A cage-specific hydrate equilibrium model for robust predictions of industrially-relevant mixtures.

Understanding the thermophysical properties and phase behaviour of gas hydrates is essential for industrial applications ranging from energy transport and storage, CO2 capture and sequestration, to gas production from hydrates found on the seabed. Current tools for predicting hydrate equilibrium boundaries typically use van der Waals-Platteeuw-type models which are over-parameterised containing terms with limited physical basis. Here we present a new model for hydrate equilibrium calculations with 40% fewer parameters than existing tools but with equivalent accuracy, including for multicomponent gas mixtures and/or systems with thermodynamic inhibitors. By eliminating multi-layered shells from the model's conceptual basis and focusing on Kihara potential parameters for guest-water interactions specific to each hydrate cavity type, this new model provides insight into the physical chemistry governing hydrate thermodynamics. The model retains the improved description of the empty lattice developed recently by Hielscher et al. but couples the hydrate model with a Cubic-Plus-Association Equation of State (CPA-EOS) to describe fluid mixtures with many more components including inhibitors such as methanol and mono-ethylene glycol used by industry. An extensive database of over 4000 data points was used to train and evaluate the new model and compare its performance against existing tools. The absolute average deviation in temperature (AADT) achieved with the new model is 0.92 K for multicomponent gas mixtures, compared with 1.00 K for the widely-known model of Ballard and Sloan, and 0.86 K for the CPA-hydrates model implemented in the MultiFlash 7.0 software package. With fewer, more physically justified parameters, this new cage-specific model provides a robust basis for improved hydrate equilibrium predictions particularly for industrially-important, multi-component mixtures containing thermodynamic inhibitors.

Surface-Enhanced Raman Scattering Imaging of Cetylpyridinium Chloride Adsorption to a Solid Surface.

Surface active agents (surfactants) have found a variety of critical technological applications, from helping infant lungs breathe to fugitive dust control at industrial sites. Surfactant molecules adsorb to an interface and facilitate a decrease in the surface free energy (interfacial tension) between two immiscible phases. However, a limited number of methods (e.g., holography and fluorescence microscopy) achieved visualization of surfactant molecule distribution in multiphase systems qualitatively. To probe the efficacy and/or adsorption density of surfactants at such interfaces quantitatively, we demonstrate here a direct observation of surfactant adsorption by surface-enhanced Raman scattering (SERS). This work details the development of a research platform to study surfactant adsorption using Raman imaging. The imaging and analysis were successfully benchmarked against conventional interfacial tension measurements and thermodynamic theory employed to estimate surfactant adsorption at equilibrium. This in situ Raman-based experimental method provides a platform to interrogate structure-function relationships that inform the design process for new surfactant species.

Gas hydrates in sustainable chemistry.

Gas hydrates have received considerable attention due to their important role in flow assurance for the oil and gas industry, their extensive natural occurrence on Earth and extraterrestrial planets, and their significant applications in sustainable technologies including but not limited to gas and energy storage, gas separation, and water desalination. Given not only their inherent structural flexibility depending on the type of guest gas molecules and formation conditions, but also the synthetic effects of a wide range of chemical additives on their properties, these variabilities could be exploited to optimise the role of gas hydrates. This includes increasing their industrial applications, understanding and utilising their role in Nature, identifying potential methods for safely extracting natural gases stored in naturally occurring hydrates within the Earth, and for developing green technologies. This review summarizes the different properties of gas hydrates as well as their formation and dissociation kinetics and then reviews the fast-growing literature reporting their role and applications in the aforementioned fields, mainly concentrating on advances during the last decade. Challenges, limitations, and future perspectives of each field are briefly discussed. The overall objective of this review is to provide readers with an extensive overview of gas hydrates that we hope will stimulate further work on this riveting field.

Hydrate Growth on Methane Gas Bubbles in the Presence of Salt.

Methane bubble dispersions in a water column can be observed in both vertical subsea piping as well as subsea gas seepages. Hydrate growth has been shown to occur at the gas-water interface under flowing conditions, yet the majority of the current literature is limited to quiescent systems. Gas hydrate risks in subsea piping have been shown to increase in late life production wells with increased water content and with gas-in-water bubble dispersions. The dissolution of subsea methane seepages into seawater, or methane release into the atmosphere, can be affected by hydrate film growth on rising bubbles. A high-pressure water tunnel (HPWT), was used to generate a turbulent, continuous water flow system representative of a vertical jumper line to study the relationship between bulk methane hydrate growth and bubble size during a production-well restart. The HPWT comprises a flow loop of 19.1 mm inner diameter and 4.9 m length, with a vertical section containing an optical window to enable visualization of the bubble and hydrate flow dynamics via a high-speed, high-resolution video camera. Additional online monitoring includes the differential pressure drop, viscosity, temperature, flow rates, and gas consumption. Experimental conditions were maintained at 275 K and 6.2 MPa during hydrate formation and 298 K and 1.4 MPa during hydrate dissociation. Hydrate growth using freshwater and saltwater (3.5 wt % NaCl) was measured at four flow velocities (0.8, 1.2, 1.6, and 1.9 m s-1). The addition of salt is shown in this work to alter the surface properties of bubbles, which introduces changes to bubble dynamics of dispersion and coalescence. Hydrate volume fractions and growth rates in the presence of salt were on average ∼32% lower compared to that in freshwater. This was observed and validated to be due to bubble size and dynamic factors and not due to the 1.5 K thermodynamic inhibition effect of salt. Throughout hydrate growth, methane bubbles in pure freshwater maintained larger diameters (2.4-4.2 mm), whereas the presence of salt promoted fine gas bubble dispersions (0.1-0.7 mm), increasing gas-water interfacial area. While gas bubble coalescence was observed in all freshwater experiments, the addition of salt limited coalescence between gas bubbles and reduced bubble size. Consequently, earlier formation of solid hydrate shells in saltwater produced early mass-transfer barriers reducing hydrate growth rates. While primarily directed toward flow assurance, the observed relationship between hydrates, bubble size, and saltwater also applies to broader research fields in subsea gas seepages and naturally occurring hydrates.

Hydrate nucleation and growth on water droplets acoustically-levitated in high-pressure natural gas.

Hydrate formation was studied using water droplets acoustically levitated in high-pressure natural gas. Despite the absence of solid interfaces, the droplets' area-normalised nucleation rate was about four times faster than in steel autoclave measurements with interfacial areas roughly 200 times larger. Multiple stages of stochastic, template-free hydrate growth were observed.

Gas Hydrate Formation Probability Distributions: The Effect of Shear and Comparisons with Nucleation Theory.

Gas hydrate formation is a stochastic phenomenon of considerable significance for any risk-based approach to flow assurance in the oil and gas industry. In principle, well-established results from nucleation theory offer the prospect of predictive models for hydrate formation probability in industrial production systems. In practice, however, heuristics are relied on when estimating formation risk for a given flowline subcooling or when quantifying kinetic hydrate inhibitor (KHI) performance. Here, we present statistically significant measurements of formation probability distributions for natural gas hydrate systems under shear, which are quantitatively compared with theoretical predictions. Distributions with over 100 points were generated using low-mass, Peltier-cooled pressure cells, cycled in temperature between 40 and -5 °C at up to 2 K·min-1 and analyzed with robust algorithms that automatically identify hydrate formation and initial growth rates from dynamic pressure data. The application of shear had a significant influence on the measured distributions: at 700 rpm mass-transfer limitations were minimal, as demonstrated by the kinetic growth rates observed. The formation probability distributions measured at this shear rate had mean subcoolings consistent with theoretical predictions and steel-hydrate-water contact angles of 14-26°. However, the experimental distributions were substantially wider than predicted, suggesting that phenomena acting on macroscopic length scales are responsible for much of the observed stochastic formation. Performance tests of a KHI provided new insights into how such chemicals can reduce the risk of hydrate blockage in flowlines. Our data demonstrate that the KHI not only reduces the probability of formation (by both shifting and sharpening the distribution) but also reduces hydrate growth rates by a factor of 2.

Interfacial phenomena in gas hydrate systems.

Gas hydrates are crystalline inclusion compounds, where molecular cages of water trap lighter species under specific thermodynamic conditions. Hydrates play an essential role in global energy systems, as both a hinderance when formed in traditional fuel production and a substantial resource when formed by nature. In both traditional and unconventional fuel production, hydrates share interfaces with a tremendous diversity of materials, including hydrocarbons, aqueous solutions, and inorganic solids. This article presents a state-of-the-art understanding of hydrate interfacial thermodynamics and growth kinetics, and the physiochemical controls that may be exerted on both. Specific attention is paid to the molecular structure and interactions of water, guest molecules, and hetero-molecules (e.g., surfactants) near the interface. Gas hydrate nucleation and growth mechanics are also presented, based on studies using a combination of molecular modeling, vibrational spectroscopy, and X-ray and neutron diffraction. The fundamental physical and chemical knowledge and methods presented in this review may be of value in probing parallel systems of crystal growth in solid inclusion compounds, crystal growth modifiers, emulsion stabilization, and reactive particle flow in solid slurries.

Hydrate Shell Growth Measured Using NMR.

Benchtop nuclear magnetic resonance (NMR) pulsed field gradient (PFG) and relaxation measurements were used to monitor the clathrate hydrate shell growth occurring in water droplets dispersed in a continuous cyclopentane phase. These techniques allowed the growth of hydrate inside the opaque exterior shell to be monitored and, hence, information about the evolution of the shell's morphology to be deduced. NMR relaxation measurements were primarily used to monitor the hydrate shell growth kinetics, while PFG NMR diffusion experiments were used to determine the nominal droplet size distribution (DSD) of the unconverted water inside the shell core. A comparison of mean droplet sizes obtained directly via PFG NMR and independently deduced from relaxation measurements showed that the assumption of the shell model-a perfect spherical core of unconverted water-for these hydrate droplet systems is correct, but only after approximately 24 h of shell growth. Initially, hydrate growth is faster and heat-transfer-limited, leading to porous shells with surface areas larger than that of spheres with equivalent volumes. Subsequently, the hydrate growth rate becomes mass-transfer-limited, and the shells become thicker, spherical, and less porous.

Adhesion force interactions between cyclopentane hydrate and physically and chemically modified surfaces.

Interfacial interactions between liquid-solid and solid-solid phases/surfaces are of fundamental importance to the formation of hydrate deposits in oil and gas pipelines. This work establishes the effect of five categories of physical and chemical modification to steel on clathrate hydrate adhesive force: oleamide, graphite, citric acid ester, nonanedithiol, and Rain-X anti-wetting agent. Hydrate adhesive forces were measured using a micromechanical force apparatus, under both dry and water-wet surface conditions. The results show that the graphite coating reduced hydrate-steel adhesion force by 79%, due to an increase in the water wetting angle from 42 ± 8° to 154 ± 7°. Two chemical surface coatings (nonanedithiol and the citric acid ester) induced rapid hydrate growth in the hydrate particles; nonanedithiol increased hydrate adhesive force by 49% from the baseline, while the citric acid ester coating reduced hydrate adhesion force by 98%. This result suggests that crystal growth may enable a strong adhesive pathway between hydrate and other crystalline structures, however this effect may be negated in cases where water-hydrocarbon interfacial tension is minimised. When a liquid water droplet was placed on the modified steel surfaces, the graphite and citric acid ester became less effective at reducing adhesive force. In pipelines containing a free water phase wetting the steel surface, chemical or physical surface modifications alone may be insufficient to eliminate hydrate deposition risk. In further tests, the citric acid ester reduced hydrate cohesive forces by 50%, suggesting mild activity as a hybrid anti-agglomerant suppressing both hydrate deposition and particle agglomeration. These results demonstrate a new capability to develop polyfunctional surfactants, which simultaneously limit the capability for hydrate particles to aggregate and deposit on the pipeline wall.

Surfactant adsorption and interfacial tension investigations on cyclopentane hydrate.

Gas hydrates represent an unconventional methane resource and a production/safety risk to traditional oil and gas flowlines. In both systems, hydrate may share interfaces with both aqueous and hydrocarbon fluids. To accurately model macroscopic properties, such as relative permeability in unconventional systems or dispersion viscosity in traditional systems, knowledge of hydrate interfacial properties is required. This work presents hydrate cohesive force results measured on a micromechanical force apparatus, and complementary water-hydrocarbon interfacial tension data. By combining a revised cohesive force model with experimental data, two interfacial properties of cyclopentane hydrate were estimated: hydrate-water and hydrate-cyclopentane interfacial tension values at 0.32 ± 0.05 mN/m and 47 ± 5 mN/m, respectively. These fundamental physiochemical properties have not been estimated or measured for cyclopentane hydrate to date. The addition of surfactants in the cyclopentane phase significantly reduced the cyclopentane hydrate cohesive force; we hypothesize this behavior to be the result of surfactant adsorption on the hydrate-oil interface. Surface excess quantities were estimated for hydrate-oil and water-oil interfaces using four carboxylic and sulfonic acids. The results suggest the density of adsorbed surfactant may be 2× larger for the hydrate-oil interface than the water-oil interface. Additionally, hydrate-oil interfacial tension was observed to begin decreasing from the baseline value at significantly lower surfactant concentrations (1-3 orders of magnitude) than those for the water-oil interfacial tension.

Adhesion force between cyclopentane hydrate and mineral surfaces.

Clathrate hydrate adhesion forces play a critical role in describing aggregation and deposition behavior in conventional energy production and transportation. This manuscript uses a unique micromechanical force apparatus to measure the adhesion force between cyclopentane hydrate and heterogeneous quartz and calcite substrates. The latter substrates represent models for coproduced sand and scale often present during conventional energy production and transportation. Micromechanical adhesion force data indicate that clathrate hydrate adhesive forces are 5-10× larger for calcite and quartz minerals than stainless steel. Adhesive forces further increased by 3-15× when increasing surface contact time from 10 to 30 s. In some cases, liquid water from within the hydrate shell contacted the mineral surface and rapidly converted to clathrate hydrate. Further measurements on mineral surfaces with physical control of surface roughness showed a nonlinear dependence of water wetting angle on surface roughness. Existing adhesive force theory correctly predicted the dependence of clathrate hydrate adhesive force on calcite wettability, but did not accurately capture the dependence on quartz wettability. This comparison suggests that the substrate surface may not be inert, and may contribute positively to the strength of the capillary bridge formed between hydrate particles and solid surfaces.

Nanostructure of Locally Concentrated Ionic Liquids in the Bulk and at Graphite and Gold Electrodes.

The physical properties of ionic liquids (ILs) have led to intense research interest, but for many applications, high viscosity is problematic. Mixing the IL with a diluent that lowers viscosity offers a solution if the favorable IL physical properties are not compromised. Here we show that mixing an IL or IL electrolyte (ILE, an IL with dissolved metal ions) with a nonsolvating fluorous diluent produces a low viscosity mixture in which the local ion arrangements, and therefore key physical properties, are retained or enhanced. The locally concentrated ionic liquids (LCILs) examined are 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIM TFSI), 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (HMIM FAP), or 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (BMIM FAP) mixed with 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFTFE) at 2:1, 1:1, and 1:2 (w/w) IL:TFTFE, as well as the locally concentrated ILEs (LCILEs) formed from 2:1 (w/w) HMIM TFSI-TFTFE with 0.25, 0.5, and 0.75 m lithium bis(trifluoromethylsulfonyl)imide (LiTFSI). Rheology and conductivity measurements reveal that the added TFTFE significantly reduces viscosity and increases ionic conductivity, and cyclic voltammetry (CV) reveals minimal reductions in electrochemical windows on gold and carbon electrodes. This is explained by the small- and wide-angle X-ray scattering (S/WAXS) and atomic force microscopy (AFM) data, which show that the local ion nanostructures are largely retained in LCILs and LCILEs in bulk and at gold and graphite electrodes for all potentials investigated.

Evolution of the Macondo well blowout: simulating the effects of the circulation and synthetic dispersants on the subsea oil transport.

During the Deepwater Horizon incident, crude oil flowed into the Gulf of Mexico from 1522 m underwater. In an effort to prevent the oil from rising to the surface, synthetic dispersants were applied at the wellhead. However, uncertainties in the formation of oil droplets and difficulties in measuring their size in the water column, complicated further assessment of the potential effect of the dispersant on the subsea-to-surface oil partition. We adapted a coupled hydrodynamic and stochastic buoyant particle-tracking model to the transport and fate of hydrocarbon fractions and simulated the far-field transport of the oil from the intrusion depth. The evaluated model represented a baseline for numerical experiments where we varied the distributions of particle sizes and thus oil mass. The experiments allowed to quantify the relative effects of chemical dispersion, vertical currents, and inertial buoyancy motion on oil rise velocities. We present a plausible model scenario, where some oil is trapped at depth through shear emulsification due to the particular conditions of the Macondo blowout. Assuming effective mixing of the synthetic dispersants at the wellhead, the model indicates that the submerged oil mass is shifted deeper, decreasing only marginally the amount of oil surfacing. In this scenario, the oil rises slowly to the surface or stays immersed. This suggests that other mechanisms may have contributed to the rapid surfacing of oil-gas mixture observed initially. The study also reveals local topographic and hydrodynamic processes that influence the oil transport in eddies and multiple layers. This numerical approach provides novel insights on oil transport mechanisms from deep blowouts and on gauging the subsea use of synthetic dispersant in mitigating coastal damage.

Surface evolution of the deepwater horizon oil spill patch: combined effects of circulation and wind-induced drift.

Following the Deepwater Horizon blowout, major concerns were raised about the probability that the Loop Current would entrain oil at the surface of the Gulf of Mexico toward South Florida. However, such a scenario did not materialize. Results from a modeling approach suggest that the prevailing winds, through the drift they induced at the ocean surface, played a major role in pushing the oil toward the coasts along the northern Gulf, and, in synergy with the Loop Current evolution, prevented the oil from reaching the Florida Straits. This implies that both oceanic currents and surface wind-induced drift must be taken into account for the successful forecasting of the trajectories and landfall of oil particles, even in energetic environments such as the Gulf of Mexico. Consequently, the time range of these predictions is limited to the weather forecasting range, in addition to the range set up by ocean forecasting capabilities.

Micromechanical Force Measurement of Clotted Blood Particle Cohesion: Understanding Thromboembolic Aggregation Mechanisms.

PURPOSE: Arterial shear forces may promote the embolization of clotted blood from the surface of thrombi, displacing particles that may occlude vasculature, with increased risk of physiological complications and mortality. Thromboemboli may also collide in vivo to form metastable aggregates that increase vessel occlusion likelihood. METHODS: A micromechanical force (MMF) apparatus was modified for aqueous applications to study clot-liquid interfacial phenomena between clotted porcine blood particles suspended in modified continuous phases. The MMF measurement is based on visual observation of particle-particle separation, where Hooke's Law is applied to calculate separation force. This technique has previously been deployed to study solid-fluid interfacial phenomena in oil and gas pipelines, providing fundamental insight to cohesive and adhesive properties between solids in multiphase flow systems. RESULTS: This manuscript introduces distributed inter-particle separation force properties as a function of governing physio-chemical parameters; pre-load (contact) force, contact time, and bulk phase chemical modification. In each experimental campaign, the hysteresis and distributed force properties were analysed, to derive insight as to the governing mechanism of cohesion between particles. Porcine serum, porcine albumin and pharmaceutical agents (alteplase, tranexamic acid and hydrolysed aspirin) reduced the measurement by an order of magnitude from the baseline measurement-the apparatus provides a platform to study how surface-active chemistries impact the solid-fluid interface. CONCLUSION: These results provide new insight to potential mechanisms of macroscopic thromboembolic aggregation via particles cohering in the vascular system-data that can be directly applied to computational simulations to predict particle fate, better informing the mechanistic developments of embolic occlusion.

Self-assembled nanostructure induced in deep eutectic solvents via an amphiphilic hydrogen bond donor.

HYPOTHESIS: Popular deep eutectic solvents (DESs) typically lack amphiphilic molecules and ions and therefore do not have the useful self-assembled nanostructures prevalent in many ionic liquids. We hypothesise that nanostructure in DESs can be induced via an amphiphilic hydrogen bond donor (HBD), and that nanostructure becomes better defined with HBD chain length. EXPERIMENTS: The structure of DESs formed from choline chloride mixed with either butyric acid (ChCl/BuOOH) or hexanoic acid (ChCl/HeOOH) in a 1:4 M ratio were studied using atomic force microscopy (AFM) imaging, force curves, and friction measurements combined with bulk rheology. FINDINGS: DESs formed with both the C4 and C6 acids are nanostructured. As the length of the acid group is increased from C4 to C6, AFM images reveal the nanostructure becomes larger and better defined due to the longer acid chain, and AFM force curves show the interfacial nanostructure extends further from the surface. Self-assembled nanostructure in these systems is a consequence of choline cations, chloride anions, and acid alcohol groups clustering together due to electrostatic attractions and hydrogen bonding to form polar domains. Acid alkyl chains are solvophobically excluded from the polar domains and aggregate into apolar domains.

Nanostructure, electrochemistry and potential-dependent lubricity of the catanionic surface-active ionic liquid [P6,6,6,14] [AOT].

HYPOTHESIS: A catanionic surface-active ionic liquid (SAIL) trihexyltetradecylphosphonium 1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-2-sulfonate ([P6,6,6,14] [AOT]) is nanostructured in the bulk and at the interface. The interfacial nanostructure and lubricity may be changed by applying a potential. EXPERIMENTS: The bulk structure and viscosity have been investigated using small angle X-ray scattering (SAXS) and rheometry. The interfacial structure and lubricity as a function of potential have been investigated using atomic force microscopy (AFM). The electrochemistry has been investigated using cyclic voltammetry. FINDINGS: [P6,6,6,14] [AOT] shows sponge-like bulk nanostructure with distinct interdigitation of cation-anion alkyl chains. Shear-thinning occurs at 293 K and below, but becomes less obvious on heating up to 313 K. Voltammetric analysis reveals that the electrochemical window of [P6,6,6,14] [AOT] on a gold micro disk electrode exceeds the potential range of the AFM experiments and that negligible redox activity occurs in this range. The interfacial layered structure of [P6,6,6,14] [AOT] is weaker than conventional ILs and SAILs, whereas lubricity is better, confirming the inverse correlation between the near-surface structure and lubricity. The adhesive forces of [P6,6,6,14] [AOT] are lower at -1.0 V than at open circuit potential and +1.0 V, likely due to reduced electrostatic interactions caused by shielding of charge centres via long alkyl chains.

Interfacial mechanisms governing cyclopentane clathrate hydrate adhesion/cohesion.

The present work uses a micromechanical force apparatus to directly measure cyclopentane clathrate hydrate cohesive force and hydrate-steel adhesive force, as a function of contact time, contact force and temperature. We present a hydrate interparticle force model, which includes capillary and sintering contributions and is based on fundamental interparticle force theories. In this process, we estimate the cyclopentane hydrate tensile strength to be approximately 0.91 MPa. This hydrate interparticle force model also predicts the effect of temperature on hydrate particle cohesion force. Finally, we present the first direct measurements of hydrate cohesive force in the gas phase to be 9.1 ± 2.1 mN/m at approximately 3 °C (as opposed to 4.3 ± 0.4 mN/m in liquid cyclopentane).

The choice of droplet size probability distribution function for oil spill modeling is not trivial.

The droplet size distribution (DSD) formed by gas-saturated oil jets is one of the most important characteristics of the flow to understand and model the fate of uncontrolled deep-sea oil spills. The shape of the DSD, generally modeled as a theoretical lognormal, Rosin-Rammler or non-fundamental distribution function, defines the size and the mass volume range of the droplets. Yet, the fundamental DSD shape has received much less attention than the volume median size (d50) and range of the DSD during ten years of research following the Deepwater Horizon (DWH) blowout. To better understand the importance of the distribution function of the droplet size we compare the oil rising time, surface oil mass, and sedimented and beached masses for different DSDs derived from the DWH literature in idealized and applied conditions, while keeping d50 constant. We highlight substantial differences, showing that the probability distribution function of the DSD for far-field modeling is, regardless of the d50, consequential for oil spill response.

Stem Cell Mechanosensation on Gelatin Methacryloyl (GelMA) Stiffness Gradient Hydrogels.

Stiffness gradient hydrogels are a useful platform for studying mechanical interactions between cells and their surrounding environments. Here, we developed linear stiffness gradient hydrogels by controlling the polymerization of gelatin methacryloyl (GelMA) via differential UV penetration with a gradient photomask. Based on previous observations, a stiffness gradient GelMA hydrogel was created ranging from ~ 4 to 13 kPa over 15 mm (0.68 kPa/mm), covering the range of physiological tissue stiffness from fat to muscle, thereby allowing us to study stem cell mechanosensation and differentiation. Adipose-derived stem cells on these gradient hydrogels showed no durotaxis, which allowed for the screening of mechanomarker expression without confounding directed migration effects. In terms of morphological markers, the cell aspect ratio showed a clear positive correlation to the underlying substrate stiffness, while no significant correlation was found in cell size, nuclear size, or nuclear aspect ratio. Conversely, expression of mechanomarkers (i.e., Lamin A, YAP, and MRTFa) all showed a highly significant correlation to stiffness, which could be disrupted via inhibition of non-muscle myosin or Rho/ROCK signalling. Furthermore, we showed that cells plated on stiffer regions became stiffer themselves, and that stem cells showed stiffness-dependent differentiation to fat or muscle as has been previously reported in the literature.