Spontaneous Formation of Structures with Micro- and Nano-Scopic Periodic Ripple Patterns

Abstract

We report the first study on the formation of structures with micro- and nano-scopic periodic surface patterns created by the spontaneous flow of liquid metal over thin metallic solid films. Minute details of the flow of liquid gallium over gold are captured in situ at very high magnifications using a scanning electron microscope, and a series of experiments and microstructural characterization are performed to understand the underlying principles of the liquid flow and the pattern formation. This phenomenon is solely driven by wetting, with little influence of gravity, and is aided by a tenacious semi-solidus envelope of the intermetallic compound formed due to the reaction between the liquid metal and the metallic substrate. This complex flow creates highly periodic patterns with features ranging from hundreds of nanometers to tens of micrometers, which can be tuned a priori . We propose a model capturing the essential mechanics of the ripple formation and apply it to simulate the formation of a single ripple, along with its essential asymmetry, that forms the basis for generating the observed patterns.

Manipulating liquid metal flow for creating standalone structures with micro-and nano-scale features in a single step

“…Width of the substrate metal track. The Laplace pressure, which limits the height of the ripple and determines the nucleation of the new ripple, depends on the principal radii of curvature of the liquid [24]. Therefore, the width of the substrate metal track, which limits one of the principal radii of curvature, may act as a critical control parameter for manipulating the ripple patterns.…”

“…Ga, Sn, etc, on a metallic substrate, e.g. Pt, Au, etc [24,25]; this type of flow is termed as 'rippled flow'. These small-scale patterns are promising with possibilities of applications in the areas of microelectronics [26], microfluidics [27], etc.…”

“…An amalgamation of wetting-driven pull at the flowfront and the worsening of the wetting due to the chemical reaction between liquid metal and the metallic substrate has been attributed as the primary driving force for this complex phenomenon [24]. In the particular case of the ripple flow of liquid Ga over Au (or Pt) deposited on SiO 2 /Si substrate with Ti (or Cr) adhesion layer, the wetting drives the flow of liquid metal over the metallic path and, in the process, the liquid metal dissolves the substrate metallic layer forming an intermetallic compound (IMC), with approximately an equiatomic composition, that moves upward towards the open surface [24]. However, this process establishes direct contact between liquid metal and the adhesion layer, which does not wet the liquid metal.…”

“…Accordingly, here we discuss the effects of various process parameters on the nature of ripple flow with a fixed volume and static reservoir. Hence, while our previous study [24] discusses the fundamental mechanism of the formation of standalone structures with periodic surface patterns or ripples, here we build on those fundamentals to tailor the process parameters to create ripples for applications at microand nano-scales. The patterns studied here show promise with dimensions ranging from 500 nm to 10s μm on a substrate using a single-step process.…”

Image quality enhancement of PMUT-based photoacoustic imaging

“…Micro-electro-mechanical systems (MEMS) technology-based piezoelectric micro-machined ultrasound transducer (PMUT), [1][2][3] that can be directly bonded to application-specific integrated circuits (ASICs) with ease − are ideally suited for such miniaturized devices facilitating several unique technological advantageous features such as PMUTs should have a low cost and good reproducibility due to batch fabrication processing, due to reduced power requirements and ease of on-chip integration, reduction in size and weight. [4][5][6] Micromachined Ultrasound Transducers (MUTs) [7][8][9][10] are becoming more common as a replacement for conventional transducers. 11 There has been a recent trend in employing such devices to make low-cost ultra-portable photoacoustic imaging (PAI) systems that will be useful for point-of-care imaging applications and thus complement more conventional imaging technologies like positron emission tomography (PET) and magnetic resonance imaging (MRI) because of its unique and promising technical features − more specifically, not only to achieve higher spatial resolution (∼ 150µm) at the depth ∼ cm for recovery of various and complex tissue pathophysiological information (molecular ([Hb], [HbO 2 ], SO, and total Hb), physical (acoustic and mechanical properties), tissue anatomy or morphology, and micro-vasculature structures non-destructively and non-invasively but also to facilitate development of multi-modality imaging systems 12,13 − that are of utmost clinical importance and thus, significant scientific impact.…”