Alloy Junction 1952 Wet the Germanium Faces

# Wet the Germanium Faces: Reaching Equilibrium Isn't Instant

Step nine established the equilibrium condition wetting needs — a clean surface the molten metal would eventually spread across completely — but equilibrium and "eventually" are doing a lot of work in that sentence, because nothing about a favorable contact angle guarantees the liquid gets there quickly. Now that step twenty has brought both disks into the liquid state, this step is where that spreading actually happens, in real time, against whatever duration the furnace schedule allots for it. A surface clean enough to wet perfectly in principle can still end up incompletely wetted in practice if the liquid simply isn't given enough time to finish spreading before the schedule moves on.

## 1. Spreading Takes a Real, Calculable Amount of Time

The characteristic time a molten droplet of radius $r$ takes to spread and relax toward its equilibrium contact angle scales with its viscosity $\eta$ divided by its surface tension $\gamma_{LV}$ — a genuine capillary-viscous balance, not an instantaneous snap to the angle step nine's Young equation predicts. A more viscous melt, or a larger pellet, simply takes longer to finish spreading, which means this step's required duration isn't a fixed universal number — it depends on exactly the pellet geometry step eleven chose and the melt properties step twenty's heating established, and the furnace schedule has to allow for it explicitly rather than assuming wetting is effectively instantaneous once melting starts.

## 2. Real Diagram: The Front Advances, It Doesn't Jump

## 3. A Trapped Pocket's Fate Depends on Its Own Size, Not Just Its Existence

As the wetting front advances over a surface that is clean but never perfectly flat, it inevitably encounters small-scale irregularities — and whether a gas pocket at one of them gets absorbed or survives as a permanent trapped void depends on the Laplace pressure the surrounding liquid meniscus can exert, which itself scales inversely with the pocket's own radius $r_{\text{pocket}}$. Small pockets see high Laplace pressure and tend to collapse or dissolve into the melt; larger ones see less pressure and are more likely to survive as a real defect right at the interface. This is why the damage-removal standard set back at step eight matters again here in a new way: not because residual damage threatens depth as it did there, but because it sets the size distribution of exactly the irregularities this step's advancing front now has to either bridge or trap.

## Wet the Germanium Faces's Place in the Process Lineage

Wetting the germanium faces is step twenty-one of RCA's forty-two-step alloy-junction manufacturing sequence — immediately after both disks reached the liquid state, and before germanium actually begins dissolving in earnest. It is the step that converts step nine's equilibrium wetting condition into a real, time-bounded spreading event, governed by a genuine capillary-viscous timescale, and it decides the fate of whatever small surface irregularities step eight's damage removal left behind by comparing each one's size against the Laplace pressure the advancing liquid can bring to bear. Step twenty-two, dissolving germanium locally, only begins once this step's front has finished sweeping across the intended footprint.