Spray Drift Control Agents: Chemistry and Formulation

Spray Drift and Droplet Size: Why Control Agents Exist

Drift starts at the nozzle, where a liquid sheet breaks into droplets of many sizes. The smaller a droplet, the longer it stays airborne and the more easily a crosswind carries it away. The spray drift review in Emilia Hilz's Wageningen thesis, shortened in a 2013 paper with Vermeer, relates drift to the percentage of fine droplets. Drift control agents exist to reduce that fine fraction at the formulation level.

• Fine droplets — the review reports that droplets below 100 µm contribute significantly to drift losses. Other researchers it cites treat droplets below 50, 150 or 200 µm as the most drift-prone.
• Evaporation — droplets of a few microns can evaporate before they settle, and evaporation shrinks fine droplets further depending on humidity and temperature.
• Operating factors — wind speed, application height, driving speed and nozzle spacing all influence the fate of droplets. Air-assisted and shielded sprayers can reduce drift.
• Nozzle design — orifice size, operating pressure and entrained air set droplet size and velocity. Air-induction nozzles incorporate air bubbles into the liquid and produce coarser sprays than a conventional flat fan.
• Formulation and tank additives — a 2019 open-access field study lists nozzle type, nozzle size, formulation type and tank additives as drift reduction technologies. Each one modifies the droplet size distribution at atomisation.

The same 2019 study notes that formulation and adjuvant choice can have effects equivalent to nozzle type on downwind deposition. That is why this article treats the additive as an engineering lever. The classes above come from the review, so check the current edition of the classification standard before quoting limits in a specification. To see how an additive moves a spray from one class to the next, look at how the liquid sheet breaks up.

How Spray Drift Control Agents Change Atomisation

Atomisation is a contest between the forces that stretch a liquid sheet and the forces that resist stretching. A drift control agent works by changing how the liquid resists. The review identifies three physical properties that control spray formation: shear and extensional viscosity, surface tension, and the presence of inhomogeneities such as emulsion droplets or solid particles. Each drift control chemistry acts through one or more of them.

• Viscosity route — polymer-based drift control additives were commonly used in the United States and Australia when the review was written. The viscosity increase shifts the droplet size distribution toward larger sizes.
• Sheet breakup — a spray liquid without inhomogeneities breaks up through waves that grow into ligaments and then droplets. Viscous liquids sustain high stretching before breaking and suppress the growth of those disturbances.
• Extensional viscosity — this is the resistance of a fluid to stretching in an extensional flow. For a Newtonian liquid it is three times the shear viscosity, a ratio known as the Trouton ratio.
• Polymer efficiency — polymer liquids that show elongational thickening increase droplet size most efficiently. Their shear viscosity is typically much smaller than their extensional viscosity, so far less polymer is needed when extensional viscosity rises.
• Emulsion route — dilute emulsions perforate the liquid sheet at an early stage, shorten the sheet length at breakup and give coarser sprays.

The third row is not a drift control chemistry, yet surfactants are part of many pesticide formulations and additives and can undo the first two. Which chemistry suits a product depends on stability and nozzle behaviour, which the next section compares.

Polymer, Emulsion and Oil Chemistry in Drift Control Agents

Three chemical families dominate drift control: water-soluble polymers, oil emulsions and surfactant-oil adjuvants. Each one pulls the droplet spectrum coarser through a different physical route. They also fail in different ways, and that matters more to a formulator than the headline drift reduction. The review and the recent studies allow a comparison on mechanism and stability.

• Polymer structure — three parameters set the extensional viscosity: chain rigidity, molecular weight and concentration. Rigid, low molecular weight polymers show constant extensional viscosity, while high molecular weight flexible polymers show a strong rise with strain rate.
• Critical elongational rate — extensional viscosity begins to rise only above a critical rate, which is lower for higher molecular weight and higher concentration.
• Synthetic polymers — polyacrylamide serves as the industry reference in a 2024 open-access study of an alginate-based adjuvant. Non-ionic and anionic polyacrylamides differ in shear stability, according to the review.
• Natural polysaccharides — stiffer polysaccharides are less sensitive to shear. The 2024 study tested sodium alginate across pressures, doses and nozzles against polyacrylamide and concluded that it has significant potential to mitigate drift.
• Emulsified oils — vegetable oil, mineral oil and organosilicone emulsions increase droplet size and reduce the fine fraction through flat fan, hollow cone and twin-fluid nozzles. Recirculation in the tank does not remove this effect.
• Surfactant-oil adjuvants — in a three-year ground-sprayer field study, adjuvants based on a paraffinic petroleum oil and on a modified vegetable oil, each with nonionic surfactant, reduced deposition. The vegetable-oil adjuvant reduced drift more in that study.
• Pesticide formulation type — the same field study measured 37 percent less drift from a suspension concentrate than from a wettable powder of the same active ingredient. The authors call generalisations premature.

Our guides to agricultural adjuvants and to methylated seed oil and crop oil concentrate adjuvants cover the oil and surfactant side in depth. The 2019 field study adds evidence on oil type. The vegetable-oil result matches earlier work the authors cite, although other researchers found the effect of oil type on droplet size to be small.

Chemistry sets the potential, and the pump and the water decide how much of it reaches the nozzle.

Shear Stability, Water Quality and Tank-Mix Limits

A drift control polymer is specified at the bench but stressed in the pump and the tank. The long chains that give the best extensional viscosity are the ones that break under shear. The review quotes estimated nozzle shear rates from roughly 16,000 to 700,000 per second, against about 50 per second in the tank. Recirculation, fertiliser and water quality then decide whether the viscosity measured on the bench survives in the field.

• Recirculation — intensive recirculation, especially at the end of a spray session, can easily destroy weak polymeric structures, lower the viscosity and produce finer sprays than before shearing.
• Pump type — ASTM E2408 states that many drift control polymers are irreversibly destroyed by recirculation through high-shear pumps such as gear or centrifugal pumps.
• Chain type — long-chain polymers are highly sensitive to recirculation, while stiffer polysaccharides are less so.
• pH and salts — polymer viscosity depends on salt concentration and pH when fertiliser is added to the tank. A higher pH can neutralise polyelectrolyte chains and make them contract.
• Water quality — ASTM E2408 notes that results are affected by the quality of the make-up water, which should be stated with the result.
• Emulsion systems — the drift reduction from emulsified oils is not affected by tank recirculation.

Surviving the pump is necessary, but the nozzle and the crop decide whether the coarser spray is useful.

Nozzle Interaction and the Efficacy Trade-Off

Drift control agents are never used alone, because the nozzle decides how much of the effect survives. A 2026 study of pulse-width-modulated nozzles found that droplet size depends on adjuvant type, duty cycle and nozzle type together. Coarser sprays also change coverage on the target. Formulators therefore design for a droplet size window and not for a maximum.

• Flat fan and hollow cone — polymer and emulsion routes coarsen the spray, while lower dynamic surface tension makes it finer.
• Air induction — additives that lower surface tension often produce coarser sprays than water in these nozzles. Dilute emulsions can prevent air intake, whereas microemulsions can enhance it, and emulsions can reduce mean droplet size.
• Spray geometry — higher polymer concentration decreases the spray cone angle and fan width, so the footprint shrinks. In an extreme case the nozzle spacing has to be readjusted.
• Pulse-width modulation — in the 2026 study a drift control agent markedly increased droplet size and reduced fine droplets, while a non-ionic surfactant gave a moderate increase. Both made the spectra more uniform.
• Coverage and retention — impaction increases with droplet size, while retention generally increases with decreasing droplet size and decreasing dynamic surface tension.

The review warns that coarser sprays from low-drift nozzles or additives can reduce biological efficacy, because large droplets give relatively low coverage and can rebound or scatter. It concludes that a narrow droplet size spectrum is required for an optimised application. The 2026 authors reach the same practical conclusion from their data, which is that adjuvant formulations should be tailored to each nozzle type.

Measurement is how those claims are turned into evidence.

Testing Drift Control Agents: Droplet Size, Screen Factor and Wind Tunnels

Drift cannot be read from a bench instrument the way viscosity can, so drift control claims rest on a ladder of tests. Droplet spectra come first, extensional viscosity second, wind tunnel work third and field trials last. Each rung answers a different question, and none replaces the next. A formulator needs all four to support a claim.

The ASTM test gives a number a formulator can use at the bench. Its scope covers tank mixes with dissolved, emulsified or dispersed materials. Its significance section lists uses such as comparing polymers for drift control potential and comparing tank mixes. It also covers finding how far polymer solutions break down during recirculation. The 2019 field study found that wind-tunnel droplet spectra agreed strongly with field deposition trends, which supports using droplet statistics to estimate the drift reduction of spray solutions.

Regulatory positions on additives vary by country and change over time. When the review was written, drift control additives were permitted in the United Kingdom and, more recently, in the Netherlands. Formulation effects were not yet built into European drift mitigation scenarios. A formulator should confirm the current rules for each target market. With the evidence ladder in place, the remaining job is design.

Formulating a Drift Control Product: Practical Constraints

A drift control product has to coarsen the spray, survive shear, stay compatible with the tank mix and avoid hurting coverage. Those aims pull against each other, so formulation is a negotiation and not a maximisation. The sources above translate into a short list of design decisions. Each one links back to a property that can be measured.

• Polymer selection — balance chain rigidity, molecular weight and concentration against shear stability, because the most efficient long chains are the most fragile.
• Surface tension control — polymers have little effect on surface tension, so surfactants control it. Check that any wetter in the product or the tank does not erase the coarsening.
• Pesticide formulation fit — the 2019 study found that the effect of an adjuvant depended on the formulation type it was combined with. Its authors suggest that selecting a formulation with good drift potential lowers the need for drift adjuvants, whose performance varies between combinations.
• Fertiliser and pH tolerance — polyelectrolyte polymers change viscosity with salt and pH, so test in the target tank mix.
• Test matrix — cover flat fan, pre-orifice and air-induction nozzles, the operating pressure range, defined water quality and a post-pump sample.
• Coverage check — confirm that the coarser spray does not cost efficacy on the intended crop and target.

Our guides to tank mix compatibility testing and to suspension concentrate formulation and wet milling cover the compatibility and formulation-type questions in more detail. Treat the finished product as a system, because the polymer or oil, the surfactant, the carrier and the tank all interact. A drift claim holds only for the combinations that were tested.

Working With an Agrochemical Formulation Consultant

Formulation decisions made at the bench shape how a drift control product behaves in the field. A polymer that thins after prolonged recirculation, a wetter that cancels the effect or a product tested on a single nozzle all trace back to bench-scale choices. An agrochemical formulation consultant tests those decisions against realistic nozzles and tank mixes before launch. Agrochemical product development services then carry the work through documentation and manufacturing.

• Route selection — polymer, emulsion or oil-surfactant chemistry matched to the target market and sprayer fleet.
• Shear stability programme — screen factor measured after a defined pumping regime, in the target water and fertiliser mix.
• Nozzle matrix studies — droplet size spectra across nozzle classes and pressures, reported as median diameter, span and fine fraction.
• Compatibility mapping — behaviour with the pesticide formulation types and tank-mix partners the customer uses.
• Claim substantiation — a test file that names the nozzle, pressure and tank mix, and checks the rules of each market.
• Scale-up and contract manufacturing — moving a bench product to a contract formulation agrochemical manufacturer with defined quality checks.

Anyone planning a new range can start with our guide to starting a herbicide and pesticide formulation unit. The broader picture sits in our overview of agrochemical technology and formulations. The full range of services falls under agro chemicals. Before committing to a polymer, run a short trial that sprays the product through a recirculating pump for a fixed period and compare droplet size before and after.