We present our findings on the changes to electroosmotic flow outside glass nanopores with respect to the choice of Group 1 cation species. In contrast with standard electrokinetic theory, flow reversal was observed for all salts under a negative driving voltage. Moving down Group 1 resulted in weaker flow when the driving voltage was negative, in line with the reduction in the zeta potential on the glass surface going down the periodic table. No trend emerged with a positive driving voltage, however for Cs, flow was uniquely found to be in reverse. These results are explained by the interplay between the flow inside the nanopore and flow along the outer walls in the vicinity of the nanopore.Nanopores are sensors based on the resistive-pulse technique 1 . Sensing is achieved by monitoring the ionic current through a nanoscale aperture in electrolytic solution. Nanopores exist in a variety of forms, the earliest used for sensing being the biological nanopore α-haemolysin 2,3 . Today many solid state nanopore systems are known, primary examples of these being Si 3 N 4 4 , quartz glass 5 and graphene 6 . They have all proven capable of single molecule sensing 7 , detecting proteins 8,9 , DNA sequencing 10 and, in conjunction with DNA nanotechnology, detection of single nucleotide polymorphisms 11 and specific proteins from mixtures 12 .Hydrodynamic and electrokinetic phenomena dictate the behavior of analytes in nanopores. There are many works theoretically and experimentally probing the details of these phenomena with regards to micro-and nanofluidic systems [13][14][15][16] . Here, of prime importance is electroosmosis. Si 3 N 4 and glass nanopores have a negative surface charge in solution at biological pH. This results in a build-up of positive ions proximate to the surface 17 . Applying an electric field to drive an analyte through a nanopore causes the charges at the surface to move. The moving charges couple to the fluid medium and result in electroosmotic flow (EOF). This effect is depicted in Fig. 1(a). The force a target molecule experiences in nanopores thus depends sensitively on the direction and strength of EOF 18,19 ; it may slow the target down in a manner useful for sensing, or it may deny entry to molecules, hampering throughput 18,20,21 . As such, EOF in nanopores has been extensively studied 22-24 , including reports of enhancement of molecular binding within an α-haemolysin nanopore with EOF 25 , facilitated protein capture in Fragaceatoxin C nanopores using EOF 26 , and recently the demonstration that EOF can be used to control the folding state of DNA entering glass nanopores 27 .Applying an electric field through the nanopore not only drives flow from within the pore, it establishes a flow field in the region outside the pore that is several microns a) Email: ufk20@cam.ac.uk in extent. This field can be quantified by a single parameter, P , the force required to generate this field in an otherwise calm fluid. This force originates from an immersed fluid jet which is described by the Landau-Sq...

Luminescent colloidal CdSe nanorings are a new type of semiconductor structure that have attracted interest due to the potential for unique physics arising from their non-trivial toroidal shape. However, the exciton properties and dynamics of these materials with complex topology are not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature-resolved photoluminescence (PL), and single particle measurements to study these materials. We find that on transformation of CdSe nanoplatelets to nanorings, by perforating the center of platelets, the emission lifetime decreases and the emission spectrum broadens due to ensemble variations in the ring size and thickness. The reduced PL quantum yield of nanorings (~10%) compared to platelets (~30%) is attributed to an enhanced coupling between: (i) excitons and CdSe LO-phonons at 200 cm -1 and (ii) negatively charged selenium-rich traps which give nanorings a high surface charge (~-50 mV).Population of these weakly emissive trap sites dominates the emission properties with an increased trap emission at low temperatures relative to excitonic emission. Our results provide a detailed picture of the nature of excitons in nanorings and the influence of phonons and surface charge in explaining the broad shape of the PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that the excitonic properties of nanorings are not solely a consequence of the toroidal shape but are also a result of traps introduced by puncturing the platelet center.

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Understanding (de)lithiation heterogeneities in battery materials is key to ensuring optimal electrochemical performance and developing better energy storage devices. However, this remains challenging due to the complex three dimensional morphology of microscopic electrode particles, the involvement of both solid and liquid phase reactants, and range of relevant timescales (seconds to hours). Here, we overcome this problem and demonstrate the use of bench-top laser scanning confocal microscopy for simultaneous three-dimensional operando measurement of lithium ion dynamics in single particles, and the electrolyte, in batteries. We examine two technologically important cathode materials that are known to suffer from intercalation heterogeneities: LixCoO2 and LixNi0.8Mn0.1Co0.1O2. The single-particle surface-to-core transport velocity of Li-phase fronts, and volume changes – as well as their inter-particle heterogeneity – are captured as a function of C-rate, and benchmarked to previous ensemble measurements. Additionally, we visualise heterogeneities in the bulk and at the surface of particles during cycling, and image the formation of spatially non-uniform concentration gradients within the liquid electrolyte. Importantly, the conditions under which optical imaging can be performed inside absorbing and multiply scattering materials such as battery intercalation compounds are outlined.

Most animal cells are surrounded by a cell membrane and an underlying actomyosin cortex. Both structures are linked with each other, and they are under tension. Membrane tension and cortical tension both influence many cellular processes, including cell migration, division, and endocytosis. However, while actomyosin tension is regulated by substrate stiffness, how membrane tension responds to mechanical substrate properties is currently poorly understood. Here, we probed the effective membrane tension of neurons and fibroblasts cultured on glass and polyacrylamide substrates of varying stiffness using optical tweezers. In contrast to actomyosin-based traction forces, both peak forces and steady state tether forces of cells cultured on hydrogels were independent of substrate stiffness and did not change after blocking myosin II activity using blebbistatin, indicating that tether and traction forces are not directly linked with each other. Peak forces on hydrogels were about twice as high in fibroblasts if compared to neurons, indicating stronger membrane-cortex adhesion in fibroblasts. Finally, tether forces were generally higher in cells cultured on hydrogels compared to cells cultured on glass, which we attribute to substrate-dependent alterations of the actomyosin cortex and an inverse relationship between tension along stress fibres and cortical tension. Our results provide new insights into the complex regulation of membrane tension, and they pave the way for a deeper understanding of biological processes instructed by it.