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Rough surfaces immersed under water remain practically dry if the liquid-solid contact is on roughness peaks, while the roughness valleys are filled with gas. Mechanisms that prevent water from invading the valleys are well studied. However, to remain practically dry under water, additional mechanisms need consideration. This is because trapped gas (e.g. air) in the roughness valleys can dissolve into the water pool, leading to invasion. Additionally, water vapor can also occupy the roughness valleys of immersed surfaces. If water vapor condenses, that too leads to invasion. These effects have not been investigated, and are critically important to maintain surfaces dry under water. In this work, we identify the critical roughness scale, below which it is possible to sustain the vapor phase of water and/or trapped gases in roughness valleys – thus keeping the immersed surface dry. Theoretical predictions are consistent with molecular dynamics simulations and experiments.

≈500 nm thick)-that are believed to lend GO papers their extraordinary ductility and mechanical toughness. [ 8 ] Lamellae consist of hundreds of highly aligned nanosheets, whereas superlamellae consist of tens of aligned lamellae. From the inspection of swollen GO papers lamellae are found to be organized into a highly interconnected, branched, lattice-like structure.In a previous study, we suggested that the formation mechanism of GO papers formed by vacuum-assisted fi ltration progresses in two stages whereby a semiordered aggregate (SOA) of loosely packed nanosheets forms and grows due to the fl ow of water through the fi ltration membrane. This is followed by compaction of the SOA, induced by the lowering air/ liquid interface into the fi nal free-standing paper with a layered, hierarchical structure containing preferentially aligned nanosheets and lamellae (groups of tens of nanosheets). [ 9 ] In a subsequent study, we discovered an additional length scale in the hierarchical structure, whereby groups of lamellae form superlamellae. [ 7a ] However, the evidence to support the two-stage paper-formation mechanism was indirect and did not provide any mechanism for the evolution of lamellae or superlamellae. In this study, we provide direct experimental describing the formation mechanism of GO papers as well as the origin of the hierarchical structure of GO papers.Taking inspiration from a process used for the fabrication of GO sponges, foams, and other structures, [ 10 ] we present a novel method that preserves the paper formation process in sequential snapshots akin to stop-motion animation. Several different methods of fl ash freezing (see Experimental Section and Note S2, Supporting Information) were implemented to rapidly arrest the formation of the paper at various stages. The partially formed material is subsequently cleaved from the resulting ice puck, maintained at a low temperature and then lyophilized to remove water without surface tension driven aggregation or collapse. Samples are then imaged by SEM to observe the interconnected GO nanosheet network. It is important to note that by controlling both the direction and rate of ice crystal formation prior to lyophilization, the structure of porous materials can be modifi ed, a process known as ice templating. To reduce the infl uence of ice templating the samples were frozen as rapidly as possible. In addition, the direction of ice crystal formation with respect to the direction of fi ltration was varied (Note S2, Supporting Information). In each of the cases, the structure obtained in the vicinity of the fi lter membrane was consistent, implying that the overall structure of the nanosheet network is preserved by this method.In order to form a time-elapsed picture of the formation mechanism for GO papers, several samples were taken atThe need for materials with favorable properties that can be used across many scientifi c and technological disciplines has motivated a burgeoning interest in graphene and its derivatives over the last decade. In this p...

Magnetophoretic isolation of biochemical and organic entities in a microfluidic environment is a popular tool for a wide range of bioMEMS applications, including biosensors. An experimental and numerical analysis of magnetophoretic capture of magnetic microspheres in a microfluidic channel under the influence of an external field is investigated. For a given microfluidic geometry, the operating conditions for marginal capture is found to be interrelated in such a manner that a unique critical capture parameter [Pi(crit) = ((Iota(crit)a))(2)/Q(eta)], that is proportional to the ratio of the magnetic force to viscous force, can be identified. Influences of the flow rate, magnetic field and other parameters on the particle trajectories in the microfluidic channel are investigated both numerically and through bright-field imaging under a microscope. Like the event of critical capture, particle trajectories are also found to be guided by a similar parameter, pi. Magnetophoretic capture efficiency of the device is also evaluated as a function of a nondimensional number [Pi(*) = chiP(2)a(2) / (U(null)etah(5)], when both numerical and experimental results are found to agree reasonably well. Results of this investigation can be applied for the selection of the operating parameters and for prediction of device performance of practical microfluidic separators.

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