Chemistry

This article describes the use of embossing and "cutand-stack" methods of assembly, to generate microfluidic devices from omniphobic paper and demonstrates that fluid flowing through these devices behaves similarly to fluid in an open-channel microfluidic device. The porosity of the paper to gases allows processes not possible in devices made using PDMS or other nonporous materials. Droplet generators and phase separators, for example, could be made by embossing "T"-shaped channels on paper. Vertical stacking of embossed or cut layers of omniphobic paper generated threedimensional systems of microchannels. The gas permeability of the paper allowed fluid in the microchannel to contact and exchange with environmental or directed gases. An aqueous stream of water containing a pH indicator, as one demonstration, changed color upon exposure to air containing HCl or NH 3 gases.

Soft robots have emerged as a new set of machines capable of manipulation and locomotion. [6] Pneumatic expansion of a network of microchannels (pneu-nets) fabricated in organic elastomers, using low-pressure air (< 10 psi; 0.7 atm; 71 kPa), provides a simple method of achieving complex movements: grasping and walking. Despite their advantages (simplicity of fabrication, actuation, and control; low cost; light weight), pneu-nets have the disadvantage that actuation using them is slow, in part because the viscosity of air limits the rate at which the gas can be delivered through tubes to fill and expand the microchannels. Herein, we demonstrate the rapid actuation of pneu-nets using a chemical reaction (the combustion of methane) to generate explosive bursts of pressure.

Analysis of rates of tunneling across self-assembled monolayers (SAMs) 8 of n-alkanethiolates SC n (with n = number of carbon atoms) incorporated in junctions 9 having structure Ag TS -SAM//Ga 2 O 3 /EGaIn leads to a value for the injection tunnel 10 current density J 0 (i.e., the current flowing through an ideal junction with n = 0) of 11 10 3.6±0.3 A·cm −2 (V = +0.5 V). This estimation of J 0 does not involve an extrapolation 12 in length, because it was possible to measure current densities across SAMs over the 13 range of lengths n = 1−18. This value of J 0 is estimated under the assumption that 14 values of the geometrical contact area equal the values of the effective electrical contact 15 area. Detailed experimental analysis, however, indicates that the roughness of the 16 Ga 2 O 3 layer, and that of the Ag TS -SAM, determine values of the effective electrical 17 contact area that are ∼10 −4 the corresponding values of the geometrical contact area. 18 Conversion of the values of geometrical contact area into the corresponding values of 19 effective electrical contact area results in J 0 (+0.5 V) = 10 7.6±0.8 A·cm −2 , which is 20 compatible with values reported for junctions using top-electrodes of evaporated Au, 21 and graphene, and also comparable with values of J 0 estimated from tunneling through single molecules. For these junctions, the 22 = × β − J V J V ( ) ( ) 10 d 0 /2.303 33 (1) 34 with the falloff in current density J(V) (A·cm −2 ) with increasing 35 length d of the n-alkyl group giving (for even-numbered carbon 36 chains, and at voltages in the range V = ±0.5 V) approximately 37 the same value of the tunneling decay factor β by most or all 38 methods of measurement (β = 0.73−0.89 Å −1 ; for d = nC = 39 number of carbon atoms, β = 0.90−1.1 nC −1 ). Using mercury 40 drops as top-electrodes, measurements of rates of tunneling 41 across n-alkanes anchored to heavily doped silicon surfaces led 42 to β = 0.9 ± 0.2 nC −1 , similar to the values observed for n-43 alkanethiolates on Au and Ag substrates. 3,4 44 By contrast, values of the injection current J 0 (V = +0.5 V) 45 the limiting value of current for an ideal junction with no 46 hydrocarbon present (d = 0), but with all the interfaces and 47 characteristics of junctions containing the SAMsvary from 48 ∼10 8 A·cm −2 , estimated from single-molecules approaches 5−10 49 and measured for graphene 11 and evaporated gold 12 top-50 electrodes, to ∼10 2 A·cm −2 , observed in large-area junctions 51 using, as top-electrodes, conductive polymers, 13 Hg-drops 52 supporting an insulating organic film (Hg-SAM), 14−16 and 53 Ga 2 O 3 /EGaIn tips. 17−20 Why is there high consistency in values 54 of β, but broad inconsistency in values of J 0 (V) within these 55 systems? 56 A priori, at least four factors might contribute to differences 57 in J 0 (V) among methods of measurements: 58 (i) In large-area junctions, assuming that the effective 59 electrical contact area (A elec )the area through which current 60 actually passescoincides with the geometrical contact area 61 (A geo ) estimated by optical microscopy could result in errors in 62 the conversions of values of current into current densities. 63 Contact between surfaces occurs only through asperities 64 distributed on the surfaces, which are always rough to some 65 extent; in addition, only a fraction of the true, physical contact 66 area is conductive. 21−24 Estimations of the effective contact area 67 from measurements of adhesion and friction between surfaces 68 indicate that values of A elec /A geo vary in the range 10 −2 −10 −4 , 69 depending on the hardness of the materials, the heights, widths, 70 and number of asperities on both surfaces, and loads applied to 71 the contacts. 22,23,25−27

A novel method for batch fabrication of substrates for surface-enhanced Raman scattering (SERS) has been developed. A modified platen that fits in a commercial electron beam evaporator enables the simultaneous deposition of Ag nanorod arrays onto six microscope slides by glancing angle deposition. Following removal of substrates from the evaporator, patterned wells are formed by contact printing of a polymer onto the surface. Well dimensions are defined by penetration of the polymer into the nanorod array and subsequent photochemical curing. Inherent advantages of this method include: (1) simultaneous production of several nanorod array substrates with high structural uniformity, (2) physical isolation of nanorod arrays from one another to minimize cross contamination during sample loading, (3) dimensional compatibility of the patterned array with existing SERS microscope, (4) large SERS enhancement afforded by the nanorod array format, (5) small fluid volumes, and (6) ease of use for manual delivery of fluids to each element in the patterned array. In this article, the well-to-well, slide-to-slide, and batch-to-batch variability in physical characteristics and SERS response of substrates prepared via this method is critically examined.