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Open Access Publications from the University of California

A Study on the Characteristics of Liquid Based Mechanical Interfaces

  • Author(s): Zhuang, Jinda
  • Advisor(s): Ju, Yongho
  • et al.

Most existing approaches for flexible and stretchable electronics are based on directly bonding semiconductor devices onto stretchable substrates. They require fragile 3D interconnects, stretchable conducting polymeric materials, or liquid metals to accommodate mechanical deformation while often providing limited optoelectronic functionality and mechanical robustness. An intriguing alternative strategy for realizing flexible electronics is to integrate brittle semiconductor chips onto flexible substrates using capillary confined microscale liquid bridges as mechanical interfaces. This approach allows the use of standard electronic or optoelectronic devices fabricated on “thick” substrates as opposed to simplified circuits built on highly customized ultra-thin semiconductor membranes.

For the first part of this thesis, studies on the statics and dynamics of the liquid based mechanical interface for rigid devices are present. We first introduce a design of the mechanical interface using capillary confined microscale liquid bridges between a rigid component and a flexible substrate. Numerical modeling methods are developed and are validated by corresponding experiments to precisely predict the liquid bridge topologies, capillary forces under various loading conditions. This work establishes the engineering and scientific foundation for fabrication and optimization of such liquid interfaces for flexible electronics in future research.

We then continue to investigate the dynamics of such mechanical liquid interfaces. A liquid bridge is the basic element of the mechanical liquid interface. The dynamic behaviors of a liquid bridge confined between two coaxial disks are comprehensively investigated through a combined modeling and experimental study. The effects of the stretching velocity, liquid properties, and liquid volume on the dynamics of liquid bridges are systematically studied to provide a direct experimental validation of our numerical model as well as offer further physical insights for stretching velocities as high as 3 m/s.

We also present a numerical modeling approach that fully captures the dynamics of a capillary self-alignment process, where a solid object floating on a liquid bridge is aligned by capillary forces of the underlying liquid bridge onto the target position. By directly coupling fluid dynamics, solid mechanics along with the additional line friction from moving contact line, model predictions have shown good agreement with experimental measurements. This work provides an experimentally validated modeling approach and physical insights to help establish foundation for systematic further studies and applications of capillary self-alignment and other related applications of the mechanical liquid interfaces.

The second part of this thesis discusses the applications, designs and experimental characterizations of such mechanical interface for flexible devices. We introduce a design concept for a deployable planar microdevice based on a thin film liquid bridge and the modeling and experimental validation of its mechanical behaviors. We develop and experimentally validate theoretical models based on the energy minimization approach to examine the conformality and figures of merit of the device. This study establishes an early foundation for the mechanical design of this and related deployable planar microdevice concepts.

We lastly present a tunable platform for incorporating flexible and yet non-stretching device layers on a hemispherical structure. A mechanical model is developed to elucidate the dependence of the conformality of the petal structures on their elastic modulus and thickness and the liquid surface tension. This platform will enable facile integration of non-stretching electronic and optoelectronic components prepared using established planar fabrication techniques on tunable hemispherical surfaces.

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