V-Ray VolumeGrid Foam

This component is intended to shade particles such as splash and foam but may be used to render sparks, sand or thin cigarette smoke as well.

Mode – Specifies how the particles are represented internally for raytracing when rendering on the CPU.

Volumetric –  Faster to render but does not export deep images and many standard V-Ray render elements such as velocity, multi matte, zdepth, etc. Additinally, other V-Ray volumetric effects (e.g. Environment Fog, Aerial Perspective) may not be able to blend together with the Particle shader when rendering in Volumetric mode.
Volumetric Geometry – Slightly slower to render - produces procedural geometry that contains multiple transparent layers. Supports the majority of the standard V-Ray render elements. As a drawback, note that the Approximate and Approximate + Shadows options for the Scattering Type parameter are not supported.

Lights

Use Volumetric Light Cache – Enables light caching, which can speed up bucket rendering considerably.

Light Cache Speedup – (Available only when Use Volumetric Light Cache is enabled.) Reduces the quality of the Volume Light Cache and increases the rendering speed. You can increase this and gain render speed as long as you don't start getting artifacts and excessive flickering in animation. Note that when using complex lighting with many light sources or dome lights with HDRI maps, combined with Bubbles, Cellular or Splashes mode rendering with Highlights enabled, increasing this option causes the highlights to jitter over the surfaces of bubbles and this may cause flickering or noise in animation.

Scattering Type – Controls how the light rays are scattered inside the particle volume.

Ray-traced (GI only) – Enables physically accurate scattering of light rays. This mode produces the most realistic results but it's the slowest to render. It requires V-Ray with enabled Global Illumination, otherwise the rendered result would be the same as if the option is Disabled. The Diffuse Multiplier does not affect the rendering in this mode.
Disabled – Disables scattering. The Diffuse Multiplier value can be used to correct the brightness because without light scattering the particles would generally render darker.
Approximate – Uses an approximate formula which is faster than Ray-Traced scattering and produces good-looking results.
Approximate+Shadows – Same as Approximate, but also affects the strength of shadows over the scene geometry.

Motion Blur

Multiplier – Can be used to strengthen or weaken the motion blur effect. This value can also be negative and would change the motion blur direction. It would also affect the V-Ray Velocity Render Element, even if motion blur is disabled.

Type – Controls the motion blur effect.

From Render Settings – The current renderer's motion blur setting is used.
Force Enabled – The content is rendered with motion blur regardless of the global setting.
Force Disabled – The content is rendered without motion blur regardless of the global setting.

Global Illumination

Generate GI – Controls whether the object generates indirect illumination. Note that to obtain the caustics effect you must set the appropriate value for the Caustics multiplier below as well as put some objects that generate caustics in the scene.

Receive GI – Controls whether the object receives indirect illumination. A multiplier can be specified for the received indirect illumination.

Generate Caustics – When enabled, the selected objects refract the light coming from light sources that are caustics generators, so that caustics are produced.

Receive Caustics – When enabled, the selected objects become caustic receivers. When light is refracted by objects that generate caustics the resulting caustics are only visible when they are projected on caustics receivers.

Generate GI Multiplier – Specifies a multiplier for the amount GI generated by the material.

Receive GI Multiplier – Specifies a multiplier for the amount GI received by the material.

Caustics Multiplier – Specifies a multiplier for the caustics.

Type – Sets the shading mode:

Bubbles – Each particle is shaded as a spherical, transparent, reflective foam bubble. Surfaces of bubbles that intersect one another are visible inside the bubbles. If you get flickering or noisy renders of tightly packed masses of foam such as beer heads, you should switch to Cellular mode. Otherwise, in animation, pairs of bubbles would appear with one bubble completely in front of another in one frame, and completely behind on the next frame, once the bubble's center goes behind the other bubble's center. This would cause abrupt changes in the overall look of the foam mass in animation.
Cellular – Similar to Bubble mode, but replaces intersecting walls between bubbles with a curved wall. This mode is about twice as slow as simple Bubbles but is suitable for close-up foam.
Splashes – Each particle is shaded as a spherical, opaque, reflective droplet. Surfaces of droplets that intersect one another are not visible. Note that when a Liquid Simulator is connected, particles do not render as Splashes when they are inside the liquid volume of the Liquid Simulator. This is because Splashes represents small liquid droplets used to add more detail to the Liquid mesh, so it does not make sense to render them underwater.
Points – Particles are shaded as flat discs, each the size of one pixel by default. This mode is faster than Bubbles, Cellular, and Splashes and thus is suitable for large scale foam and splashes over a large surface such as an ocean surface. This mode is also suitable for rendering non-foamy fluids such as smoke or ink. Because the points are pixel sized, moving the camera away or decreasing the render resolution makes them appear denser, while moving the camera in closer or increasing the render resolution makes the particles look more scattered. Note that Point mode does not do reflections or refractions like the Bubbles, Cellular, and Splashes, so the particles look diffuse and sometimes darker if you render them next to strongly reflective materials.

Count Multiplier – Increases or decreases the number of particles for rendering. When the value is below 1, the particle count decreases, skipping the particles randomly. When the value is above 1, new particles are created and placed randomly among the original ones in a way that attempts to preserve the group's overall shape. Note that you have to export the particle ID channel if you want to render animation, otherwise the particles would start appearing and disappearing in different places during the animation.

Color

Foam Color – Specifies the particle color.

Diffuse Multiplier – Provides an additional option to multiply the color brightness.

Bubbles|Cellular|Splashes

Refraction Index – The IOR of the bubble material. Higher refractive indices produce more pronounced reflection and refraction. 

Bounces – Specifies the maximum depth of reflection/refraction branches. When the limit is reached, the color of the Environment texture is used instead of starting new rays. Using more bounces slows down the rendering considerably, but reduces any flickering that may appear with a higher Refractive Index.

Reflection Cut Off – Starting a reflection ray is an expensive operation because it produces an avalanche of rays that can consume resources very quickly. Because of this, a reflection ray is only started if its result is very visible. This parameter is used to determine the critical visibility at which new reflection rays will start. If the visibility is less than the specified value, the Environment map is used instead of tracing a new ray. This option has no effect if the Bounces are 0.

Highlights Width – Specifies the width of the specular highlights.

Highlights Strength – Specifies a multiplier to control the brightness of the highlights.

Pressure Variation – Used when Type is set to Cellular, which produces a wall between each two bubbles in contact. In nature, no two bubbles in contact have exactly the same internal pressure, and the bubble with the higher pressure pushes against the lower-pressure bubble to produce a curved wall between the two. In the simulation, a random pressure difference is assigned to bubbles in contact, with the Pressure Variation value as the maximum. Larger values result in a more pronounced curve between the bubbles.

Optimize Congestion – When the particles overlap significantly in tight bunches such as beer head simulations, the render speed may drop significantly. With this option, an optimization pre-process is performed that deletes the bubbles that are fully inside other bubbles, and decreases the sizes of significantly overlapped bubbles. This reduces render times with a minimal impact on quality. Enabling this option is highly recommended.

Textures

Override Reflections – Enables reflection map.

Reflection Coloring – Used to represent the coloring of the bubble reflection due to interference. The texture is sampled using the direction instead of the explicit coordinates.

Override Environment – Enables environment map.

Environment – This map is used when the visibility is less than the Reflection Cut Off value or when the renderer's reflection depth is reached. The texture is sampled using the direction instead of the explicit coordinates.

Points

Ignore Particle Size – By default, the point shader calculates the alpha by considering the size of the particle and the distance to it. This option is used when the particle system contains particles that are too large and leave hard tracks with motion blur.

Motion Blur Step –  Unlike the Bubbles, Cellular, and Splashes modes, motion blur in Point mode is calculated by cloning the particle several times and placing the copies along the trajectory. This parameter controls the distance between the copies. The smaller the step, the higher the quality, though at the cost of render time.

Motion Blur Limit – If the particle system contains very fast particles, or if the motion blur step is too small, there is a possibility of rendering overload due to the huge count of particle copies. This parameter sets a limit on the number of clones to keep a reasonable rendering time.

Parameters visible when Type is Bubbles/Cellular/Splashes

Parameters visible when Type is Points