Solid capillarity of soft materials

Figure: Reconstruction of the 3D confocal image of a deformed soft surface.

Surface stress, also known as surface tension, is a fundamental material property of any interface. However, measurements of solid surface stress in traditional engineering materials, such as metals and oxides, have proven to be very challenging. Consequently, our understanding relies heavily on untested theories, especially regarding the strain dependence of this properties. Here, we take the advantage of high compliance and large deformability of a soft polymer gel to directly measure solid surface stress as a function of strain. Under biaxial stretch, we find the surface stress depends on the strain via a surface modulus, which, remarkably, is many times larger than the zero-strain surface tension. Further, we find the solid surface stress becomes anisotropic under uniaxial stretch. These results suggest that solid surface stress, as a strain-dependent tensor, can play a dominant role in solid mechanics at much larger length scales than previously anticipated.

Related Reference:
[1] Qin Xu, Katharine E. Jensen, Rostislav Boltyanskiy, Raphael Sarfati, Robert Style, and Eric R. Dufresne Nature Communications 8, 555(2017)
[2] Katharine E. Jensen, Robert Style, Qin Xu and Eric R. Dufresne arXiv:1707.03089 (2017) Submitted
[3] Robert W. Style, Tianqi Sai, Nicolo Fanelli, Katrina Smith-Mannschott, Robert Style, Qin Xu, Larry A. Willen and Eric R. Dufresne arXiv:1709.00500 (2017) Submitted

Shear thickening in granular suspensionsImageJ=1.46r
Dense granular suspensions can exhibit a strong ST that can become discontinuous as a critical packing fraction is approached. Recent works have related this to frictional particle interactions and dilation as dry granular materials. In this granular mechanism, the frictional stress between solid particles is the dominant contributor to ST. The suspending liquid mainly acts as a boundary constraint to prevent expansion. Nevertheless, viscous hydrodynamic interactions, as another dissipation mechanism, still exist in the bulk, and lubrication and viscous drag could become significant when the suspending liquid is highly viscous. Here, we try to perform experimental characterization of how the hydrodynamics coupled with dilation and affects the shear thickening of nearly jammed granular suspensions.

Related references:
Qin Xu, Sayantan Majumdar, Eric Brown and Heinrich M. Jaeger, Europhys. Lett. 107, 68004 (2014)

Impact dynamics of non-Newtonian fluids
Drop impact onto a solid surface is a key process in many applications involving electronic fabrication, spray coating, and inkjet printing. Impact dynamics of Newtonian drops have been extensively studied and is still attracts many interests from scientists. Non-Newtonian fluids impact, on the other hand, is poorly understood due to their complex physical properties. Here, we focus on two types of non-Newtonian fluids: oxidized liquid metals and densely packed granular suspensions.

Liquid metal drops: liquid metal impact
Most liquid metals develop an oxide skin when exposed to air, resulting in non-Newtonian behavior. One of the direct consequences is that the oxide layer consumes a portion of kinetic energy and thus deviates their spreading dynamics from ordinary liquids in the impact. Also, oxidized liquid metals can generally form non-spherical drops since the surface skin not only prevents contact between air and the bulk of the material but also the minimization of the surface energy. Our findings show how the presence of the oxide skin leads to two dynamical phases: capillary and viscous regimes. The results from both regimes can be collapsed in an impact phase diagram by considering the surface elasticity.

Dense Granular Suspensions: Splat_2
We depart from the dilute cases, and instead focus on the limit of dense granular suspensions with volume packing fractions φ = 0.62 +/- 0.03, where the droplets are sustained by capillary pressure. We found the splashing onset is predicted by an energy balance at the level of the particles in the suspensions. Also, by comparing the experimental results with a theoretical simulation, we can model the spreading dynamics while a monolayer of particles is formed after impact.

Related references:
[1] Qin Xu, Nikolai Oudalov, Qiti Guo, Heinrich M. Jaeger and Eric Brown, Physics of Fluids, 24, 063101 (2012) pdf

[2] Qin Xu, Eric Brown and Heinrich M. Jaeger, Physical Review E, 87, 043012 (2013) pdf
[3] Ivo R. Peters, Qin Xu and Heinrich M. Jaeger, Physical Review Letters, 111, 028301 (2013) pdf
[4] Qin Xu, Ivo R. Peters, Sam Wilken, Eric Brown and Heinrich M. Jeager, Journal of  Visualized Experiments, 85, e51249 (2014) pdf
[5] Luuk A. Lubbers, Qin Xu, Sam Wilken, Wendy W. Zhang and Henrich M. Jaeger, Submitted to Physical Review Letters (2014)

DNA-functionalized colloidal systemsScreenshot 2014-04-21 14.58.28
Colloids coated with complementary single-stranded DNA “sticky ends” associate and dissociate upon heating. Recently, microscopy experiments have been carried out where this association-dissociation transition has been investigated for different types of DNA and different DNA coverages. We have studied a model system, colloidal particles functionalized with DNA ‘‘sticky ends’’ diffusing on a complementary coated surface, and observed a crossover from subdiffusive to conventional behavior as temperature is increased near the particle-surface melting temperature consistent with a simple Gaussian distribution of sticky ends. In addition, permanently bonding some or all of the complementary pairs may allow for flexibility in design and construction. Substitution of a Cinnamate group for a pair of complementary bases provides an efficient, addressable, ultraviolet light-based method to bond complementary DNA covalently.

Related references:
[1] Q. Xu, L. Feng, R. Sha, N. C. Seeman and P. M. Chaikin, Physical Review Letters, 106, 228102 (2011) pdf 
[2] Lang Feng, Joy Romulus, Minfeng Li,Ruojie Sha, John Royer, Kun-Ta Wu, Qin Xu, Nadrian Seeman, Marcus Weck and Paul Chaikin, Nature Materials, 12, 747-753 (2013) pdf

Gradient-index meta-surfacesGradient-index
The arbitrary control of electromagnetic waves is a key aim of photonic research. Although, for example, the control of freely propagating waves and surface waves has separately become possible using transformation optics and metamaterials, a bridge linking both propagation types has not yet been found. We demonstrate theoretically and experimentally that a specific gradient-index meta-surface can convert a PW to a SW with nearly 100% efficiency. Distinct from conventional devices such as prism or grating couplers, the momentum mismatch between PW and SW is compensated by the reflection-phase gradient of the meta-surface, and a nearly perfect PW–SW conversion can happen for any incidence angle larger than a critical value. Experiments in the microwave region, including both far-field and near-field characterizations, are in excellent agreement with full-wave simulations. Our findings may pave the way for many applications, including high-efficiency surface plasmon couplers, anti-reflection surfaces, light absorbers, and so on.

Related references:
[1] Shulin Sun, Qiong He, Shiyi Xiao, Qin Xu, Xin Li and Lei Zhou, Nature Materials, 11, 426-431 (2012) pdf
[2] Shulin Sun, Qiong He, Shiyi Xiao, 
Qin Xu, Xin Li, Che Qu and Lei Zhou, Laser & Optoelectronic Progress, 50, 080009 (2013) pdf

[3] Qiong He, Shiyi Xiao, Xin Li, Che Qu, Qin Xu, Shulin Sun and Lei Zhou, SPIE Newsroom – Optical Design & Engineering, 10.1117/2.1201402.005337 (2014) pdf