Strongly Accelerated and Humidity-Independent Drying of Nanochannels Induced by Sharp Corners

Abstract

Measurements are shown indicating that the drying rate of nanochannels can be enhanced by up to 3 orders of magnitude relative to drying by vapor diffusion, and that the drying rate is independent of the relative humidity of the environment up to a relative humidity of more than 90%. Micromachined Pyrex glass nanochannels of 72 nm height and with sharp corners (corner angles 7 degrees) were used. Available theory shows that the sharp corners function as a low-resistance pathway for liquid water, siphoning (wicking) the water to a location close to the channel exit before it evaporates. The described phenomena are of importance for the understanding of drying processes in industry and agriculture. The introduction of sharp corners or grooves can furthermore be beneficial for the functioning of microheat pipes and capillary-pumped loops. DOI: 10.1103/PhysRevLett.95.256107 PACS numbers: 68.03.Fg Understanding the drying mechanism of porous materials is of importance in many industries such as the food, paper, pharmaceutical, and textile industry [1][2][3]. It has previously been observed that microporous media dried approximately 1 order of magnitude faster than can be expected from vapor diffusion alone [4,5]. Flow in liquid water films held on surfaces and flow of water held in corners or grooves were thought to cause this acceleration [6]. The contribution of film flow to drying has been experimentally investigated in cylindrical nanocapillaries, where it caused a tenfold increase of drying rate, [7] but the contribution of corner flow has never been investigated. Here we report on experiments using noncylindrical micromachined nanochannels to quantify corner flow.Drying results from three water transport mechanisms: vapor diffusion, film flow, and corner flow. To specifically investigate corner flow we designed an array of high aspect ratio (widthheight) noncylindrical channels of equal height but different width (Fig. 1). The three water transport mechanisms schematically are shown in Fig. 2. When corner flow dominates the drying process in a channel, the drying rate will depend on the inverse of the channel width, because the number of corners is independent of width but the total water volume inside the channel proportional with width. Drying due to film flow (for widthheight) and vapor diffusion in contrast does not depend on the channel width. Arrays of Pyrex channels, open on two sides, were manufactured in a clean room. Channels were wet etched (hydrofluoric acid) into one Pyrex wafer using a photolithographic mask. This wafer was bonded by thermal fusion to a second wafer which had access holes for filling. The channels were 4 mm long, 72:4 0:8 nm high (determined by AFM) and the width in the array differed from 2 to 30 m. The channel shape was an isosceles trapezoid of very high aspect ratio (width=height > 40) (Fig. 1 bottom). The angle of the sharp corner was determined to be 6:6 0:7 degrees by SEM measurements of bonded chips and by AFM measurements prior to bonding of the Pyrex plates. The shar...

Evaporation-induced cavitation in nanofluidic channels

“…In contrast, bubbles were never observed in nanochannels without the local expansions (where the SF 6 etch step was omitted). Instead, menisci receded from one or both sides of such uniform-height channels, being consistent with the previous studies on negative pressure and evaporation in nanochannels (7,11). These observations suggest that the expansion at the entrance of the nanochannels played a role in remerging of the menisci, resulting in the entrapped bubble that subsequently moved into the nanochannel and expanded.…”

“…4A). In the nanochannels where the cavitation was absent, evaporation occurred faster than the theoretical prediction based on pure vapor diffusion in the nanochannel, which may be attributed to the presence of a water film along the corners of the rectangular cross-sections of these nanochannels (11). However, the total water evaporation length, L, still followed L ∝ ffi ffi t p behavior indicative of a diffusion-governed process.…”

“…In previous experiments on evaporation in microchannels and nanochannels (7,11), it was observed that the menisci recede from the channel entrance until all the water was evaporated. In contrast, we found that the menisci remained pinned at the channel entrance while a small bubble appeared at one end of the channels and rapidly moved into the channel (Fig.…”

Nano-slit electrospray emitters fabricated by a micro- to nanofluidic via technology

“…The nanochannels filled rapidly around 4.4 s, with a visible corner flow (Dong and Chatzis 1995) preceding the bulk fluid. After 6.2 s air bubbles are trapped by fluid appearing from the opposite end and only slowly dissolve until the nanochannels are completely filled at 42 s. The liquid appearing on opposite ends is caused by additional transport mechanisms, possibly vapour diffusion, film flow and corner flow (Eijkel et al 2005). The pinning of the fluid contact line (Ondarçuhu and Piednoir 2005) at the fluidic via connections on the opposite ends of the nanochannels prevents further capillary filling of the connected microchannels.…”

“…A redesign of the nanochannel electrospray emitter was made based on experimental filling results, taking the effect of additional transport mechanisms (Dong and Chatzis 1995;Eijkel et al 2005) and gas-flow restriction on the filling behaviour into consideration. Electrospray chips were batch fabricated containing microchannel probes with integrated nano-slit emitters, where the nanochannel length was designed to be 3 lm (see Fig.…”