Visualizing Evaporation of Femtoliter
Water Capillary Bridges
KUN CHO AND BYUNG MOOK WEON
SOFT MATTER PHYSICS LABORATORY, SCHOOL OF ADVANCED MATERIALS SCIENCE
AND ENGINEERING, SKKU ADVANCED INSTITUTE OF NANOTECHNOLOGY (SAINT)
SUNGKYUNKWAN UNIVERSITY, KOREA
Femtoliter, abbreviated as fL, is a ultrasmall volume of liquid: 1 fL is equal to one quadrillionth liter (10-15 L) or one cubic micrometer (1 μm3) for water. Water with a few hundreds of femtoliters can be trapped between a micrometer-scale colloidal particle and a substrate. Interestingly, the trapped water usually exists as a capillary bridge, which has a catenoidal curvature. For a capillary bridge, direct visualization of evaporation process is required to clearly understand what really happens during evaporation. Recently, clear evidence for evaporation rates of femtoliter water capillary bridges was taken, thanks to ultrafast high-resolution transmission X-ray microscopy. This research will be helpful for scientists and engineers in determining evaporation dynamics of femtoliter water capillary bridges.
Capillary bridges are ubiquitous in microscopic or nanoscopic confinement. Capillary forces exerted by Laplace pressure in capillary bridges play essential roles in the bonding of microscale or nanoscale objects. Water capillary bridges with ultrasmall volumes have attracted much attention because of interesting kinetics, showing either capillarity-induced condensation or evaporation, which impacts many processes such as ink deposition, spray drying, evaporative lithography, and fluid dynamics in colloidal particles[2,3]. Importantly, capillary bridges may have a lower internal pressure than the surrounding atmospheric pressure, particularly for volumes of the order of femtoliters. For instance, negative pressure in liquid capillary bridges can reach -160 MPa for a very small volume around the AFM tip radius (e.g., ~10 nm)[4,5]. The influence of internal pressure lower than 1 atm on the evaporation kinetics of femtoliter capillary bridges remained unclear until quite recently.
To understand the details of the evaporation process for femtoliter water capillary bridges, Cho et al. suggested a feasible experimental method to directly visualize how water bridges evaporate between a microsphere and a flat glass substrate in still air using ultrafast high-resolution transmission X-ray microscopy in the 7C X-ray Nano Imaging beamline established at the Pohang Light Source II (Fig. 1). A water film that was initially thicker than a microsphere diameter gradually evaporated in still air at the early stages and eventually evolved into a catenoidal capillary bridge surrounding a polystyrene microsphere with a radius of ~3 μm on a flat glass substrate in later stages (Fig. 2a). Precise measurements for changes in meniscus profiles and volumes of water capillary bridges provided quantitative information about the evaporation kinetics for femtoliter capillary bridges (Fig. 2b).
Fig. 1: The full-field transmission X-ray microscopy at the 7C X-ray Nano Imaging (XNI) beamline established at the Pohang Light Source II.
Fig. 2: (a) Visualizing evaporation of femtoliter water capillary bridges between a microsphere on a flat solid surface, taken by transmission X-ray microscopy. (b) The time-resolved sequential images of an evaporating water film and capillary bridge during evaporation.
The main finding was as follows. The surface area of the catenoidal water bridge was estimated as ~100 μm2 at ~240 fL and equivalent to the surface area of a hemisphere sessile drop with a radius ~4.0 μm. Despite the different geometry, consideration of the evaporation rate between a bridge and a hemisphere sessile drop with an identical surface area was useful. The measured evaporation rate of 1.5 pL/s at ~240 fL for the water bridge was much smaller than the estimate of 8.9 pL/s for a 4.0-μm-radius hemisphere sessile drop (1 pL = 103 fL). The slow evaporation rate for the catenoidal water bridge was attributed to the lower internal pressure of the bridge. The pressure inside the bridge was estimated as 0.6 atm for ~240 fL, and 1.2 atm for the equivalent-surface-area hemisphere drop. Here, the slow evaporation from the water bridge would be plausible through the suppression of the vapor diffusion from 0.6 to 1.0 atm (to the surrounding atmosphere), compared with the evaporation from 1.2 to 1.0 atm for the hemisphere sessile drop.
Finally, the feasibility of transmission X-ray microscopy was successfully demonstrated for exploring in detail the evaporation kinetics of femtoliter water capillary bridges. X-ray microscopy and precise image analysis provided a high spatial resolution of 46 nm, which was enough for direct visualization of capillary bridge evaporation in still air between a microsphere and a flat solid surface. Detailed information on the evaporation kinetics of capillary bridges in microscopic or nanoscopic confinement will be useful in many natural and industrial situations. At femtoliter scales, capillary bridges are well known to have lower internal pressure than the ambient surrounding pressure. The recent work shows that lower internal pressure than the surrounding pressure can significantly decrease evaporation through the suppression of vapor diffusion. This finding provides an insight for the better understanding of the evaporation of ultrasmall capillary bridges.
 K. Cho, et al. Scientific Reports 6, 22232 (2016).
 N. Maeda, et al. Proc. Natl. Acad. Sci. USA 100, 803-808 (2003).
 M. J. Neeson, et al. Soft Matter 10, 8489-8499 (2014).
 J. W. van Honschoten, et al. Chem. Soc. Rev. 39, 1096-1114 (2009).
 N. R. Tas, et al. Nano Lett. 3, 1537-1540 (2003).
*Corresponding to: Prof. Byung Mook Weon (firstname.lastname@example.org)
*Soft Matter Physics Laboratory: www.softphys.org
Kun Cho is a PhD candidate in the School of Advanced Materials Science and Engineering at Sungkyunkwan University (SKKU). His research interests are associated with applications of X-ray and optical microscopic observations for understanding of small liquid volumes and colloidal particles.
Byung Mook Weon is an assistant professor of the School of Advanced Materials Science and Engineering and SKKU Advanced Institute of Nanotechnology (SAINT) at Sungkyunkwan University (SKKU). He received his PhD (2008) in materials science from Pohang University of Science and Technology (POSTECH), Korea. After his postdoctoral research in the Department of Physics at Harvard University, he joined SKKU in 2013 and then became an editorial board member of Scientific Reports. His research interests include soft matter physics, aging dynamics, and X-ray microscopy.