The spelling "ui" and the pronunciation /uː/ in juice, fruit, bruise, cruise, sluice, suit, nuisance, recruit, bruit

The words juice, fruit, bruise, cruise, sluice, suit, pursuit, suitcase, lawsuit, nuisance, recruit, bruit are spelled with ui and pronounced with the IPA phoneme /uː/.

Full pronunciations from OED:

nuisance: Brit. /ˈnjuːsns/, U.S. /ˈn(j)us(ə)ns/
juice:         Brit. /dʒuːs/, U.S. /dʒus/
cruise:      Brit. /kruːz/, U.S. /kruz/
bruise:      Brit. /bruːz/, U.S. /bruz/
suit:           Brit. /s(j)uːt/, U.S. /sut/
recruit:     Brit. /rᵻˈkruːt/, U.S. /rəˈkrut/, /riˈkrut/
fruit:         Brit. /fruːt/, U.S. /frut/
sluice:       Brit. /sluːs/, U.S. /slus/
bruit:        Brit. /bruːt/, U.S. /brut/ (In sense 5 also) Brit. /ˈbruːi/, U.S. /ˈbrui/

The pronunciation differs from some other similar words spelled with ui like guise, disguise, guide, cuisine, suicide.

Full pronunciations from OED:

guide:       Brit. /ɡʌɪd/, U.S. /ɡaɪd/
guise:       Brit. /ɡʌɪz/, U.S. /ɡaɪz/
cuisine:   /kwiːˈziːn/
suicide:   /ˈs(j)uːɪsʌɪd/

Ruin is kind of an exception to an exception because the pronunciation is different in British English and American English:

Full pronunciations from OED:

ruin: Brit. /ˈruːɪn/, U.S. /ˈruən/, /ˈruˌɪn/

Note: There can be other similar words I might have missed and there can be dialectal/regional differences also.

When I checked the etymology of the word sluice in OED, I found this note:

The spelling with ui (compare juice) did not come into general use until the 18th century.

In fact, when you check the older (pre-1600s) spellings of the word sluice in OED, you can see that it is mostly spelled with u instead of ui. (Of course, there are other irregular spellings also). In OED, the first citation where the word is spelled with ui is from 1611. The word is written as sluice which is same as today and the citation is from Randle Cotgrave · A dictionarie of the French and English tongues · 1st edition.

In OED, the etymology of the word doesn't mention the ui spelling also (except the current Dutch word sluis):

< Old French escluse (-clusse , -clouse , etc.; modern French écluse ), = Spanish esclusa , Portuguese esclusa , late and medieval Latin exclusa (also sclusa , etc.), feminine singular of Latin exclūsus , past participle of exclūdere to shut out, exclude v.
_{Old French is also the source of Middle Dutch sluse , sluyse , sluus (Dutch sluis , West Frisian slús ), Middle Low German sluse , sluze (Low German slüse , slüs , German schleuse ), Danish sluse , Swedish slus . For the English forms which represent the late Latin clūsa see clow n.1}

Historical spellings of sluice from OED:

α.
1340: scluse
1449: Scluces
c1480: scluse
1558: sklus
1583: scluse
1609: scluces
1665 : Scluse
β.
1533: slewssis
1541–2: sloweses
1582: sleuss
?1677: slewces
γ.
1538: sluse
1568: slus
1577: Sluse
1611: sluce
1648: Sluses
1695: Sluces
δ.
1596: Sluyce
1611: Sluice
1699: sluice
1745: Sluices
1785: sluices
1839: sluice
1879: sluices
1794: sluices

You can see a similar pattern for juice, bruise and somewhat cruise also.

In OED, the first citation where juice is spelled as today is from a1626.

Etymology of juice from OED:

< French jus, < Latin jūs broth, sauce, juice of animal or plant. The β forms are normal from French; with the others compare those of duke, flute, jupe, and bruit, fruit.

OED has the note below in the etymology of cruise:

The word is thus ultimately identical with croise v. and cross v.; the current spelling with ui seems to be after Dutch; but the vowel sound is as in Spanish and Portuguese.

Although there isn't an exact pattern, the historical spelling change pattern explained above differs in the words like suit, fruit, recruit but the question and the answer might cover them also. The words in the question is mostly the ones where it has the ui spelling followed by ce and se.

After all these historical notes, what is the question?

Why was ui spelling introduced in words that used to be spelled with u and where the pronunciation is with IPA phoneme /uː/?

Why did it become more prevalent after 18th century?

Note: I know that historical "why" questions are hard to answer. English has a lot of irregularities and changes throughout the history also. There isn't an exact pattern or rule in spelling and orthography; but the spelling is mostly fixed in the traditional orthography.

• The answer might be related to the English spelling reforms or how Dutch influenced the spelling of the words in these reforms or in the history.
• Or is there any influential person or publication that might have affected these spellings?
• Or might the spelling of the word fruit or any other word have influenced the words like juice, sluice? Fruit is an older word but it has historical spellings like frut and the first citation with fruit is from 1549 in OED.
• Was there a need or a proposition of indicating longer u sound in the spelling reforms?

Solution 1:

Words with "gui" like "guide" and "guise" don't contain the "ui" digraph: instead, they have the "gu" digraph that functions to mark "hard g" before a front vowel letter (I or E).

The word suicide doesn't contain the "ui" digraph either: it has U followed by I in hiatus. This is the usual pronunciation of "ui" before a consonant letter in words from Latin, like suicide (from latin suicida).

Further background information about the "ui" digraph

As a digraph, "ui" acts as an alternative spelling of the "long u" sound in English. The basic pronunciation of the "long u" sound is /juː/, but because of "yod-dropping", the pronunciation /uː/ is used instead after /dʒ/ or /r/, or after a consonant cluster ending in /l/. The current pronunciations of the English "long u" sound are thought to have developed from a falling diphthong pronounced something like [ɪu̯]. The diphthong [ɪu̯] had several different sources:

• various diphthongs in native English words
• words from French that had in that language the front rounded monophthong /y/
• words from French with various diphthongs, such as beauty, deuce, suit from French beawté/beuté/bewté, deux, siute/sute/seute/suite

In modern Dutch, "ui" is used as a digraph representing a front rounded diphthong typically transcribed /œy̯/. This sound developed from an earlier long monophthong, as exemplified by the Dutch word bruin from Proto-Germanic *brūnaz (English brown). If the Dutch word sluis was borrowed from French before the diphthongization of long high vowels, that would explain why it has "ui".

The various changes that occurred between diphthongs and monophthongal long vowels, between and within languages, probably contributed to the diversity of spellings of the "long u" sound in present-day English.

The words bruise, juice and cruise

The word bruise is an unusual "ui" word in that it has a Germanic origin. The "ui" here is thought to have developed irregularly or dialectally from an Old English [yː] sound, which in most words lost its rounding (e.g. Old English [myːs] became modern English mice /maɪs/ via a Middle English form /miːs/). I talk a little bit more about it in my answer to the question "Relation of RUFFLER and RUYFLER".

With cruise, as the OED mentions, it seems pretty likely that the spelling was just taken from Dutch. Although the OED entry mentions cruse as a historical alternative spelling, I don't think it says that cruse used to be the more common spelling: both spellings (as well as cruize) are marked as being used starting from the 17th century. The pronunciation with /uː/ could be related to the pronunciation of Spanish/Portuguese cruzar, or it could be based on the spelling.

I don't know of any special reason for why the spelling juice won out over spellings like juce or juse. Neither of those spellings has any particular issue with representing the sound of the word: -⁠uce and -⁠use are used in the spelling of some words that rhyme with juice, such as puce, spruce, truce, reduce, use, abuse. Your suggestion of influence from other words spelled with "ui" seems the best explanation to me. I don't know of any way Dutch could have influenced the spelling of juice, since I haven't found any Dutch words spelled with ui that are related to French jus or English juice.

I'm not sure about the history of the spelling sluice.