Slime
Slime refers to any moist, sticky, and viscous substance, often with a slippery or gelatinous consistency, such as mud, mucus, or ooze. In nature, it occurs in biological contexts like the secretions of snails or the extracellular matrix of biofilms and slime molds. Scientifically, slime exemplifies non-Newtonian fluids whose viscosity changes under stress, studied in chemistry, physics, and materials science. Slime also appears in technology, culture, and slang. A prominent modern use is as a children's toy, popularized by Mattel in 1976 as a green, squishy putty sold in plastic trash cans for sensory play.[1] This toy slime, a viscoelastic non-Newtonian fluid, gained renewed popularity in the mid-2010s through social media DIY videos and ASMR content, sparking a "slime economy" of custom creations.[2] On November 6, 2025, the toy was inducted into the National Toy Hall of Fame at The Strong National Museum of Play in Rochester, New York, alongside Battleship and Trivial Pursuit.[3]Science and nature
Biology
In biology, slime refers to mucus or extracellular polymeric substances (EPS) secreted by various organisms, including animals, plants, and microorganisms, primarily serving functions such as protection against environmental threats, lubrication for movement, and facilitation of locomotion.[4] These secretions form viscous layers that create barriers, aid in adhesion, or enable gliding over surfaces, adapting to diverse ecological niches.[5] In humans, mucus in the respiratory and digestive systems exemplifies protective slime, composed mainly of water (approximately 95%) and mucin glycoproteins that form a gel-like matrix to trap inhaled pathogens and facilitate their removal via mucociliary clearance.[6] In the digestive tract, this mucus layer lubricates food passage while shielding epithelial cells from mechanical damage and acidic conditions, preventing infections by binding microbes and promoting their expulsion.[7] Gastropods, such as slugs and snails, produce pedal mucus that enables efficient locomotion across rough or vertical surfaces by providing lubrication and temporary adhesion, reducing friction and allowing wave-like foot contractions to propel the animal without energy loss.[8] This slime also offers protection against desiccation and predators in terrestrial environments.[9] Slime molds, classified within the protist group Myxogastria, exhibit a distinctive life cycle featuring a multinucleate plasmodial stage—a mobile, slime-like mass that engulfs food particles for nutrient absorption—before forming fruiting bodies to release spores.[10] Historically, these organisms were misclassified as fungi in the 19th century due to superficial similarities in spore production, but 20th-century microscopy and genetic studies reclassified them as protists in the Amoebozoa phylum, emphasizing their amoeboid motility and lack of chitinous cell walls.[11] Bacterial biofilms, often described as "slime layers," consist of microbial communities embedded in a self-produced EPS matrix, forming through initial attachment to surfaces, microcolony development, maturation into a structured architecture, and eventual dispersion for colonization elsewhere.[4] These biofilms contribute to infections, such as dental plaque where Streptococcus mutans and other bacteria accumulate on teeth to cause caries, or on medical devices like catheters, where they resist antibiotics and immune responses, leading to persistent device-related infections.[12][13] Evolutionary adaptations highlight slime's role in survival across environments; in aquatic settings, hagfish deploy slime rapidly as a defense by secreting vesicles containing keratin threads that unravel and swell in water to form a suffocating net, clogging predators' gills and composed of intermediate filament proteins for tensile strength.[14] In terrestrial contexts, slime in gastropods and plant-associated microbes (like EPS in root mucilage) aids hydration retention and symbiotic interactions, contrasting with aquatic expansions but similarly enhancing resilience to desiccation or predation.[5] These glycoproteins, such as mucins, underpin the viscous properties briefly noted here but analyzed molecularly elsewhere.[15]Chemistry
Slime substances, both natural and synthetic, are characterized by their polymeric compositions that confer viscoelastic properties. In synthetic slimes, commonly used in educational and toy contexts, the primary polymer is polyvinyl alcohol (PVA), derived from white school glue, which consists of long chains of repeating vinyl alcohol units.[16] These chains are cross-linked by borax, or sodium tetraborate decahydrate (Na₂B₄O₇·10H₂O), which dissociates in water to form borate ions that bind to hydroxyl groups on adjacent PVA chains, creating borate ester linkages responsible for the material's elasticity. Alternative synthetic formulations employ guar gum, a natural polysaccharide polymer extracted from guar beans, which similarly cross-links with borate ions to form a gel-like network.[1] Natural analogs of slime, such as biological mucus, rely on mucopolysaccharides like hyaluronic acid, a glycosaminoglycan composed of repeating disaccharide units of D-glucuronic acid and N-acetyl-D-glucosamine, which provides hydration and lubrication through its ability to bind large amounts of water.[17] Mucins, high-molecular-weight glycoproteins featuring a protein core rich in proline, threonine, and serine residues heavily glycosylated with sialic acid and other sugars, form the structural backbone of mucus gels, enabling entanglement and cross-linking via electrostatic interactions and hydrogen bonding.[18] These components, comprising up to 95% water in native mucus, create a viscoelastic matrix analogous to synthetic slimes but tailored for biological functions.[15] The synthesis of toy slime has evolved since its commercial introduction by Mattel in 1976, initially using guar gum cross-linked with sodium borate to produce a viscous, green substance sold in plastic containers.[19] Modern PVA-based slime is prepared by first dissolving PVA glue (typically 4-8% PVA in water) in a container, often with added colorants or fillers for customization. A borax solution (4% sodium tetraborate in water) is then gradually added while stirring; the borate ions rapidly form di-diol complexes with PVA's hydroxyl groups, leading to gelation within minutes as cross-links propagate across polymer chains, resulting in a stretchy, non-Newtonian fluid.[16] For instance, combining 40 cm³ of 4% PVA solution with 10 cm³ of 4% borax solution yields a homogeneous slime, with the reaction's elasticity arising from reversible borate ester bonds.[20] Due to concerns over borax's potential skin irritation, borax-free alternatives have gained popularity, utilizing contact lens solutions containing sodium borate (often 0.1-0.2%) as a milder cross-linker, combined with baking soda (sodium bicarbonate) to adjust pH and activate the borate. In this process, PVA glue is mixed with water and baking soda to form a basic slurry (pH ~8-9), followed by incremental addition of contact lens solution until cross-linking occurs, producing slime with comparable elasticity but reduced boron concentration.[21] This method maintains the core borate-PVA chemistry while minimizing direct exposure to powdered borax.[22] The stability of borax-cross-linked slimes is highly pH-dependent, with optimal cross-linking occurring at pH 7-9, where borate ions (B(OH)₄⁻) predominate and effectively form esters with PVA.[23] At lower pH values (below 5), such as when exposed to acids like vinegar (acetic acid), protonation disrupts the borate esters, breaking cross-links and causing the slime to revert to a liquid state through hydrolysis.[24] Conversely, strong bases (pH >10) can deprotonate the polymer chains excessively, weakening hydrogen bonding and leading to degradation or excessive brittleness, though the borate buffer inherently stabilizes the system around pH 9. Analytical techniques confirm the chemical structure of slimes by probing molecular interactions. Fourier-transform infrared (FTIR) spectroscopy identifies cross-linking through shifts in the O-H stretching band (from ~3300 cm⁻¹ in pure PVA to broader, lower-intensity peaks post-borax addition) and emergence of B-O vibrations around 1400-1100 cm⁻¹, verifying borate ester formation.[25] Rheometry complements this by measuring viscoelastic moduli during gelation; for PVA-borax systems, the storage modulus (G') surpasses the loss modulus (G'') above a critical borax concentration, indicating a transition to a cross-linked network, thus confirming the extent of chemical bonding indirectly through mechanical response.[26] These methods ensure rigorous characterization without invasive sampling.Physics
Slime exhibits the properties of a shear-thinning non-Newtonian fluid, in which its apparent viscosity decreases as shear stress is applied. This results in behavior where the material flows readily when squeezed or agitated but resists deformation under low or slow stress, maintaining a more solid-like consistency. Such characteristics arise from the polymeric network structure that temporarily aligns under force, facilitating easier flow.[27] The rheological response of slime is commonly described by the Herschel-Bulkley model, which generalizes the Bingham plastic model by incorporating a yield stress and nonlinear viscosity: \tau = \tau_0 + K \dot{\gamma}^{n}, where \tau is the shear stress, \tau_0 is the yield stress below which no flow occurs, K is the consistency index, \dot{\gamma} is the shear rate, and n < 1 is the flow behavior index indicating pseudoplastic (shear-thinning) behavior. This model captures the yield stress observed in slime (from the Bingham component, where n=1 and K = \eta_p) and the post-yield shear-thinning regime where viscosity drops nonlinearly with increasing shear rate, providing an empirical fit to the relationship between shear stress and shear rate.[28] Experimental investigations of slime's properties often employ rotational viscometers to measure flow curves, revealing a rest viscosity typically ranging from $10^1 to $10^3 Pa·s, reflecting its gel-like stasis that transitions to fluid flow under stress. A representative analog is Oobleck, a cornstarch-water suspension that demonstrates non-Newtonian dilation under sudden impact, hardening momentarily despite its shear-thickening nature, which contrasts yet illustrates the broader class of complex fluid responses akin to slime's variability.[29] In physics research, slime serves as a model system for exploring complex fluids and soft matter phenomena, including chaotic mixing dynamics and granular-like flows. Studies from the 2010s have utilized slime analogs to investigate pattern formation and instability in viscoelastic materials, providing insights into chaotic advection during mixing processes and the transition between fluid and solid states in granular systems.[30]Technology and applications
Computing
In computing, SLIME (Superior Lisp Interaction Mode) is an Emacs extension designed for interactive development in Common Lisp, originating as an extension of the earlier SLIM mode created by Eric Marsden in mid-2003.[31] Developed further as an open-source project by Luke Gorrie and Helmut Eller, SLIME integrates a read-eval-print loop (REPL) directly into Emacs, enabling seamless evaluation of Lisp code within the editor environment.[32] Key features include auto-completion for symbols, syntax highlighting, and debugging tools such as the SLDB (SLIME Debugger) stepper, which allows stepping through code execution and inspecting variables. SLIME's evolution began with its initial release supporting CMU Common Lisp and has since incorporated integrations like Quicklisp for package management, facilitating easy loading of libraries during development sessions. By the 2020s, enhancements addressed performance issues, including the Sly fork initiated in 2018 by João Távora, which introduced a redesigned REPL based on Emacs's comint.el for better concurrency support and debugging utilities like "stickers" for code annotation.[33] SLIME and its derivatives remain staples in symbolic computing and AI development, where Common Lisp's expressiveness supports rapid prototyping of algorithms in areas like machine learning and knowledge representation.[31] Technically, SLIME communicates with Lisp implementations via the Swank server, a backend process that handles requests over a TCP connection, enabling features like remote evaluation and inspector views. Common commands includeC-c C-c for compiling and evaluating the top-level form at point, and C-c C-k for loading entire files, streamlining the edit-compile cycle.
Beyond Lisp environments, the term "slime" appears in computational simulations inspired by biological slime molds, such as algorithms mimicking Physarum polycephalum for pathfinding in robotics. These models approximate shortest paths by simulating protoplasmic tube formation, as demonstrated in a 2011 mathematical framework showing Physarum's ability to compute optimal routes between nodes.[34] In swarm robotics, such algorithms enable decentralized exploration and network formation, where robot agents adapt paths based on virtual nutrient gradients, achieving efficiency comparable to biological foraging.[35]
Slime mold behaviors are also modeled using cellular automata (CA), discrete grid-based simulations that replicate growth and decision-making processes. For instance, CA rules can emulate Physarum's network optimization, where cells transition states to form fault-tolerant structures, as in models that solve transport problems through iterative neighborhood updates.[36] Variants of Conway's Game of Life incorporate slime-like growth patterns, such as diffusive expansion rules that produce amorphous, spreading entities in two-dimensional grids, useful for studying emergent computation in unconventional substrates.[37]