Person:

Liu, Qihan

Pure substances can often be cooled below their melting points and still remain in the liquid state. For some supercooled liquids, a further cooling slows down viscous flow greatly, but does not slow down self-diffusion as much. We formulate a continuum theory that regards viscous flow and self-diffusion as concurrent, but distinct, processes. We generalize Newton’s law of viscosity to relate stress, rate of deformation, and chemical potential. The self-diffusion flux is taken to be proportional to the gradient of chemical potential. The relative rate of viscous flow and self-diffusion defines a length, which, for some supercooled liquids, is much larger than the molecular dimension. A thermodynamic consideration leads to boundary conditions for a surface of liquid under the influence of applied traction and surface energy. We apply the theory to a cavity in a supercooled liquid and identify a transition. A large cavity shrinks by viscous flow, and a small cavity shrinks by self-diffusion.

When rigid and stretchable components are integrated onto a single elastic carrier substrate, large strain heterogeneities appear in the vicinity of the deformable-non-deformable interfaces. In this paper, we report on a generic approach to manufacture hybrid stretchable circuits where commercial electronic components can be mounted on a stretchable circuit board. Similar to printed circuit board development, the components are electrically bonded on the elastic substrate and interconnected with stretchable electrical traces. The substrate—a silicone matrix carrying concentric rigid disks—ensures both the circuit elasticity and the mechanical integrity of the most fragile materials.

An elastomeric seal may leak by elastic deformation without any material damage. We describe elastic leak using a theoretical model, and watch a seal deform and leak using a transparent experimental setup. The elastomer seals the fluid by forming contact with surrounding hard materials. As the fluid pressure increases, the contact stress also increases but not as much. When the fluid pressure surpasses the contact stress, the elastomer and t he hard materials lose contact in some region, forming a leaking path. The critical fluid pressure for elastic leak depends on the geometry and constraint of the seal, but is insensitive to the rate at which the fluid is injected. Our study points to the significance of elastic deformation in modes of failure that also involve material damage.

A covalent network of polymer can imbibe a solvent to form a gel. In a cavity on the surface of the gel, capillary force may suck the solvent out of the gel to form a pure liquid phase. We show that such osmocapillary phase separation occurs when capillarity balances osmosis, and when the diffusion of solvent in the gel prevails over the condensation of solvent from the vapor. Osmocapillary phase separation can occur even when the gel is isolated from the vapor, or when the solvent is nonvolatile. We relate osmocapillary phase separation to phenomena of practical significance, including the wettability of gels, the transparency of gels, the liquid bridge at the tip of an atomic force microscope, the adhesion between a gel and another substance, the surface morphology of gels, and the production of tight oil.

Materials where thermal energy is comparable to the interaction energy between molecules are called soft materials. Soft materials are everywhere in our life: food, rubber, polymer diaper, our own body, etc. The thermal fluctuation endows soft materials with fundamentally different behavior comparing to hard materials like metals and ceramics. This dissertation studies three aspects of the mechanics and physics of soft materials, as is reviewed below.

First, soft materials are generally swellable and viscous. The combination of diffusion and viscous flow gives rise to a length scale we called poroviscous length. The emergence of a length scale results in size dependent relaxation. We show that the coupling between diffusion and viscous flow explains the Brownian motion in supercooled liquids, where the classical result of Stokes-Einstein relation generally fails. The concurrent diffusion and viscous flow cannot be described by the classical hydrodynamics, where all the material transport is lumped into velocity field. We formulated a continuum theory to modify the classical hydrodynamics. In particular, the new theory predicts a new bulk viscosity that could exist in incompressible material. We generalize this idea of bulk viscosity to binary systems and study the mixing of materials that is limited by local structural rearrangement instead of diffusion. This model develops formulation of non-equilibrium thermodynamics by removing the common assumption of local equilibrium.

Second, capillarity has strong influence on the morphology of soft materials. The competition between capillarity and elasticity gives rise to the elastocapillary length, which is defined as surface tension over the shear modulus. We show that elastocapillary effect explains the complex nucleation of crease, a widely observed surface instability in soft elastic materials. We also explore the possible competition between capillarity and osmosis in gels, which defines the osmocapillary length, the surface tension divided by osmotic pressure. We show that at small enough length scale or for a gel that is nearly fully swollen, surface tension can pull liquid solvent out from the gel phase, a phenomenon we termed osmocapillary phase separation.

Third, soft materials are nearly incompressible. The incompressibility and softness makes elastomers ideal for the design of seals. Although the failure of seals has been studies for decades, existing studies mainly focus on the damage and degradation of materials. Here we study the leak of a seal due to elastic deformation without any damage. We call such a failure mode the elastic leak. We point out that elastic leak is involved in any leak event no matter whether material is damaged or not. We also show that the reversible nature of the elastic leak enable seal series to achieve higher sealing capability.

The emergence of stretchable electronics and its potential integration with textiles have highlighted a challenge: textiles are wearable and washable, but electronic devices are not. Many stretchable conductors have been developed to enable wearable active textiles, but little has been done to make them washable. Here we demonstrate a new class of stretchable conductors that can endure wearing and washing conditions commonly associated with textiles. Such a conductor consists of a hydrogel, a dissolved hygroscopic salt, and a butyl rubber coating. The hygroscopic salt enables ionic conduction, and matches the relative humidity of the hydrogel to the average ambient relative humidity. The butyl rubber coating prevents the loss and gain of water due to the daily fluctuation of ambient relative humidity. We develop the chemistry of dip coating the butyl rubber onto the hydrogel, using silanes to achieve both the crosslink of the butyl rubber and the adhesion between the butyl rubber and the hydrogel. We test the endurance of the conductor by soaking it in detergent while stretching it cyclically, and by machine-washing it. The loss of water and salt is minimal. It is hoped that these conductors open applications in healthcare, entertainment, and fashion.

When a supercooled liquid approaches glass transition, viscous flow slows down greatly, but often the Brownian motion of a molecular probe in the host liquid does not slow down as much, causing the Stokes-Einstein relation to fail by orders of magnitude. Here we formulate a theory that relates the Brownian motion of the probe to two concurrent processes in the host liquid: viscous flow and molecular hopping. Molecular hopping prevails over viscous flow when the probe is small and the temperature is low. Our theory generalizes the Stokes-Einstein relation and fits the experimental data remarkably well.

Elastomeric seals are essential to two great technological advances in oilfields: horizontal drilling and hydraulic fracturing. This paper describes a method to study elastomeric seals by using the pressure-extrusion curve (i.e., the relation between the drop of pressure across a seal and the volume of extrusion of the elastomer). Emphasis is placed on a common mode of failure found in oilfields: leak caused by a crack across the length of a long seal. We obtain an analytical solution of large elastic deformation, which is analogous to the Poiseuille flow of viscous liquids. We further obtain analytical expressions for the energy release rate of a crack and the critical pressure for the onset of its propagation. The theory predicts the pressure-extrusion curve using material parameters (elastic modulus, sliding stress, and fracture energy) and geometric parameters (thickness, length, and precompression). We fabricate seals of various parameters in transparent chambers on a desktop, and watch the seals extrude, slide, rupture and leak. The experimentally measured pressure-extrusion curves agree with theoretical predictions remarkably well.

Recently developed devices mimic neuromuscular and neurosensory systems by integrating hydrogels and hydrophobic elastomers. While different methods are developed to bond hydrogels with hydrophobic elastomers, it remains a challenge to coat and print various hydrogels and elastomers of arbitrary shapes, in arbitrary sequences, with strong adhesion. Here we report an approach to meet this challenge. We mix silane coupling agents into the precursors of the networks, and tune the kinetics such that, when the networks form, the coupling agents incorporate into the polymer chains, but do not condensate. After a manufacturing step, the coupling agents condensate, add crosslinks inside the networks, and form bonds between the networks. This approach enables independent bonding and manufacturing. We formulate oxygen-tolerant hydrogel resins for spinning, printing, and coating in the open air. We find that thin elastomer coatings enable hydrogels to sustain high temperatures without boiling.

Cardiac tissue development and pathology have been shown to depend sensitively on microenvironmental mechanical factors, such as extracellular matrix stiffness, in both in vivo and in vitro systems. We present a novel quantitative approach to assess cardiac structure and function by extending the classical traction force microscopy technique to tissue-level preparations. Using this system, we investigated the relationship between contractile proficiency and metabolism in neonate rat ventricular myocytes (NRVM) cultured on gels with stiffness mimicking soft immature (1 kPa), normal healthy (13 kPa), and stiff diseased (90 kPa) cardiac microenvironments. We found that tissues engineered on the softest gels generated the least amount of stress and had the smallest work output. Conversely, cardiomyocytes in tissues engineered on healthy- and disease-mimicking gels generated significantly higher stresses, with the maximal contractile work measured in NRVM engineered on gels of normal stiffness. Interestingly, although tissues on soft gels exhibited poor stress generation and work production, their basal metabolic respiration rate was significantly more elevated than in other groups, suggesting a highly ineffective coupling between energy production and contractile work output. Our novel platform can thus be utilized to quantitatively assess the mechanotransduction pathways that initiate tissue-level structural and functional remodeling in response to substrate stiffness.