8.3 Granular jamming

Granular jamming is a fascinating phenomenon in soft robotics. It allows materials to switch between fluid-like and solid-like states by applying external pressure. This unique property enables the creation of adaptable systems with tunable stiffness and shape-changing abilities.

Granular jamming has diverse applications in soft robotics, from adaptive grippers to variable stiffness structures. By understanding the physics and optimizing materials, researchers can harness this phenomenon to develop innovative soft robotic devices with enhanced capabilities and performance.

Granular jamming overview

• Granular jamming is a phenomenon where a collection of granular particles can transition between a fluid-like and solid-like state by applying an external stimulus, such as pressure or vacuum
• This unique ability to change material properties has made granular jamming a promising approach for developing adaptable and versatile soft robotic systems
• Granular jamming allows for tunable stiffness, shape conformity, and controllable gripping force, making it well-suited for applications in soft robotics

Definition of granular jamming

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• Granular jamming refers to the process by which a granular material, consisting of a large number of discrete particles, can switch between a loose, fluid-like state and a rigid, solid-like state
• This transition occurs when an external stimulus, such as a vacuum or pressure, is applied to the granular material, causing the particles to pack together more tightly and interlock
• In the jammed state, the granular material exhibits increased stiffness, strength, and resistance to deformation, while in the unjammed state, it can flow and conform to external shapes

Transition between fluid-like and solid-like states

• The transition between fluid-like and solid-like states in granular jamming is governed by the applied external stimulus and the properties of the granular material
• When the external stimulus is applied, the particles are forced together, increasing the interparticle contacts and friction, resulting in a jammed, solid-like state
• Removing the external stimulus allows the particles to relax and return to a loose, fluid-like state, where they can flow and rearrange
• The jamming transition is reversible and can be controlled by modulating the applied stimulus, enabling granular jamming-based devices to switch between soft, compliant states and rigid, load-bearing states as needed

Physics of granular jamming

• Understanding the underlying physics of granular jamming is crucial for designing and optimizing soft robotic systems that leverage this phenomenon
• The behavior of granular materials during jamming is governed by complex interactions between individual particles, including friction, contact forces, and geometric constraints
• Studying the physics of granular jamming helps to predict and control the mechanical properties of jammed systems, such as stiffness, strength, and deformation characteristics

Interparticle friction

• Interparticle friction plays a critical role in the jamming behavior of granular materials, as it determines the resistance to sliding and rearrangement of particles
• Higher interparticle friction leads to stronger jamming and increased stiffness, as particles are more likely to interlock and resist relative motion
• The magnitude of interparticle friction depends on factors such as particle surface roughness, material properties, and the presence of adhesive forces
• Controlling interparticle friction, through material selection or surface modification, can be used to tune the jamming response and mechanical properties of granular systems

Particle shape and size effects

• The shape and size of the granular particles significantly influence the jamming behavior and resulting mechanical properties
• Irregular particle shapes, such as angular or non-spherical grains, tend to promote stronger jamming due to increased interlocking and geometric constraints compared to spherical particles
• Particle size distribution also affects jamming, with polydisperse mixtures (particles of varying sizes) generally exhibiting higher packing densities and stronger jamming than monodisperse systems (particles of uniform size)
• Optimizing particle shape and size can be used to enhance the performance of granular jamming-based devices, such as increasing stiffness, improving shape conformity, or reducing the required jamming pressure

Packing density and volume fraction

• Packing density and volume fraction are key parameters that describe the spatial arrangement and compactness of granular materials
• Packing density refers to the fraction of the total volume occupied by the solid particles, while volume fraction is the ratio of the solid volume to the total volume (solid + void)
• Higher packing densities and volume fractions generally result in stronger jamming and increased mechanical properties, as particles have more contacts and less room for rearrangement
• The maximum achievable packing density depends on factors such as particle shape, size distribution, and the applied compressive stress during jamming
• Controlling packing density and volume fraction, through material selection, particle processing, or applied pressure, can be used to optimize the performance of granular jamming systems

Materials for granular jamming

• The selection of appropriate granular materials is essential for achieving desired performance in granular jamming-based soft robotic devices
• Granular materials used for jamming should exhibit favorable properties such as high strength, low density, good flowability, and compatibility with the intended application environment
• The choice of material also affects other aspects of the jamming system, such as the required actuation pressure, response time, and durability

Common granular materials

• Various granular materials have been explored for use in jamming-based soft robotics, each with its own advantages and limitations
• Ground coffee is a popular choice due to its irregular particle shape, low cost, and good jamming performance, but it may be susceptible to moisture and degradation over time
• Plastic beads, such as polyethylene or polystyrene, offer good flowability, chemical resistance, and reusability, but may have lower jamming strength compared to irregular particles
• Glass beads provide high stiffness and abrasion resistance, but their high density and limited compressibility may require higher jamming pressures
• Other materials, such as sand, sugar, salt, and synthetic particles (silicone, rubber) have also been investigated for specific applications or to achieve desired properties

Material properties and selection criteria

• When selecting granular materials for jamming applications, several key properties and criteria should be considered:
  • Particle size and shape: Irregular, angular particles generally provide stronger jamming than spherical particles, while size distribution affects packing density and flowability
  • Density: Lower density materials are preferred to minimize the weight of the jamming system, but may require higher jamming pressures to achieve the desired stiffness
  • Strength and stiffness: Materials with high inherent strength and stiffness can enhance the load-bearing capacity and rigidity of the jammed state
  • Flowability: Good flowability ensures easy transition between fluid-like and solid-like states and helps to avoid clogging or inhomogeneous jamming
  • Environmental compatibility: The granular material should be stable and resistant to the intended operating conditions, such as temperature, humidity, and chemical exposure
  • Cost and availability: Practical considerations, such as material cost, ease of procurement, and reusability, are important for scalable and economical soft robotic applications
• Material selection often involves trade-offs between different properties, and the optimal choice depends on the specific requirements and constraints of the target application

Granular jamming applications in soft robotics

• Granular jamming has found diverse applications in soft robotics, leveraging its unique ability to control stiffness, shape, and gripping force
• The adaptability and versatility of granular jamming make it well-suited for tasks that require conformability, variable stiffness, or controllable grasping
• Integrating granular jamming with other soft robotic technologies, such as pneumatic actuators or flexible sensors, can further enhance the capabilities and performance of soft robotic systems

Granular jamming-based grippers

• Granular jamming has been widely employed in the development of adaptive, conformable grippers for grasping and manipulation tasks
• Jamming-based grippers consist of a flexible membrane filled with granular material, which can conform to the shape of the target object when unjammed and then solidify upon jamming to provide a secure grip
• The gripping force and stiffness can be controlled by adjusting the jamming pressure, enabling the gripper to handle objects of various shapes, sizes, and fragility
• Examples of granular jamming grippers include the universal gripper developed by Empire Robotics (now Soft Robotics Inc.) and the particle-based jammable manipulator by Cornell University

Granular jamming for variable stiffness

• Granular jamming can be used to create soft robotic structures with variable stiffness, allowing them to switch between compliant and rigid states as needed
• Variable stiffness is useful for applications that require adaptability, such as conformable interfaces, impact absorption, or load-bearing structures
• By controlling the jamming pressure, the stiffness of the granular material can be modulated, enabling the soft robotic structure to adjust its mechanical properties in real-time
• Examples of variable stiffness applications include jammable soft robotic arms, adjustable exoskeletons, and conformable wearable devices

Integration with other soft robotic technologies

• Granular jamming can be combined with other soft robotic technologies to create hybrid systems with enhanced capabilities and performance
• Integrating granular jamming with pneumatic actuators allows for the development of soft robots that can change shape, stiffness, and force output, enabling more versatile and adaptive behavior
• Granular jamming can also be combined with flexible sensors, such as stretchable electronics or soft tactile sensors, to enable proprioceptive and exteroceptive sensing in soft robotic systems
• Examples of integrated systems include pneumatically actuated soft robots with jammable reinforcements, soft grippers with embedded tactile sensors, and wearable devices with adjustable stiffness and sensing capabilities

Modeling and simulation of granular jamming

• Modeling and simulation play a crucial role in understanding the behavior of granular jamming systems and optimizing their design for soft robotic applications
• Computational models can capture the complex interactions between granular particles, predict the jamming transition and resulting mechanical properties, and guide the selection of materials and operating conditions
• Various modeling approaches, ranging from discrete element methods to continuum models, have been employed to study granular jamming at different scales and levels of detail

Discrete element method (DEM)

• The discrete element method (DEM) is a popular approach for modeling granular materials, including granular jamming systems
• In DEM, each individual particle is represented as a separate element, and the interactions between particles are modeled based on contact mechanics and force-displacement laws
• DEM simulations can capture the detailed behavior of granular materials, such as particle rearrangement, force chains, and jamming transitions, providing insights into the microscopic mechanisms underlying granular jamming
• DEM models can be used to study the effects of particle properties (shape, size, friction) on jamming behavior and to optimize the design of granular jamming-based devices

Continuum modeling approaches

• Continuum models treat the granular material as a continuous medium, describing its behavior using macroscopic variables such as stress, strain, and density
• Continuum approaches, such as the Mohr-Coulomb plasticity model or the critical state soil mechanics framework, can capture the bulk behavior of granular materials and predict the jamming transition and mechanical properties
• Continuum models are computationally less expensive than DEM and are suitable for modeling larger-scale systems or for coupling with other physical phenomena (e.g., fluid-structure interaction)
• However, continuum models may not capture the detailed particle-level interactions and may require calibration based on experimental data or micromechanical models

Multiphysics modeling of coupled systems

• Granular jamming-based soft robotic systems often involve the coupling of multiple physical phenomena, such as solid mechanics, fluid dynamics, and electromechanics
• Multiphysics modeling approaches are needed to capture the complex interactions between the granular material, the soft robotic structure, and the actuation and sensing components
• Coupled models can simulate the deformation of the soft robotic structure under the influence of granular jamming, the fluid-structure interaction during pneumatic actuation, or the response of embedded sensors to external stimuli
• Multiphysics simulations can help to optimize the design of integrated granular jamming-based soft robotic systems, predict their performance, and guide the selection of materials and operating conditions

Experimental characterization of granular jamming

• Experimental characterization is essential for validating computational models, understanding the behavior of real granular jamming systems, and informing the design and optimization of soft robotic devices
• Various experimental techniques and setups have been developed to measure the key properties and performance metrics of granular jamming, such as the jamming transition pressure, mechanical properties in the jammed state, and the microstructural evolution during jamming

Measuring jamming transition pressure

• The jamming transition pressure is a critical parameter that determines the onset of jamming and the required actuation pressure for granular jamming-based devices
• Experimental setups typically involve a granular sample enclosed in a flexible membrane, which is subjected to controlled pressure or vacuum while monitoring the sample's volume or resistance to deformation
• The jamming transition pressure can be identified by the sudden change in the sample's response, such as a sharp increase in stiffness or a decrease in compressibility
• Factors influencing the jamming transition pressure, such as particle properties, packing density, and membrane material, can be systematically investigated through experimental studies

Evaluating mechanical properties in jammed state

• Characterizing the mechanical properties of granular materials in the jammed state is crucial for assessing the load-bearing capacity, stiffness, and strength of granular jamming-based devices
• Experimental techniques, such as compression testing, indentation, or three-point bending, can be used to measure the force-displacement response, elastic modulus, and yield strength of jammed granular samples
• The effects of various parameters, such as the applied jamming pressure, particle properties, or sample geometry, on the mechanical properties can be systematically investigated
• Experimental data can be used to validate computational models, develop constitutive relations, and guide the design of granular jamming-based soft robotic structures

Imaging techniques for granular media

• Imaging techniques play a vital role in understanding the microstructural evolution and particle-level interactions during granular jamming
• X-ray computed tomography (CT) can provide high-resolution 3D images of the internal structure of granular materials, allowing for the visualization and quantification of particle packing, force chains, and void space distribution
• Digital image correlation (DIC) can be used to measure the surface deformation and strain fields of granular samples during jamming, providing insights into the local deformation mechanisms and failure modes
• Optical imaging techniques, such as particle image velocimetry (PIV) or photoelastic stress analysis, can capture the flow fields, particle trajectories, and stress distributions in granular materials
• Imaging data can be used to validate and inform computational models, elucidate the fundamental mechanisms of granular jamming, and guide the optimization of particle properties and system design

Design considerations for granular jamming systems

• Designing effective and efficient granular jamming-based soft robotic systems requires careful consideration of various factors, such as particle properties, membrane material selection, and pressure control and actuation methods
• Optimizing these design elements is crucial for achieving the desired performance, reliability, and scalability of granular jamming devices in soft robotic applications

Optimizing particle properties

• The properties of the granular particles, such as size, shape, and material, significantly influence the jamming behavior and resulting mechanical properties of the system
• Particle size and size distribution can be optimized to achieve high packing densities, good flowability, and uniform jamming, while minimizing the required jamming pressure
• Particle shape can be engineered to enhance interlocking and jamming strength, for example, by using irregular, angular, or non-spherical particles
• Material selection for particles should consider factors such as density, stiffness, strength, and environmental compatibility, depending on the specific application requirements
• Computational modeling and experimental characterization can guide the optimization of particle properties for specific granular jamming applications

Membrane material selection and design

• The flexible membrane enclosing the granular material plays a critical role in the performance and durability of granular jamming-based devices
• Membrane materials should exhibit high flexibility, low stiffness, and good puncture and tear resistance to allow for large deformations and shape conformity during the unjammed state
• The membrane material should also be compatible with the granular particles and the operating environment, considering factors such as chemical resistance, temperature stability, and moisture permeability
• Membrane thickness and geometry can be optimized to achieve the desired deformation characteristics, jamming pressure, and actuation response
• Novel membrane designs, such as multi-layered structures or reinforced patterns, can be explored to enhance the performance and reliability of granular jamming systems

Pressure control and actuation methods

• Effective pressure control and actuation are essential for achieving reliable and responsive jamming transitions in granular jamming-based soft robotic devices
• Pneumatic systems, using compressed air or vacuum, are the most common method for controlling the jamming pressure, as they allow for fast, reversible, and easily controllable actuation
• Hydraulic systems, using incompressible fluids, can provide higher force output and stiffness compared to pneumatic systems, but may be more complex and less responsive
• Mechanical actuation methods, such as cable-driven or motor-driven systems, can be used to apply compressive stress to the granular material, enabling more compact and self-contained jamming devices
• Pressure control algorithms and feedback systems can be implemented to ensure precise, stable, and repeatable jamming transitions, compensating for factors such as air leakage, material relaxation, or external disturbances
• The choice of actuation method and pressure control strategy depends on the specific requirements of the application, such as force output, response time, portability, and control precision

Challenges and limitations of granular jamming

• While granular jamming offers unique advantages for soft robotic applications, it also presents several challenges and limitations that need to be addressed for reliable and scalable implementation
• Understanding and mitigating these challenges is crucial for the successful development and deployment of granular jamming-based soft robotic systems

Hysteresis and repeatability

• Granular jamming exhibits hysteretic behavior, meaning that the jamming and unjamming paths may not be identical, leading to differences in the mechanical properties and shape of the system between cycles
• Hysteresis can arise from factors such as particle rearrangement, plastic de