms of accepting or releasing certain kinds of moieties as well regulating cellular signaling) that was hitherto inconceivable.

We carry out molecular dynamics (MD) simulations to compare the equilibrium architecture and properties of nanoparticle-supported lipid bilayers (NPSLBLs) with the free vesicles of similar dimensions. Three key differences emerge. First, we witness that for a free vesicle, a much larger number of lipid molecules occupy the outer layer as compared to the inner layer; on the other hand, for the NPSLBL the number of lipid molecules occupying the inner and outer layers is identical. Second, we witness that the diffusivities of the lipid molecules occupying both the inner and the outer layers of the free vesicles are identical, whereas for the NPSLBLs the diffusivity of the lipid molecules in the outer layer is more than twice the diffusivity of the lipid molecules in the inner layer. Finally, the NPSLBLs entrap nanoscopic thin water film between the inner lipid layer and the NP and the diffusivity of this water film is nearly 1 order of magnitude smaller than the diffusivity of the bulk water; on the other hand, the water inside the free vesicles has a diffusivity that is only slightly lower than that of the bulk water. Our findings, possibly the first probing the atomistic details of the NPSLBLs, are anticipated to shed light on the properties of this important nanomaterial with applications in a large number of disciplines ranging from drug and gene delivery to characterizing curvature-sensitive molecules.

Enhancing nanoscale liquid flows remains an existing challenge in nanofluidics. Here we propose the generation of highly augmented thermoosmotic (TOS) liquid flows in soft nanochannels (or nanochannels functionalized by grafting with end-charged polyelectrolyte or PE brushes) by employing an axial temperature gradient. The TOS transport is a combination of the induced-electric-field electroosmotic (EOS) transport and a thermo-chemioosmotic (TCOS) transport with the latter resulting from an induced pressure gradient on account of the changes associated with the imposition of the axial temperature gradient. The end-charged brushes develop an electric double layer (EDL) localized at the charged, non-grafted brush end. Depending on the system parameters, this EDL localization massively augments the influence of the EOS body force and the induced pressure-gradient resulting in a TOS transport in soft nanochannels that can be more than one order of magnitude larger than that in brush-free nanochannels. Given the existing notion that the presence of the brushes invariably reduces the flow strength, this result of massive flow augmentation is extremely significant and non-trivial serving as a paradigm shift in the study of liquid transport in brush-grafted nanochannels.

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