Ultrafast/Nanoscale Dynamics by X-Ray Imaging

Ultrafast/nanoscale dynamics by X-ray imaging

J.S. Lee1, B.M. Weon2, S.J. Park3, S.-H. Lee1, K. Fezzaa4, and J.H. Je1*

1Pohang University of Science and Technology,Korea

2Sungkyunkwan University, Korea

3Stanford University, USA

4Advanced Photon Source, ANL, USA

Keywords: X-ray imaging, interface dynamics, soft matter

*e-mail:

Fluid flow is very commonly encountered in our daily lives such as rain drops, ink-jet printing, wetting, etc. Interface dynamics in fluids is important in understanding physical, chemical, and biological processes occurring at the interfaces, but largely unexplored due to lack of appropriate visualizationmethods, in particular, in extreme conditions of ultrafast time domains (~μs) and/or nanoscale regimes.

Here we employphase contrast x-ray imaging to visualize micro/nano and/or ultrafast interface dynamics, especially, in ‘drop impact’ and ‘wetting’.Phase contrast X-ray imaging is really useful for real-timevisualization of versatile systems.[1, 2] First, X-ray imaging is compared with conventional optical imaging and the principle of phase contrast X-ray imaging is explained, compared with absorption X-ray imaging.

The first subject demonstrated here by X-ray imaging is evolution of air film that is entrapped during drop impact on a solid surface. Using ultrafast phase-contrast x-ray imaging, we directly visualize the profile of an entrapped air film and its evolution into a bubble during drop impact. We identify a complicated evolution process that consists of three stages:i) inertial retraction of the air film, ii) contraction of the top air surface into a bubble, and iii) pinch-off of a daughter droplet inside the bubble [3].

A bubble reaching an air-liquid interface usually bursts and forms a liquid jet. Jetting that isone of the sources of aerosol droplets has been studied mostly for large bubbles (R > 100μm), not for small bubbles (R < 100μm) despite the existence of a large number of small bubbles in sea water. Here we reveal that jet formation is inhibited for bubblesthat are smaller than critical sizes [4]. Ultrafast X-ray imaging enables us to build a phase diagram for jetting and the absence of jetting.

A vortex is a flow phenomenon that is very commonly observed in nature. The origin of the vortices and their dynamics are a century-long issue. With ultrafast X-ray phase-contrast imaging, we reveal that the formation of vortex rings originates from the energy transfer by capillary waves generated at the moment of the drop impact [5].

One of the most questionable issues in wetting is the force balance that includes the vertical component of liquid surface tension. On soft solids, the vertical component leads to a microscopic protrusion of the contact line, that is, a ‘wetting ridge’. Here we reveal a universal wetting principle from the tip of a wetting ridge directly visualized with high spatio-temporal resolution of X-ray imaging [6]. We find that the cusp of the ridge is bent with an asymmetric tip, whose geometry is invariant during ridge growth or by surface softness. The singular asymmetry is deduced by linking the macroscopic and microscopic contact angles to Young and Neuman laws, respectively.

Finally, transient dynamics of dynamic Leidenfrost phenomenon is revealed based on using ultrafast X-ray imaging. Ultrafast/nanoscale dynamics byphase-contrast X-ray imaging will significantly contribute to resolve various unsolved puzzling problems in nature.

Acknowledgments: This work was supported by the Ministry of Trade, Industry and Energy(MOTIE) and Korea Institute for Advancement of Technology (KIAT)through the International Cooperative R&D Program.

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[1]W.-L Tsai et al., Nature, 417, 139 (2002).

[2]B.M. Weon et al, Phys. Rev. Lett, 100, 217403 (2008).

[3]J.S. Lee et al, Phys. Rev. Lett, 109, 204501 (2012).

[4]J.S. Lee et al, Nat. Comm.2, 367 (2011).

[5]J.S. Lee et al, Nat. Comm.6, 8187 (2015).

[6]S.J. Park et al, Nat. Comm.5, 4369 (2014).