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The question that comes up most often in an optics lab is a simple one: this material in my hand, which bottle should I fill the tank with? The answer depends on two things — the index of the substrate itself, and the range of index the fluid family can offer plus how finely it can be trimmed. This article collects refractive index values for common optical substrates, transparent plastics and particulate materials into one quick-reference table, maps them grade by grade against the nominal indices of IOTA phenyl silicone fluids, and closes with a blending method you can run on the bench.
Values below are visible-band references at about 589 nm and 25 °C, suitable for screening. For real experiments, measure the part in front of you.
| Material | Approximate index | Where it turns up |
|---|---|---|
| Air | 1.000 | The unmatched baseline |
| Water | 1.333 | Baseline flow medium |
| Living cells and soft tissue | 1.38 | Deep live-cell imaging |
| Dimethyl silicone fluid / PDMS | 1.40–1.41 | Microfluidic chips, elastomer models |
| Fused silica | 1.458 | Precision windows, reference parts |
| Mineral oils and waxes | 1.45–1.48 | Classical immersion media |
| Fumed silica | about 1.45 | Blended with silicone fluid to make optically transparent grease |
| PMMA (acrylic) | 1.49 | Wind tunnel and hydraulic model test sections |
| Soda-lime glass | 1.51–1.52 | Microscope slides, general windows |
| BK7 crown glass | 1.5168 | Lenses, prisms, optical windows |
| Epoxy systems | 1.55–1.60 | Cast models, potted specimens |
| Polycarbonate | 1.585 | Impact-resistant transparent parts |
| Polystyrene | 1.59 | Tracer microspheres, model stock |
| Dense flint glass | above 1.60 | High-index optical elements |
| Band | Index range | Materials covered | Reachability with silicone fluids |
|---|---|---|---|
| Lower | 1.33–1.42 | Water, living tissue, PDMS, some elastomers | Covered by dimethyl fluids; the IOTA 201 family is nominal 1.390–1.410 |
| Middle | 1.45–1.51 | Fused silica, mineral-oil territory, PMMA, soda-lime glass, BK7 | The core of the IOTA phenyl range: 250-30, 556, BJ550, the 255 family and 54 |
| Upper | 1.55–1.59 | Epoxy resins, PC, PS | IOTA 704 (1.5500–1.5600) and IOTA 705 (1.5765–1.5785) land here |
Read across the three bands and the practical point is clear: from biological specimens to engineering plastics, the IOTA phenyl range carries a grade somewhere within 1.39–1.58, so one product line covers a large share of bench matching needs.
| Target substrate | Substrate index | IOTA grades to look at | How to land it |
|---|---|---|---|
| PDMS microfluidic chip | 1.40–1.41 | IOTA 201 dimethyl fluid | Use as delivered; pick a low-viscosity grade so narrow channels fill |
| Fused silica | 1.458 | IOTA 556 (1.46) or IOTA 250-30 (1.47–1.48) | 556 is the closer neighbour; trim with 201 if measurement says so |
| PMMA model | 1.49 | IOTA BJ550 or 255-125 (1.49–1.505) | Usable almost as-is; for tight work, trim against the measured value |
| Soda-lime glass, BK7 | 1.51–1.52 | Blend the 255 family with a high-index partner such as 54 | Raise the high-index share step by step from the measured value |
| Cast epoxy part | 1.55–1.60 | IOTA 704 (1.5500–1.5600) | Blend back toward BJ550 or 255 rather than buying a costly specialist medium |
| PC and PS particles or parts | 1.585–1.59 | IOTA 705 (1.5765–1.5785) | Blend with 704 to close the remaining gap and check viscosity at the same time |
| Fumed-silica filled systems | about 1.45 | Methylphenyl fluid balanced against filler index | The formulation logic behind optically transparent coupling greases |
A single grade rarely lands exactly on the substrate value. Continuous adjustability is what makes a matching fluid genuinely useful.
| Stage | What to do | Why |
|---|---|---|
| Establish endpoints | Measure the two neat components and record them as nL and nH | Real readings can sit off the nominal figures |
| Estimate a first mix | Interpolate the volume ratio between the endpoints for your target nT | A solution exists only when nT lies between nL and nH |
| Approach in steps | Move in increments worth 0.002–0.003, then re-measure before deciding the next move | One big jump tends to overshoot, and overshoot costs time |
| Calibrate in conditions | Measure at the experiment temperature and at the experiment wavelength | Temperature coefficient about 1×10⁻⁴ per °C; dispersion shifts with wavelength |
| Degas and log | Rest or pull vacuum to release bubbles, re-measure, write down the ratio | Trapped bubbles scatter light and bias the reading |
| Parameter | Why it matters | Options in the IOTA range |
|---|---|---|
| Kinematic viscosity | Sets fill speed, bubble rise time, tracer settling rate and pump load | From 22.5 cSt (IOTA 556) through 450–550 mPa·s (255-500) up to six figures in the 201 family |
| Density | Governs tracer-particle selection and buoyancy balance | About 1.068 for BJ550 and 255-500, about 1.078 for 255-10000 |
| Volatility loss | Controls fluid level and index drift over long runs | IOTA 255-125 is specified at ≤1.0 wt% after 3 h at 150 °C |
| Aspect | What it looks like in practice |
|---|---|
| Full index span | Nominal coverage from 1.390 to 1.5785, across all three matching bands |
| Index and viscosity chosen separately | Several viscosities are available within one index band, so you need not trade matching accuracy for flow behaviour |
| Temperature headroom | IOTA 255A spans −70 °C to 320 °C and BJ550 reaches 315 °C in sealed systems, so a match set hot stays matched |
| Batch control | A documented quality system with pre-shipment testing; refractive index is a routine item |
| Supply and pack sizes | From 1 kg bottles to 200 kg drums, with standard grades held in stock |
| Technical back-up | Selection advice, sample testing and on-site support |
Viscosity, density and transmission decide whether the fluid is pleasant to work with. A correctly matched fluid that is too thin lets tracers settle in minutes; one that is too thick refuses to fill a microchannel.
Some data sheets quote index at 20 °C — IOTA 255-125, for instance, is listed at 1.490–1.505 on that basis. Leave room for the correction between conditions.
IOTA 704 and 705 are siloxane fluids like the others and can be proportioned against medium-phenyl grades to push a blend from the 1.49 region up past 1.55. Measure each component first.
Moulding route, residual plasticiser and annealing all move a plastic’s index. Slices cut in different directions from one acrylic sheet can read differently.
Set up two or three mother blends grouped around the materials you use, then trim at the bench from the low and high ends. That is faster than mixing from scratch each time.
Start with IOTA BJ550 or IOTA 255-125, whose 1.49–1.505 band sits right on PMMA at 1.49. If your target is Δn ≤ 0.005, measure the actual sheet first and trim with the low-index partner.
PC sits near 1.585 and PS near 1.59. Consider blending IOTA 705 (1.5765–1.5785) with IOTA 704 (1.5500–1.5600) and step toward the measured target, checking viscosity at the same time. If the residual gap stays wide, revisit the overall approach.
At 1.458, IOTA 556 (1.46) is the natural starting point. Adding IOTA 201 dimethyl fluid (1.390–1.410) pulls a blend down, so confirm direction and magnitude by measurement.
Two mother fluids: one from the low-index side (556 or the 250-30 direction) and one from the high side (255-500 or 54). Proportioning them gives continuous coverage across the interval.
Within one chemical family, viscosity moves the index very little, so set index by composition and viscosity by flow requirements. Across different phenyl loadings, however, grades of similar viscosity can differ in index by more than 0.01, so check each line item.
IOTA 54 at 1.50 and IOTA 56A at 1.498 are legitimate trimming components from an optical point of view. Whether to specify them depends on purity requirements, viscosity needs and cost, assessed together.
Company note: Iota Silicone Oil (Anhui) Co., Ltd. works the full silicone chain — methyl fluids, phenyl fluids, hydrogen-containing fluids, hydroxyl and vinyl fluids, vacuum diffusion pump fluids, silanes, fumed silica and platinum catalysts — and supports customers from selection through to on-site application work.
