Carbomer networks like AC-Carbomer 940 and 980 anchor oil droplets up to a 0.45 volume fraction by establishing yield stress microgels above 40 Pa at 0.20 wt% concentration within pH 6.0–7.2, preventing gravitational separation across 24-month stability trials.
Yield stress microgels formed by crosslinked polyacrylic acid determine whether dispersed oil droplets remain suspended or coalesce during long-term storage.
At an active concentration of 0.20 wt%, AC-Carbomer 940 and 980 molecules hydrate in deionized water to form coiled structures with an initial viscosity under 500 mPa·s at 25°C. Unneutralized carboxylic acid groups maintain a pH range of 2.7 to 3.2, keeping the polymer backbone tightly packed before alkaline activation.
When formulators add triethanolamine to reach pH 6.8, carboxyl groups ionize into carboxylate anions, forcing polymer chains to uncoil through electrostatic repulsion and boosting yield stress to 65 Pa.
Electrostatic repulsion transforms compact polymer chains into an interconnected spatial network capable of resisting external gravitational forces.
This neutralized microgel matrix exhibits a storage modulus G' of 180 Pa and a loss modulus G'' of 22 Pa during oscillatory rheology testing at 1 Hz frequency, confirming solid-like behavior.
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Hydration state: Unneutralized polymer suspension (pH 2.8) yields viscosity below 500 mPa·s.
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Neutralized state: Sodium hydroxide addition raises pH to 6.5, increasing zero-shear viscosity past 40,000 mPa·s.
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Network packing: Microgel units touch at critical overlap concentration c* of 0.05 wt%.
Adding light mineral oil at a 0.10 volume fraction disperses droplets averaging 3.2 microns throughout the expanded polymer matrix without disrupting the yield stress threshold.
Dispersed oil droplets occupy interstitial spaces within the continuous gel framework, exerting localized pressure on the surrounding polymer chains.
As the oil phase expands to a 0.25 volume fraction, droplets cluster within the aqueous channels, increasing system viscosity at a 1 s^-1 shear rate from 12,000 mPa·s to 28,000 mPa·s.
In a 2024 laboratory trial with 150 emulsion batches, oil volume fractions above 0.35 pushed the flow behavior index n down to 0.18, producing strong shear-thinning behavior under stirring.
| Oil Volume Fraction (ϕ) | Droplet Mean Diameter (μm) | Yield Stress τ0 (Pa) | Viscosity at 10 s−1 (mPa·s) | 90-Day Separation Rate |
| 0.10 | 2.8 | 32 | 1,400 | 0.0% |
| 0.25 | 3.5 | 58 | 2,800 | 0.0% |
| 0.40 | 4.9 | 82 | 5,100 | 0.8% |
| 0.50 | 8.2 | 41 | 3,200 | 12.4% |
Crossing the 0.45 volume fraction threshold causes droplets to press against microgel boundaries, reducing overall network elasticity and lowering G' by 35% across 60-day testing at 40°C.
Droplet crowding alters internal friction, changing how the liquid flows when pushed through a pump or spread onto skin.
When shear rates increase to 1,000 s^-1 during pumping, viscosity drops below 350 mPa·s, allowing smooth fluid transfer through standard factory pipelines.
Removing shear stress allows the polymer chains to recover 92% of their original yield stress within 45 seconds, stopping oil droplet movement before phase separation starts.
Low Shear (Storage) High Shear (Pumping) Rest Period (Recovery)
Droplets trapped Droplets align Network rebuilds
[ O O O O O ] ---> = O = O = O = O = ---> [ O O O O O ]
Yield Stress > 50 Pa Viscosity < 400 mPa·s Recovery = 92% in 45s
Introducing 50 mM sodium chloride shields the negatively charged carboxylate groups, compressing the polymer double layer and dropping overall gel volume by 52%.
Dissolved salts neutralize charge repulsion along the polymer chain, causing the structure to collapse and release suspended oil droplets.
Divalent ions like magnesium at 10 mM concentration reduce yield stress from 65 Pa to 12 Pa, causing 8.5% oil creaming in a 50-sample test cohort stored at 45°C for 30 days.
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Monovalent salt effect: 100 mM NaCl reduces storage modulus G' by 60% within 10 minutes.
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Divalent salt effect: 5 mM CaCl2 causes local polymer precipitation and instant droplet coalescence.
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pH shift effect: Dropping pH below 4.5 protonates carboxyl groups, destroying the yield stress matrix.
High-shear homogenization at 10,000 rpm for over 5 minutes breaks long polymer backbones, permanent cutting gel viscosity by 48% in batch production runs.
Mechanical force cuts polymer chains if applied after neutralization, leaving the emulsion vulnerable to temperature-induced separation.
Formulators prevent chain cleavage by dispersing AC-Carbomer 940 and 980 into the oil-water mixture at 65°C before adding neutralizing agents during the final cooling phase.
Adding 0.5 wt% non-ionic emulsifiers like Polysorbate 60 lowers interfacial tension to 4.2 mN/m, working alongside the polymer yield stress to maintain stability across 12 freeze-thaw cycles.